roxlap_render/

lib.rs

//! roxlap-render — unified CPU/GPU renderer facade.
//!
//! New to roxlap? **[The roxlap book](https://ncrashed.github.io/roxlap/)**
//! is the guide (quickstart, concepts, rendering, lighting, sprites);
//! this page is the API reference.
//!
//! One [`SceneRenderer`] hides the choice between the CPU opticast
//! path (`roxlap-core` / `roxlap-scene`, presented via `softbuffer`)
//! and the GPU compute-shader path (`roxlap-gpu`, presented via its
//! own wgpu surface). Construction picks the GPU backend when asked
//! and able, and **falls back to CPU automatically** when WGPU init
//! fails — so a host never has to branch on GPU availability or carry
//! the `Scene`→GPU upload/refresh/transform glue itself.
//!
//! Hosts stay thin: build a `Scene`, advance it from input, then call
//! [`SceneRenderer::render`] each frame. The facade owns the window
//! surface, the framebuffer/z-buffer (CPU) or the resident scene +
//! dirty-chunk tracking (GPU), and presentation.
//!
//! The per-frame flow is `render` → *(optional overlays)* → finish.
//! Between [`SceneRenderer::render`] and the finishing
//! [`SceneRenderer::present`] / [`SceneRenderer::paint_egui`] call, a
//! host may overlay depth-tested world-space lines with
//! [`SceneRenderer::draw_lines`] (editor gizmos, debug geometry — see
//! [`Line3`]); they land in the framebuffer, occluded by the rendered
//! scene, with egui still painting panels on top.
//!
//! Beyond the scene render itself, the facade owns sprites (static +
//! per-instance dynamic), animated voxel clips, billboard sprites +
//! actors, characters (`.rkc`), materials + transparency, dynamic
//! lighting ([`FrameParams::lights`]), screen→world picking, overlay
//! lines / images / egui, and the fixed-resolution post pipeline
//! ([`SceneRenderer::set_render_resolution`] / `set_ssaa` /
//! `set_posterize`).

#![forbid(unsafe_code)]
// Compile the workspace README's Rust snippets as doctests so the
// "Use it in your game" example can never rot the way the pre-QE.0
// Multicore snippet did (it kept calling API deleted releases earlier).
// `cfg(doctest)` keeps the README out of the rendered crate docs.
#[cfg(doctest)]
#[doc = include_str!("../../../README.md")]
struct ReadmeDoctests;

mod cpu;
/// WebGL2 framebuffer presenter for the CPU backend on wasm (the
/// browser has no `softbuffer`).
#[cfg(target_arch = "wasm32")]
mod cpu_blit;
#[cfg(feature = "hud")]
mod cpu_egui;
/// QE-C6 — the one place `ROXLAP_*` env overrides are read (once, at
/// construction). The user-facing variable table is on
/// [`RenderOptions`].
mod env_config;
mod gpu;
/// Dynamic lighting types (stages DL + SL) — runtime sun, point and
/// spot lights, on both backends.
mod light;
/// Particle system over dynamic sprite instances (stage PS) — see
/// `docs/porting/PORTING-PARTICLES.md`.
mod particles;

#[cfg(not(target_arch = "wasm32"))]
use std::sync::Arc;

use roxlap_core::kfa_draw::{compose_attachment, solve_kfa_limbs};
use roxlap_core::opticast::OpticastSettings;
use roxlap_core::sky::Sky;
use roxlap_core::Camera;
use roxlap_formats::material::MaterialTable;
use roxlap_formats::voxel_clip::{duration_prefix_sums, frame_at_prefix};
use roxlap_scene::Scene;

pub use light::{DirectionalLight, LightRig, PointLight, SpotLight};
pub use particles::{
    CollisionMode, ConeDef, EmitterId, EmitterShape, Particle, ParticleEmitterDef, ParticleSystem,
    SpawnMode, VelocityDef, CARVE_DEBRIS_CAP, DEFAULT_MAX_PARTICLES,
};
pub use roxlap_formats::character::{Attachment, Character, MeshRef};
pub use roxlap_formats::color::{OverlayColor, Rgb, VoxColor};
/// Animated-GIF → [`VoxelClip`] importer for Doom-style billboard sprites
/// (stage BB). Behind the `gif` feature; see `PORTING-BILLBOARD.md`.
#[cfg(feature = "gif")]
pub use roxlap_formats::gif_import;
pub use roxlap_formats::kfa::KfaSprite;
pub use roxlap_formats::kv6::Kv6;
pub use roxlap_formats::material::{BlendMode, Material};
/// PNG-sequence / APNG → [`VoxelClip`] importer (stage BB). Behind the `png`
/// feature; see `PORTING-BILLBOARD.md`.
#[cfg(feature = "png")]
pub use roxlap_formats::png_import;
pub use roxlap_formats::sprite::Sprite;
pub use roxlap_formats::voxel_clip::{
    DecodeError, DecodedClip, LoopMode, StreamingClip, VoxelClip, VoxelFrame,
};
pub use roxlap_gpu::{GpuInitError, GpuRendererSettings, PowerPreference};
// Re-exported so hosts can name the [`SceneRenderer::new`] bounds
// without adding a direct `raw-window-handle` dependency of their own.
pub use raw_window_handle::{HasDisplayHandle, HasWindowHandle};
// Re-exported so hosts feed [`SceneRenderer::paint_egui`] from the exact
// egui version the renderer was built against (`hud` feature).
#[cfg(feature = "hud")]
pub use egui;

use crate::cpu::CpuBackend;
use crate::gpu::GpuBackend;

/// Type-erased display handle stored by the CPU backend's softbuffer
/// surface. `raw-window-handle` implements `HasDisplayHandle` for
/// `Arc<H>` (`H: ?Sized`), and the bare trait object implements its
/// own object-safe trait — so `Arc<W>` coerces to `Arc<DynDisplay>`
/// for any provider `W`.
#[cfg(not(target_arch = "wasm32"))]
pub(crate) type DynDisplay = dyn HasDisplayHandle + Send + Sync + 'static;
/// Type-erased window handle counterpart to [`DynDisplay`].
#[cfg(not(target_arch = "wasm32"))]
pub(crate) type DynWindow = dyn HasWindowHandle + Send + Sync + 'static;

/// One placed sprite instance: which [`SpriteSet::models`] entry and
/// where in the world.
pub struct SpriteInstanceDesc {
    /// Positional index into [`SpriteSet::models`].
    pub model: usize,
    /// World position of the model's anchor (voxel units).
    pub pos: [f32; 3],
}

/// Stable handle to a registered sprite model, returned (one per
/// [`SpriteSet::models`] entry, in order) by
/// [`SceneRenderer::set_sprites`]. Pass it to
/// [`refresh_sprite_model`](SceneRenderer::refresh_sprite_model) to
/// re-register that model's geometry after a content edit — so callers
/// never track the positional `usize` index themselves. Opaque on
/// purpose: there is no arithmetic to do on it.
///
/// Also returned by [`SceneRenderer::add_sprite_model`] for an
/// incrementally registered model, and accepted by
/// [`remove_sprite_model`](SceneRenderer::remove_sprite_model). A handle
/// to a removed model is **stale**: it resolves to nothing, so passing
/// it anywhere is a safe no-op. The `gen` (generation) field guards a
/// future compacting registry; it stays `0` today because model slots
/// are tombstoned in place and never reused (GPU chain ids are
/// append-only).
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct SpriteModelId {
    pub(crate) slot: u32,
    pub(crate) gen: u32,
}

/// Stable handle to a **dynamically added** sprite instance — the result
/// of [`SceneRenderer::add_sprite_instance`], passed to
/// [`remove_sprite_instance`](SceneRenderer::remove_sprite_instance).
///
/// Backends remove instances by swap (O(1)), which moves another instance
/// into the freed slot; this handle survives that because the facade keeps
/// the id↔slot mapping up to date. The generation guards against a stale
/// handle aliasing a recycled slot.
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct SpriteInstanceId {
    slot: u32,
    gen: u32,
}

/// Facade-side slotmap that turns the backends' swap-remove indexing into
/// stable [`SpriteInstanceId`] handles. Both backends keep their dynamic
/// instances as a tail sublist indexed `0..n`; `order[dyn_index]` is the
/// owning slot, and a removal fixes up the one slot whose instance was
/// swapped into the hole.
#[derive(Default)]
struct DynInstanceMap {
    /// Per slot: `(generation, Some(dyn_index) while live)`.
    slots: Vec<(u32, Option<u32>)>,
    /// Per live `dyn_index`: the owning slot. Parallel to the backends'
    /// dynamic sublist (so `order.len()` == the dynamic instance count).
    order: Vec<u32>,
    free: Vec<u32>,
}

impl DynInstanceMap {
    /// Register a freshly appended instance (always at `dyn_index ==
    /// order.len()`); returns its stable handle.
    fn alloc(&mut self, dyn_index: u32) -> SpriteInstanceId {
        debug_assert_eq!(self.order.len() as u32, dyn_index);
        let slot = self.free.pop().unwrap_or_else(|| {
            self.slots.push((0, None));
            (self.slots.len() - 1) as u32
        });
        let gen = self.slots[slot as usize].0;
        self.slots[slot as usize].1 = Some(dyn_index);
        self.order.push(slot);
        SpriteInstanceId { slot, gen }
    }

    /// Resolve a handle to its current backend `dyn_index`, or `None` if
    /// it's stale / already removed.
    fn dyn_index(&self, id: SpriteInstanceId) -> Option<u32> {
        let (gen, idx) = *self.slots.get(id.slot as usize)?;
        (gen == id.gen).then_some(idx).flatten()
    }

    /// Apply a removal: the backend swap-removed `removed` and reported
    /// `moved` (the old-last `dyn_index` that slid into `removed`, or
    /// `None` if `removed` was itself the last).
    fn remove(&mut self, id: SpriteInstanceId, removed: u32, moved: Option<u32>) {
        self.slots[id.slot as usize].1 = None;
        self.slots[id.slot as usize].0 += 1; // bump generation
        self.free.push(id.slot);
        if let Some(last) = moved {
            let moved_slot = self.order[last as usize];
            self.slots[moved_slot as usize].1 = Some(removed);
            self.order[removed as usize] = moved_slot;
        }
        self.order.pop();
    }
}

/// Crate-internal contract of every epoch-slotmap handle type
/// ([`SpriteModelId`], [`VoxelClipId`], [`CharacterId`],
/// [`StreamingClipId`], [`BillboardActorId`]): mint from / split into
/// the `(slot, generation)` pair. Lets one generic [`EpochSlotMap`]
/// serve all five families while each keeps its own distinct handle
/// type (so a `CharacterId` can never be passed where a `VoxelClipId`
/// is expected).
trait SlotHandle: Copy {
    fn mint(slot: u32, gen: u32) -> Self;
    fn parts(self) -> (u32, u32);
}

macro_rules! impl_slot_handle {
    ($($t:ty),+ $(,)?) => {$(
        impl SlotHandle for $t {
            fn mint(slot: u32, gen: u32) -> Self {
                Self { slot, gen }
            }
            fn parts(self) -> (u32, u32) {
                (self.slot, self.gen)
            }
        }
    )+};
}

impl_slot_handle!(
    SpriteModelId,
    VoxelClipId,
    CharacterId,
    StreamingClipId,
    BillboardActorId,
);

/// Facade-side epoch slotmap (QE.1a — one generic replacing five
/// hand-rolled copies). Unlike [`DynInstanceMap`] there is **no**
/// swap-remove fixup: a slot maps 1:1 to the backends' positional
/// index, which is append-only and never reused. A removed entry
/// tombstones its slot *in place* (the backend frees the data but
/// keeps the index), so a stale handle resolves to `None` → a safe
/// no-op rather than aliasing another entry.
///
/// [`reset`](Self::reset) clears the slots **and bumps `epoch`**,
/// which is baked into each minted id's generation. A handle from
/// before a [`SceneRenderer::set_sprites`] therefore carries the old
/// epoch and resolves to `None` rather than silently aliasing the
/// entry that re-took its slot. ([`reset_live`](Self::reset_live) is
/// the model map's deliberate exception.)
struct EpochSlotMap<I> {
    /// Per slot (== backend positional index): `(epoch_at_alloc, live)`.
    slots: Vec<(u32, bool)>,
    epoch: u32,
    _handle: std::marker::PhantomData<I>,
}

impl<I> Default for EpochSlotMap<I> {
    fn default() -> Self {
        Self {
            slots: Vec::new(),
            epoch: 0,
            _handle: std::marker::PhantomData,
        }
    }
}

impl<I: SlotHandle> EpochSlotMap<I> {
    /// Register a freshly appended entry at positional `index` (always
    /// the current `slots.len()`); returns its stable handle.
    fn alloc(&mut self, index: u32) -> I {
        debug_assert_eq!(self.slots.len() as u32, index);
        self.slots.push((self.epoch, true));
        I::mint(index, self.epoch)
    }

    /// The handle a live slot at `index` resolves from — used by
    /// [`SceneRenderer::set_sprites`] to hand back ids for the `0..n`
    /// entries seeded by [`reset_live`](Self::reset_live).
    fn minted(&self, index: u32) -> I {
        I::mint(index, self.epoch)
    }

    /// Resolve a handle to its backend positional index, or `None` if
    /// it's stale / already removed.
    fn index(&self, id: I) -> Option<usize> {
        let (slot, gen) = id.parts();
        let (epoch, live) = *self.slots.get(slot as usize)?;
        (epoch == gen && live).then_some(slot as usize)
    }

    /// Tombstone a slot in place. Returns `false` if the handle is
    /// stale / already removed.
    fn remove(&mut self, id: I) -> bool {
        let (slot, gen) = id.parts();
        let Some(entry) = self.slots.get_mut(slot as usize) else {
            return false;
        };
        if entry.0 != gen || !entry.1 {
            return false;
        }
        entry.1 = false;
        true
    }

    /// Drop every entry and invalidate every outstanding handle (the
    /// epoch bump).
    fn reset(&mut self) {
        self.slots.clear();
        self.epoch = self.epoch.wrapping_add(1);
    }

    /// Positional indices of every live entry, ascending — the
    /// iteration [`SceneRenderer::tick`] drives collection updates
    /// from.
    fn live_indices(&self) -> impl Iterator<Item = usize> + '_ {
        self.slots
            .iter()
            .enumerate()
            .filter_map(|(i, &(_, live))| live.then_some(i))
    }

    /// Re-seat `n` live entries with positional ids `0..n` **without**
    /// bumping the epoch — [`SceneRenderer::set_sprites`]' model-map
    /// semantics, where model index = chain id on both backends and a
    /// pre-rebuild positional id deliberately keeps meaning "model
    /// `i` of the current set".
    fn reset_live(&mut self, n: usize) {
        self.slots.clear();
        self.slots.resize(n, (self.epoch, true));
    }
}

/// Stable handle to a registered animated voxel clip (VCL.4) — the
/// result of [`SceneRenderer::add_voxel_clip`], passed to
/// [`add_clip_instance_posed`](SceneRenderer::add_clip_instance_posed)
/// and [`remove_voxel_clip`](SceneRenderer::remove_voxel_clip). Like
/// [`SpriteModelId`], a removed clip's handle is stale → a safe no-op.
/// Reset by [`set_sprites`](SceneRenderer::set_sprites) (which drops the
/// dynamic + clip layers).
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct VoxelClipId {
    slot: u32,
    gen: u32,
}

/// Stable handle to a registered animated character (VCL.6) — the result
/// of [`SceneRenderer::add_character`], advanced each frame with
/// [`advance_character`](SceneRenderer::advance_character) and dropped with
/// [`remove_character`](SceneRenderer::remove_character). Reset by
/// [`set_sprites`](SceneRenderer::set_sprites).
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct CharacterId {
    slot: u32,
    gen: u32,
}

/// Stable handle to a registered **streaming** voxel clip (follow-up #3) —
/// the result of [`SceneRenderer::add_streaming_clip`], advanced with
/// [`set_streaming_clip_frame`](SceneRenderer::set_streaming_clip_frame) and
/// dropped with
/// [`remove_streaming_clip`](SceneRenderer::remove_streaming_clip). Reset by
/// [`set_sprites`](SceneRenderer::set_sprites).
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct StreamingClipId {
    slot: u32,
    gen: u32,
}

/// Handle to an instance of a streaming clip
/// ([`add_streaming_clip_instance`](SceneRenderer::add_streaming_clip_instance)).
///
/// Deliberately **distinct** from [`SpriteInstanceId`]: a streaming clip's
/// frame is per-*clip* (all its instances share one re-uploaded model,
/// advanced by
/// [`set_streaming_clip_frame`](SceneRenderer::set_streaming_clip_frame)), so
/// a streaming instance is *not* accepted by the per-instance
/// [`set_clip_instance_frame`](SceneRenderer::set_clip_instance_frame) —
/// trying to scrub two instances of one streaming clip independently is a
/// compile error, not a silent coupling. (Use a flipbook clip for
/// per-instance frames.) Move it with
/// [`set_streaming_instance_transform`](SceneRenderer::set_streaming_instance_transform)
/// and drop it with
/// [`remove_streaming_instance`](SceneRenderer::remove_streaming_instance).
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct StreamingInstanceId(SpriteInstanceId);

/// One registered streaming clip: the seekable cursor + the single sprite
/// model it re-uploads each frame, plus the dims/pivot used to rebuild it.
struct StreamingClipState {
    cursor: StreamingClip,
    model: SpriteModelId,
    dims: [u32; 3],
    pivot: [f32; 3],
    /// Colour→material map (TV.3), empty for an all-opaque streaming clip.
    /// Re-applied on every per-frame re-upload so the streamed model keeps
    /// its per-voxel materials as it advances.
    material_map: Vec<(Rgb, u8)>,
    /// PF.5 — the clip frame currently resident on `model`. A player driven
    /// at render rate re-applies the same clip frame most ticks (10 fps clip
    /// on a 144 fps render = 14×); when it matches, the dense `to_kv6`
    /// rebuild + LOD chain + GPU re-upload are skipped entirely.
    last_applied: Option<usize>,
}

/// Per-clip-attachment playback clock (VCL.6): the timing it needs to
/// resolve a frame, plus its own accumulating clock.
struct ClipClock {
    /// PF.13 (S4) — inclusive duration prefix sums (ms), built once per
    /// timeline via [`duration_prefix_sums`] so every tick resolves its
    /// frame by binary search instead of re-summing the duration list.
    prefix: Vec<u64>,
    loop_mode: LoopMode,
    /// Playback rate, Q8 (256 = 1×).
    speed_q8: i32,
    /// Accumulated playback time (ms), seeded from the attachment's
    /// `start_phase_ms`.
    clock_ms: f64,
}

impl ClipClock {
    /// Build a clock over `durations` starting at `clock_ms`.
    fn new(durations: &[u32], loop_mode: LoopMode, speed_q8: i32, clock_ms: f64) -> Self {
        Self {
            prefix: duration_prefix_sums(durations),
            loop_mode,
            speed_q8,
            clock_ms,
        }
    }

    /// Frame shown at `elapsed_ms` on this clock's timeline —
    /// [`frame_at`](roxlap_formats::voxel_clip::frame_at) over the
    /// cached prefix sums.
    fn frame_for(&self, elapsed_ms: u32) -> u32 {
        frame_at_prefix(&self.prefix, self.loop_mode, elapsed_ms) as u32
    }

    /// Advance the clock by `dt` seconds at its Q8 `speed` and return the
    /// frame to show. Shared by character attachments and standalone clip
    /// players. A negative clock (rewind past 0) reads as frame 0 but is
    /// kept signed so resuming forward is continuous.
    #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
    fn tick(&mut self, dt: f64) -> u32 {
        self.clock_ms += dt * 1000.0 * f64::from(self.speed_q8) / 256.0;
        self.frame_for(self.clock_ms.max(0.0) as u32)
    }

    /// Retarget this clock to a different clip's timeline (BB.1): swap the
    /// per-frame `durations` + `loop_mode` and restart at `0`, **preserving
    /// the playback rate** (`speed_q8`). Used by
    /// [`SceneRenderer::set_clip_instance_clip`] so swapping a billboard's
    /// animation keeps its speed / pause policy.
    fn retarget(&mut self, durations: &[u32], loop_mode: LoopMode) {
        self.prefix = duration_prefix_sums(durations);
        self.loop_mode = loop_mode;
        self.clock_ms = 0.0;
    }
}

/// Facade-side metadata captured for a registered flipbook clip, so editor
/// queries + the auto-player don't shadow the `DecodedClip`.
struct ClipMeta {
    dims: [u32; 3],
    pivot: [f32; 3],
    voxel_world_size: f32,
    durations: Vec<u32>,
    loop_mode: LoopMode,
    /// Colour→material map the clip was registered with (TV.3), empty for an
    /// all-opaque clip. Retained so an in-place
    /// [`update_clip_frame`](SceneRenderer::update_clip_frame) re-classifies
    /// the edited frame's voxels instead of dropping its per-voxel materials.
    material_map: Vec<(Rgb, u8)>,
}

/// Public metadata for a registered clip — the inspector view returned by
/// [`SceneRenderer::clip_metadata`].
#[derive(Clone, Debug, PartialEq)]
pub struct ClipMetadata {
    /// Fixed bounding box (voxels).
    pub dims: [u32; 3],
    /// Model pivot (the kv6 pivot frames share).
    pub pivot: [f32; 3],
    /// Render scale (1 voxel = this many world units).
    pub voxel_world_size: f32,
    /// Playback wrap behaviour.
    pub loop_mode: LoopMode,
    /// Number of frames.
    pub frame_count: usize,
    /// Per-frame durations (ms), one per frame.
    pub durations: Vec<u32>,
    /// Total loop length (ms) — sum of `durations`.
    pub total_ms: u32,
}

/// What an auto-advancing [`ClipPlayer`] (#6) drives each
/// [`advance_voxel_clips`](SceneRenderer::advance_voxel_clips). A flipbook
/// clip's frame is per-instance; a streaming clip's is per-clip (its
/// instances share one model), so the targets differ.
#[derive(Clone, Copy)]
enum PlayerTarget {
    Flipbook(SpriteInstanceId),
    Streaming(StreamingClipId),
}

/// A standalone clip given its own playback clock (#6): the host calls
/// `advance_voxel_clips(dt)` once instead of hand-driving `frame_at` +
/// `set_clip_instance_frame`.
struct ClipPlayer {
    target: PlayerTarget,
    clock: ClipClock,
    /// When `true`, [`advance_voxel_clips`](SceneRenderer::advance_voxel_clips)
    /// leaves the clock (and frame) untouched — the editor's play/pause.
    paused: bool,
}

/// One live bone attachment: which bone drives it, its local offset, the
/// renderer instance it owns, and (for a clip target) its playback clock.
struct AttachInst {
    bone: usize,
    local_offset: roxlap_formats::xform::BoneXform,
    inst: SpriteInstanceId,
    clip: Option<ClipClock>,
}

/// A live animated character: the hinge skeleton (the bone-transform
/// solver) + one [`AttachInst`] per bone attachment.
struct CharInstance {
    skeleton: KfaSprite,
    attaches: Vec<AttachInst>,
    /// Sprite models + voxel clips this character registered, so
    /// [`remove_character`](SceneRenderer::remove_character) can free them
    /// (otherwise they leak until the next `set_sprites`).
    models: Vec<SpriteModelId>,
    clips: Vec<VoxelClipId>,
}

/// Orientation + position for a dynamic sprite instance — the per-frame
/// pose passed to [`SceneRenderer::add_sprite_instance_posed`] and
/// [`set_sprite_instance_transform`](SceneRenderer::set_sprite_instance_transform).
///
/// `right`/`up`/`forward` are the instance's local axes expressed in
/// world space (the columns of the model→world rotation), mapping
/// directly onto the underlying [`Sprite`]'s `s`/`h`/`f` (kv6 local
/// +x/+y/+z). They **must** be non-singular (`det ≠ 0`) but need not be
/// orthonormal — a uniform/non-uniform scale or shear is fine. A
/// near-singular basis falls through the renderer's degenerate-basis
/// guards and the instance silently skips that frame rather than
/// panicking. [`Default`] is the identity basis (axis-aligned).
#[derive(Clone, Copy, Debug)]
pub struct DynSpriteTransform {
    /// Instance world position (the kv6 pivot maps here).
    pub pos: [f32; 3],
    /// Local +x in world space ↦ [`Sprite::s`].
    pub right: [f32; 3],
    /// Local +y in world space ↦ [`Sprite::h`].
    pub up: [f32; 3],
    /// Local +z in world space ↦ [`Sprite::f`].
    pub forward: [f32; 3],
}

impl Default for DynSpriteTransform {
    fn default() -> Self {
        Self {
            pos: [0.0, 0.0, 0.0],
            right: [1.0, 0.0, 0.0],
            up: [0.0, 1.0, 0.0],
            forward: [0.0, 0.0, 1.0],
        }
    }
}

impl DynSpriteTransform {
    /// Stamp this pose onto a [`Sprite`] in place: `pos → p`,
    /// `right/up/forward → s/h/f` (a direct copy — the basis is the
    /// model→world columns). Both backends keep the rest of the template
    /// (`kv6`, `flags`) and only overwrite the pose.
    pub(crate) fn apply_to(self, s: &mut Sprite) {
        s.p = self.pos;
        s.s = self.right;
        s.h = self.up;
        s.f = self.forward;
    }
}

/// How a billboard instance turns to face the camera (BB.2). Set per
/// instance via [`SceneRenderer::add_billboard_instance`] /
/// [`set_billboard_mode`](SceneRenderer::set_billboard_mode); applied each
/// [`face_billboards_to`](SceneRenderer::face_billboards_to).
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum BillboardMode {
    /// Not auto-oriented — the host drives its transform directly. Default
    /// (so a billboard record with no mode is inert).
    #[default]
    None,
    /// Yaw-only: the slab stays vertical (image up = world up) and rotates
    /// about the vertical axis to face the camera. The Doom/Build default —
    /// its cast shadow stays sane (a vertical card) as the camera orbits.
    Cylindrical,
    /// Full face: the slab is always perpendicular to the camera direction
    /// (pitches with the view). Ideal head-on, but its cast shadow rotates
    /// as you orbit.
    Spherical,
}

/// How a sprite/billboard instance derives its **shading normal** (BB.2b) —
/// a per-instance choice that rides the sprite `flags`. A camera-facing
/// billboard's DDA face normal tracks the camera, so its `N·L` would shift as
/// you orbit; `WorldUp` / `AmbientOnly` tame that. Only affects the dynamic
/// lighting path (a disabled rig is unaffected). Set via
/// [`set_sprite_instance_lighting`](SceneRenderer::set_sprite_instance_lighting)
/// or [`BillboardActorDef::lighting`].
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum BillboardLighting {
    /// The DDA hit-face normal — today's DL.7 look (default).
    #[default]
    FaceNormal,
    /// A fixed world-up normal: stable directional shading regardless of the
    /// camera angle.
    WorldUp,
    /// Ambient only — no sun / point-light direct term, the flattest,
    /// most Doom-faithful cutout look (still scaled by the scene's ambient
    /// level, so it dims in a dim scene).
    AmbientOnly,
    /// Full-bright / **emissive** — the voxel colour at full intensity,
    /// ignoring all lighting. The right look for glows (fire, spell auras,
    /// muzzle flashes) and markers that shouldn't darken in shadow.
    FullBright,
}

/// One camera-facing billboard instance (BB.2): the clip/sprite instance it
/// drives, its world position, and how it orients.
struct BillboardRec {
    id: SpriteInstanceId,
    pos: [f32; 3],
    mode: BillboardMode,
}

/// roxlap world up — voxlap is z-down, so up is `-z` (matches the
/// scene-demo camera builder + the lighting bake's z convention). Billboard
/// orientation assumes this; an app with a different up convention would
/// need this generalised (not exposed yet — YAGNI).
const BILLBOARD_UP: [f32; 3] = [0.0, 0.0, -1.0];

fn bb_norm(v: [f32; 3]) -> Option<[f32; 3]> {
    let m = (v[0] * v[0] + v[1] * v[1] + v[2] * v[2]).sqrt();
    (m > 1e-6).then(|| [v[0] / m, v[1] / m, v[2] / m])
}

fn bb_cross(a: [f32; 3], b: [f32; 3]) -> [f32; 3] {
    [
        a[1] * b[2] - a[2] * b[1],
        a[2] * b[0] - a[0] * b[2],
        a[0] * b[1] - a[1] * b[0],
    ]
}

/// The camera-facing basis for a billboard at `pos` (the slab's local axes:
/// `+x` = image horizontal, `+y` = normal toward the camera, `+z` = image
/// vertical). Returns `None` for [`BillboardMode::None`] or a degenerate
/// pose (camera on the sprite's vertical axis for cylindrical; looking
/// straight along world-up for spherical) — the caller then skips it.
fn billboard_transform(
    pos: [f32; 3],
    cam: [f64; 3],
    mode: BillboardMode,
) -> Option<DynSpriteTransform> {
    #[allow(clippy::cast_possible_truncation)]
    let to_cam = [
        cam[0] as f32 - pos[0],
        cam[1] as f32 - pos[1],
        cam[2] as f32 - pos[2],
    ];
    // `+y` = slab normal toward the camera (horizontal-only for
    // cylindrical). Degenerate view axes (camera exactly overhead /
    // exactly at the pos — e.g. a third-person camera the instant
    // before its boom applies) fall back to a fixed facing instead
    // of returning None: the card is edge-on/invisible right then
    // anyway, but the POSITION update must never be dropped, or the
    // actor freezes at its last pose (the CC.4 third-person bug).
    const FALLBACK: [f32; 3] = [0.0, 1.0, 0.0];
    let ny = match mode {
        BillboardMode::Cylindrical => bb_norm([to_cam[0], to_cam[1], 0.0]).unwrap_or(FALLBACK),
        BillboardMode::Spherical => bb_norm(to_cam).unwrap_or(FALLBACK),
        BillboardMode::None => return None,
    };
    // `+x` = image horizontal = screen-right (non-mirrored): up × normal.
    let nx = bb_norm(bb_cross(BILLBOARD_UP, ny)).unwrap_or([1.0, 0.0, 0.0]);
    // `+z` = image vertical (≈ world up; exactly world up for cylindrical).
    let nz = bb_cross(ny, nx);
    Some(DynSpriteTransform {
        pos,
        right: nx,
        up: ny,
        forward: nz,
    })
}

/// Apply shadow cast/receive booleans to a sprite `flags` word in place
/// (XS.4 bits 4/5), preserving the other bits. Shared by both backends'
/// per-instance shadow-flag setters (BB.3).
pub(crate) fn apply_shadow_flags(flags: &mut u32, casts: bool, receives: bool) {
    use roxlap_formats::sprite::{SPRITE_FLAG_NO_SHADOW_CAST, SPRITE_FLAG_NO_SHADOW_RECEIVE};
    if casts {
        *flags &= !SPRITE_FLAG_NO_SHADOW_CAST;
    } else {
        *flags |= SPRITE_FLAG_NO_SHADOW_CAST;
    }
    if receives {
        *flags &= !SPRITE_FLAG_NO_SHADOW_RECEIVE;
    } else {
        *flags |= SPRITE_FLAG_NO_SHADOW_RECEIVE;
    }
}

/// Apply a [`BillboardLighting`] mode to a sprite `flags` word in place
/// (BB.2b bits 6/7), preserving the other bits. Shared by both backends'
/// per-instance lighting setters.
pub(crate) fn apply_lighting_flags(flags: &mut u32, mode: BillboardLighting) {
    use roxlap_formats::sprite::{SPRITE_FLAG_LIGHT_AMBIENT_ONLY, SPRITE_FLAG_LIGHT_WORLD_UP};
    *flags &= !(SPRITE_FLAG_LIGHT_WORLD_UP | SPRITE_FLAG_LIGHT_AMBIENT_ONLY);
    match mode {
        BillboardLighting::FaceNormal => {}
        BillboardLighting::WorldUp => *flags |= SPRITE_FLAG_LIGHT_WORLD_UP,
        BillboardLighting::AmbientOnly => *flags |= SPRITE_FLAG_LIGHT_AMBIENT_ONLY,
        // Full-bright is encoded as both bits set (the decoders check it first).
        BillboardLighting::FullBright => {
            *flags |= SPRITE_FLAG_LIGHT_WORLD_UP | SPRITE_FLAG_LIGHT_AMBIENT_ONLY;
        }
    }
}

// ---- billboard actors (BB.4) --------------------------------------------

/// Stable handle to a [`BillboardActor`](SceneRenderer::add_billboard_actor)
/// — a high-level directional billboard managed by the renderer (it owns one
/// clip instance, picks the directional clip by view angle, and plays a
/// named-state animation). Reset by [`set_sprites`](SceneRenderer::set_sprites);
/// a removed actor's handle is stale → a safe no-op.
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct BillboardActorId {
    slot: u32,
    gen: u32,
}

/// One animation state of a [`BillboardActorDef`]: its name plus the clips
/// for each viewing direction. `dirs.len()` may be `1` (non-directional),
/// `8` (classic Doom rotations), or any `N` (uniform angular bins). Index 0
/// is the view-from-front (camera in the actor's facing direction),
/// increasing counter-clockwise.
pub struct ActorState {
    /// State name [`SceneRenderer::set_actor_state`] selects by
    /// (e.g. `"walk"`, `"attack"`). Owned since QE.7b, so actor
    /// definitions can come from data files (monster definitions
    /// loaded at runtime) without `Box::leak`.
    pub name: String,
    /// One clip per view direction, counter-clockwise from "facing the
    /// viewer" — 1 (billboard), 4, or 8 entries.
    pub dirs: Vec<VoxelClipId>,
}

/// How a sprite instance participates in dynamic-light shadows
/// (QE.7b — replaces the unreadable
/// `set_sprite_instance_shadow_flags(id, true, false)` bool pair).
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub struct ShadowFlags {
    /// The instance's voxels block dynamic-light shadow rays.
    pub casts: bool,
    /// Terrain / other-caster shadows darken the instance's voxels.
    pub receives: bool,
}

impl Default for ShadowFlags {
    /// Both on — the participating default every spawn starts with.
    fn default() -> Self {
        Self {
            casts: true,
            receives: true,
        }
    }
}

/// Recipe for [`add_billboard_actor`](SceneRenderer::add_billboard_actor).
pub struct BillboardActorDef {
    /// Animation states (≥1, each with ≥1 directional clip). The first is
    /// the initial state.
    pub states: Vec<ActorState>,
    /// How the slab turns to face the camera (default [`BillboardMode::Cylindrical`]).
    pub mode: BillboardMode,
    /// Shading-normal mode (BB.2b; default [`BillboardLighting::FaceNormal`]).
    pub lighting: BillboardLighting,
    /// Playback rate of the state animation (`1.0` = authored speed,
    /// negative = reverse). QE.7b — was Q8 fixed point (`speed_q8:
    /// 256`); migrate with `speed = speed_q8 as f32 / 256.0`.
    pub speed: f32,
    /// World units per slab voxel (`1.0` = classic 1:1). Applied to
    /// the instance basis every tick. Composes multiplicatively with
    /// the clip's own `voxel_world_size` (both backends honour both).
    pub scale: f32,
    /// Shadow participation for the actor's voxels.
    pub shadows: ShadowFlags,
}

impl Default for BillboardActorDef {
    fn default() -> Self {
        Self {
            states: Vec::new(),
            mode: BillboardMode::Cylindrical,
            lighting: BillboardLighting::FaceNormal,
            speed: 1.0,
            scale: 1.0,
            shadows: ShadowFlags::default(),
        }
    }
}

/// QE.7b — the public API speaks `f32` speed (`1.0` = authored rate);
/// the clip clocks keep their Q8 fixed-point internals.
#[allow(clippy::cast_possible_truncation)]
fn speed_to_q8(speed: f32) -> i32 {
    (speed * 256.0).round() as i32
}

/// A live directional billboard: one clip instance whose directional clip is
/// reselected by view angle and whose animation plays a named state.
struct BillboardActor {
    /// World units per slab voxel (from the def).
    scale: f32,
    inst: SpriteInstanceId,
    states: Vec<ActorState>,
    cur_state: usize,
    pos: [f32; 3],
    /// World yaw the actor "faces" (radians); the dir picker compares the
    /// camera's bearing against it.
    facing_yaw: f64,
    mode: BillboardMode,
    clock: ClipClock,
    /// The directional clip currently shown, to avoid redundant clip swaps.
    showing: Option<VoxelClipId>,
    /// The def's playback rate, kept for state-switch clock rebuilds.
    speed: f32,
}

impl BillboardActor {
    /// Pick the directional clip index for a camera at `cam` (world). See
    /// [`dir_index`].
    fn pick_dir(&self, cam: [f64; 3]) -> usize {
        dir_index(
            self.pos,
            self.facing_yaw,
            cam,
            self.states[self.cur_state].dirs.len(),
        )
    }
}

/// Bin a camera's bearing (relative to an actor at `pos` facing `facing_yaw`)
/// into one of `n` viewing-direction sectors. Index 0 = viewed-from-front
/// (camera in the actor's facing direction), increasing counter-clockwise.
/// `n <= 1` or a camera directly above/below ⇒ 0.
fn dir_index(pos: [f32; 3], facing_yaw: f64, cam: [f64; 3], n: usize) -> usize {
    if n <= 1 {
        return 0;
    }
    let dx = cam[0] - f64::from(pos[0]);
    let dy = cam[1] - f64::from(pos[1]);
    if dx * dx + dy * dy < 1e-12 {
        return 0; // camera directly above/below → no horizontal bearing
    }
    let rel = (dy.atan2(dx) - facing_yaw).rem_euclid(std::f64::consts::TAU);
    let sector = std::f64::consts::TAU / n as f64;
    #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
    let idx = (rel / sector).round() as usize % n;
    idx
}

/// Backend-agnostic sprite description. The facade builds the CPU
/// per-instance draw list and the GPU instanced registry from the
/// same data, so both backends show identical sprites. The host owns
/// content (which models, where, recolouring) — building a recoloured
/// variant is just a second [`Sprite`] model with edited `kv6.voxels`.
pub struct SpriteSet {
    /// Distinct voxel models (KV6 + base orientation). Instances index
    /// into this; their position overrides the model's.
    pub models: Vec<Sprite>,
    /// Initial placements — one drawn sprite per entry, each binding a
    /// [`models`](Self::models) index to a world position.
    pub instances: Vec<SpriteInstanceDesc>,
    /// Model the [`SceneRenderer::carve_active_sprite`] hotkey edits
    /// (GPU only, mirroring the demo's `G`-carve). `None` disables it.
    pub carve_model: Option<usize>,
}

/// Per-frame inputs both backends consume. The host builds the
/// [`OpticastSettings`] (it owns scan distance, projection etc.); the
/// facade does everything else (pool config, sky fill, render,
/// present).
///
/// **Both backends derive their projection from
/// [`settings`](Self::settings)** (QE.2a): the vertical field of view
/// is `2·atan(yres/2 / hz)` and the GPU outer-DDA step budget follows
/// [`OpticastSettings::max_scan_dist`], so the two backends can no
/// longer disagree about what they show. Pick an explicit FOV with
/// [`OpticastSettings::with_fov_y`].
///
/// `#[non_exhaustive]`: construct with [`FrameParams::new`] and
/// override fields — a future field addition is then not a breaking
/// change (pre-QE.2, every added field broke every host's struct
/// literal).
#[non_exhaustive]
pub struct FrameParams<'a> {
    /// Render settings both backends share: framebuffer geometry +
    /// projection (`hx`/`hy`/`hz`), scan distances, and the mip
    /// ladder. (Named for the CPU renderer it configured first; the
    /// GPU backend derives its FOV + step budget from it too.)
    pub settings: &'a OpticastSettings,
    /// Packed engine sky colour: the CPU sky-miss fill + skycast, and
    /// the clear colour if no scene renders.
    pub sky_color: Rgb,
    /// Optional sky panorama for the CPU rasterizer's sky sampling.
    pub sky: Option<&'a Sky>,
    /// CPU fog: the packed colour distant voxels fade toward.
    pub fog_color: Rgb,
    /// CPU fog: full-fog distance (voxels). `0` disables CPU fog.
    pub fog_max_scan_dist: i32,
    /// CPU: treat z=255 as air (avoids the S1.X bedrock path for
    /// out-of-bounds cameras).
    pub treat_z_max_as_air: bool,
    /// Whether to draw the renderer's sprites this frame. Both backends
    /// draw KV6 sprites flat-lit (the clean-room DDA sprite raycaster on
    /// CPU; uploaded model colours on GPU), so no host-supplied lighting
    /// is needed — this is just the on/off opt-in. `false` skips sprite
    /// drawing.
    pub draw_sprites: bool,
    /// Per-face directional shading for the voxel grids — voxlap's
    /// `setsideshades(top, bot, left, right, up, down)`, the grid-scan
    /// analogue of [`draw_sprites`](Self::draw_sprites). Each
    /// entry darkens the faces pointing that way; the host typically
    /// passes its engine's `side_shades()`. The default `[0; 6]` keeps
    /// `sideshademode` off (no per-side shading), so existing hosts and
    /// the oracle goldens are unaffected. Applied each frame by **both**
    /// backends: the CPU rasteriser via `gcsub`, and the GPU scene-DDA
    /// pass by darkening a hit voxel's brightness by the hit face's
    /// shade (the face taken from the DDA's last-stepped axis).
    pub side_shades: [i8; 6],
    /// Dynamic lighting (stages DL + SL) — runtime sun + point + spot
    /// lights + stylized shadows, applied by **both** backends (GPU
    /// shaders since DL; the CPU renderer since CPU.1/CPU.2). `None`
    /// (the default for hosts that don't set it) ⇒ exactly the pre-DL
    /// render, both backends. The baked brightness byte is
    /// reinterpreted as the ambient/AO channel; direct light composites
    /// on top (`albedo*ambient + Σ direct`).
    pub lights: Option<LightRig<'a>>,
}

impl<'a> FrameParams<'a> {
    /// Frame params with sensible defaults for everything except
    /// [`settings`](Self::settings) (which the host owns): the default
    /// sky colour ([`RenderOptions::clear_sky`]'s default), no
    /// panorama, CPU fog off, sprites on, no per-side shading, no
    /// dynamic lights. Both backends' projection comes from
    /// `settings` (see the struct docs).
    ///
    /// `treat_z_max_as_air` defaults to `true` (what every demo passes;
    /// out-of-bounds cameras skip the bedrock path). Hosts that want
    /// the pre-QE literal-struct behaviour set it back to `false`.
    ///
    /// Every field stays public — construct with `new` and override
    /// what differs:
    ///
    /// ```ignore
    /// let mut fp = FrameParams::new(&settings);
    /// fp.sky = Some(&sky);
    /// fp.lights = Some(rig);
    /// ```
    #[must_use]
    pub fn new(settings: &'a OpticastSettings) -> Self {
        let default_sky = RenderOptions::default().clear_sky;
        Self {
            settings,
            sky_color: default_sky,
            sky: None,
            fog_color: default_sky,
            fog_max_scan_dist: 0,
            treat_z_max_as_air: true,
            draw_sprites: true,
            side_shades: [0; 6],
            lights: None,
        }
    }

    /// The vertical field of view both backends render `settings`
    /// with: `2·atan(yres/2 / hz)` — resolution-independent, so it
    /// holds at any window / logical size.
    // Screen dims cast to f32 are bounded by realistic resolutions.
    #[allow(clippy::cast_precision_loss)]
    #[must_use]
    pub fn fov_y_rad(&self) -> f32 {
        2.0 * ((self.settings.yres as f32) * 0.5 / self.settings.hz).atan()
    }

    /// The GPU outer-DDA step budget (chunks) derived from
    /// [`OpticastSettings::max_scan_dist`]: enough chunk steps to cover
    /// the scan distance, plus slack for the entry/exit chunks.
    #[allow(clippy::cast_sign_loss)]
    #[must_use]
    pub fn gpu_outer_steps(&self) -> u32 {
        (self.settings.max_scan_dist.max(1) as u32) / roxlap_scene::CHUNK_SIZE_XY + 4
    }
}

/// Backend-agnostic scene bookkeeping the facade owns **once** (QE.3a
/// — previously each backend kept a duplicate copy, and every new
/// feature needed three coordinated edits: facade arm + cpu + gpu,
/// with parity drift as the failure mode: the GPU copy had grown
/// change-detection the CPU copy lacked).
///
/// Backends receive `&SceneState` where they need it (the render
/// passes); the truly divergent reactions (CPU flipbook draw, GPU
/// instance-buffer/model-id writes, device palette mirror) stay in the
/// backends behind small `apply_*` hooks.
pub(crate) struct SceneState {
    /// Global voxel-material palette (TV stage): per-voxel material
    /// ids resolve to opacity + blend mode here. The CPU compositor
    /// reads it live each frame; the GPU mirrors it to device
    /// palettes when [`materials_dirty`](Self::materials_dirty).
    pub(crate) materials: MaterialTable,
    /// Terrain colour→material map (TV.4) — matching-colour world
    /// voxels render with that material (glass walls, water).
    pub(crate) terrain_materials: Vec<(Rgb, u8)>,
    /// PF.5 — set when `materials` / `terrain_materials` change;
    /// cleared after each `render` (the GPU mirrors device palettes
    /// only then). Starts `true` so the first GPU frame seeds them.
    pub(crate) materials_dirty: bool,
    /// Per **dynamic** instance (parallel to the backends' dynamic
    /// sublist): `Some((clip_index, current_frame))` for a clip
    /// instance, `None` for a plain model instance. The CPU render
    /// picks each instance's flipbook frame from this; the GPU keeps
    /// its instance buffer in sync via the `apply_clip_*` hooks.
    pub(crate) dyn_clip: Vec<Option<(usize, usize)>>,
}

impl Default for SceneState {
    fn default() -> Self {
        Self {
            materials: MaterialTable::new(),
            terrain_materials: Vec::new(),
            materials_dirty: true,
            dyn_clip: Vec::new(),
        }
    }
}

/// Result of [`SceneRenderer::pick`] — a resolved screen→world voxel
/// hit. `world` is the surface point (`cam.pos + t · normalize(ray)`);
/// `grid` + `voxel` are the owning grid and its **grid-local** voxel
/// (transform-correct for rotated / translated grids).
#[derive(Clone, Copy, PartialEq, Debug)]
pub struct PickHit {
    /// World-space surface point of the hit.
    pub world: [f32; 3],
    /// The grid owning the hit voxel.
    pub grid: roxlap_scene::GridId,
    /// The hit voxel, in `grid`-local coordinates.
    pub voxel: glam::IVec3,
}

/// A world-space view ray: the canonical unproject output of
/// [`SceneRenderer::view_ray`]. `dir` is unit-length. Feed it straight
/// to [`roxlap_scene::Scene::raycast`] for depth-free, backend-agnostic
/// voxel picking (`scene.raycast(ray.origin, ray.dir, max_dist)`), or
/// intersect it with a plane for tile selection.
#[derive(Clone, Copy, PartialEq, Debug)]
pub struct Ray {
    /// World-space ray origin (the camera position for view rays).
    pub origin: glam::DVec3,
    /// Unit-length world-space ray direction.
    pub dir: glam::DVec3,
}

/// A world-space line segment to draw over a rendered frame via
/// [`SceneRenderer::draw_lines`] — editor gizmos (bounding boxes, floor
/// grids, axes, hover wireframes), debug paths, etc.
#[derive(Clone, Copy, PartialEq, Debug)]
pub struct Line3 {
    /// World-space endpoints (voxel units), in the same frame the
    /// rendered scene + `camera` use.
    pub a: [f64; 3],
    /// The segment's other endpoint, same space as `a`.
    pub b: [f64; 3],
    /// `0xAARRGGBB` — the high byte is an alpha blend factor (`0xFF`
    /// opaque, `0x00` invisible), the low 24 bits the RGB colour.
    pub color: OverlayColor,
    /// Screen-space thickness in pixels (`<= 1.0` draws a 1px line).
    pub width_px: f32,
    /// `true`: the segment is occluded by nearer rendered geometry
    /// (depth-tested against the frame's z-buffer). `false`: always on
    /// top (e.g. a hover highlight that should show through the model).
    pub depth_test: bool,
}

/// A handle to an uploaded image-sprite texture, returned by
/// [`SceneRenderer::upload_image`]. **Generational** (QE-B6): image
/// slots are reused after [`drop_image`](SceneRenderer::drop_image),
/// so each handle carries the slot's generation — a stale handle
/// (kept across a drop) resolves to a safe no-op instead of aliasing
/// whatever texture re-took the slot. Pass it in an [`ImageSprite`]
/// for [`SceneRenderer::draw_images`]. Opaque on purpose — there's no
/// arithmetic to do on it.
#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
pub struct ImageId {
    slot: u32,
    gen: u32,
}

/// QE-B6 — generational guard for image slots. Unlike
/// [`EpochSlotMap`] (whose backend indices are append-only), image
/// slots ARE reused: `drop_image` frees the texture slot and the next
/// upload may take it. Each slot therefore carries its own
/// generation; a stale [`ImageId`] resolves to `None`.
#[derive(Default)]
struct ImageSlotMap {
    /// Per backend slot: (current generation, live).
    slots: Vec<(u32, bool)>,
}

impl ImageSlotMap {
    /// Register the slot the backend just filled (append or reuse).
    fn alloc(&mut self, slot: usize) -> ImageId {
        if slot >= self.slots.len() {
            self.slots.resize(slot + 1, (0, false));
            self.slots[slot] = (0, true);
        } else {
            let e = &mut self.slots[slot];
            e.0 = e.0.wrapping_add(1);
            e.1 = true;
        }
        ImageId {
            slot: u32::try_from(slot).expect("image slots fit u32"),
            gen: self.slots[slot].0,
        }
    }

    /// Resolve to the backend slot, or `None` when stale / removed.
    fn index(&self, id: ImageId) -> Option<usize> {
        let e = *self.slots.get(id.slot as usize)?;
        (e.0 == id.gen && e.1).then_some(id.slot as usize)
    }

    /// Tombstone; `false` when the handle was already stale.
    fn remove(&mut self, id: ImageId) -> bool {
        let Some(e) = self.slots.get_mut(id.slot as usize) else {
            return false;
        };
        if e.0 != id.gen || !e.1 {
            return false;
        }
        e.1 = false;
        true
    }
}

/// How an [`ImageSprite`]'s quad is oriented in the world.
#[derive(Clone, Copy, PartialEq, Debug)]
pub enum ImageFacing {
    /// Fixed in world space: the quad lies in the plane spanned by `u`
    /// (the image's +column / width direction) and `v` (its +row /
    /// height direction). Both are world-space directions; their length
    /// is ignored (the quad is sized by [`ImageSprite::size`]), so pass
    /// the plane's axes directly. Row 0 of the image is the `origin`
    /// edge and rows grow along `v`.
    World {
        /// World direction of the image's +column (width) axis.
        u: [f32; 3],
        /// World direction of the image's +row (height) axis.
        v: [f32; 3],
    },
    /// Always faces the camera (billboard); `up` is the world direction
    /// the image's top edge points toward (e.g. world `-Z` for the
    /// scene-demo's z-down world, or any "up" the host prefers).
    Billboard {
        /// World direction the image's top edge points toward.
        up: [f32; 3],
    },
}

/// One placed 2D image sprite for the current frame: a flat textured
/// quad in world space, composited over the rendered scene with the
/// frame's depth buffer (so the voxel model can occlude it). Built per
/// frame and passed to [`SceneRenderer::draw_images`], mirroring
/// [`Line3`] / [`SceneRenderer::draw_lines`]. The texture is uploaded
/// once via [`SceneRenderer::upload_image`] and referenced by [`image`].
///
/// [`image`]: ImageSprite::image
#[derive(Clone, Copy, PartialEq, Debug)]
pub struct ImageSprite {
    /// The uploaded texture to draw (from [`SceneRenderer::upload_image`]).
    pub image: ImageId,
    /// World position of the quad's **top-left** corner — the image's
    /// `(column 0, row 0)` texel. The quad extends `size[0]` along the
    /// facing's `u` and `size[1]` along its `v`.
    pub origin: [f32; 3],
    /// World orientation of the quad — fixed in world or camera-facing.
    pub facing: ImageFacing,
    /// World size of the quad along `u` and `v`. For pixel-art traced at
    /// 1 texel = 1 voxel, pass `[width as f32, height as f32]`.
    pub size: [f32; 2],
    /// Multiplied into every sampled texel (tint + opacity) — real
    /// alpha, so [`OverlayColor`]. `OverlayColor(0xFFFF_FFFF)` draws the
    /// texture unchanged; the high byte scales the texel alpha
    /// (`OverlayColor(0x80FF_FFFF)` = 50 % opacity).
    pub tint: OverlayColor,
    /// Alpha cutoff in `0.0..=1.0`. Texels whose **own** alpha is below
    /// this are discarded outright (not blended) — crisp pixel-art edges
    /// instead of a semi-transparent haze, and the same threshold decides
    /// what [`SceneRenderer::pick_image`] treats as solid. `0.0` keeps the
    /// plain straight-alpha over-blend (every non-zero texel draws).
    pub alpha_cutoff: f32,
    /// `true`: occluded by nearer rendered geometry (depth-tested against
    /// the frame's depth buffer, with a bias so a quad resting on a
    /// coincident voxel face doesn't z-fight). `false`: always on top.
    pub depth_test: bool,
    /// `true`: draw regardless of which way the quad faces (no backface
    /// cull) — what reference images usually want. `false`: cull when the
    /// quad faces away from the camera. Ignored for
    /// [`ImageFacing::Billboard`] (it always faces the camera).
    pub double_sided: bool,
}

/// Backend-agnostic resolved quad: four world corners (`TL, TR, BL, BR`,
/// with UVs `(0,0) (1,0) (0,1) (1,1)`) + the texture to map. The facade
/// resolves [`ImageSprite::facing`] into corners and culls back-facing
/// quads once, so both backends draw from the same geometry.
#[derive(Clone, Copy, Debug)]
pub(crate) struct QuadDraw {
    pub corners: [[f32; 3]; 4],
    /// Backend texture SLOT (facade-resolved; see `draw_images`).
    pub image: usize,
    pub tint: OverlayColor,
    pub depth_test: bool,
    pub alpha_cutoff: f32,
}

/// Result of [`SceneRenderer::pick_image`] — a resolved screen→sprite hit.
/// `uv` is the normalised position within the quad (`(0,0)` = top-left
/// corner); `texel` is the matching source-image pixel; `world` is the
/// hit point; `t` is its euclidean distance from the camera.
#[derive(Clone, Copy, PartialEq, Debug)]
pub struct ImagePickHit {
    /// The hit sprite's texture handle.
    pub image: ImageId,
    /// Normalised position within the quad (`(0,0)` = top-left).
    pub uv: [f32; 2],
    /// The matching source-image pixel (column, row).
    pub texel: (u32, u32),
    /// World-space hit point on the quad.
    pub world: [f32; 3],
    /// Euclidean distance from the camera to `world`.
    pub t: f32,
}

/// Which renderer a [`SceneRenderer`] resolved to at construction.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum Backend {
    /// `roxlap-core` opticast, presented via `softbuffer`.
    Cpu,
    /// `roxlap-gpu` compute marcher, presented via wgpu.
    Gpu,
}

/// A backend-dependent capability, probed via
/// [`SceneRenderer::supports`] (QE.7a). Every method below the parity
/// line degrades to a silent no-op on the unsupporting backend; this
/// enum turns the tribal knowledge into a queryable answer.
///
/// | Feature | CPU | GPU |
/// |---|---|---|
/// | [`Capture`](Self::Capture) | ✅ | ✅ native (❌ wasm — WebGPU can't block) |
/// | [`SkyPanorama`](Self::SkyPanorama) | ❌ (use [`FrameParams::sky`]) | ✅ |
/// | [`CarveActiveSprite`](Self::CarveActiveSprite) | ❌ | ✅ |
/// | [`TranslucentSpriteMaterials`](Self::TranslucentSpriteMaterials) | ✅ | ✅ (TV.3) |
/// | [`TranslucentTerrain`](Self::TranslucentTerrain) | ✅ | ✅ (TV.6) |
/// | [`FreePickDepth`](Self::FreePickDepth) | ✅ | ❌ (blocking readback) |
#[non_exhaustive]
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum Feature {
    /// [`SceneRenderer::request_capture`] / `take_capture` produce
    /// frames.
    Capture,
    /// [`SceneRenderer::set_sky_panorama`] uploads a sampled sky (the
    /// CPU backend samples [`FrameParams::sky`] instead).
    SkyPanorama,
    /// [`SceneRenderer::carve_active_sprite`] carves.
    CarveActiveSprite,
    /// Per-voxel / per-instance **sprite** materials composite
    /// translucently (an unsupporting backend renders them opaque).
    TranslucentSpriteMaterials,
    /// [`SceneRenderer::set_terrain_materials`] terrain translucency
    /// composites (an unsupporting backend renders terrain opaque).
    TranslucentTerrain,
    /// [`SceneRenderer::pick_depth`] is cheap enough for per-frame use
    /// (an in-memory read). Unsupporting = it works but **blocks** on
    /// a device readback — hotkey-grade, not reticle-grade.
    FreePickDepth,
}

/// Which backend [`SceneRenderer::try_new`] should build (QE.7b —
/// replaces `RenderOptions.want_gpu: bool`, which couldn't express
/// "GPU or fail": headless CI rigs and benchmarks silently fell back
/// to a software render and measured the wrong thing).
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default)]
pub enum BackendPreference {
    /// The CPU renderer, unconditionally (the pre-QE.7 `want_gpu:
    /// false`). The default.
    #[default]
    Cpu,
    /// Try the GPU; fall back to CPU with a logged `warn` when WGPU
    /// init fails (the pre-QE.7 `want_gpu: true`).
    PreferGpu,
    /// GPU or [`RenderError::GpuInit`] — no silent software fallback.
    /// (The wasm constructor has no `Result` channel and treats this
    /// as [`PreferGpu`](Self::PreferGpu).)
    RequireGpu,
}

/// Construction failure from [`SceneRenderer::try_new`] (QE.2b).
///
/// Under [`BackendPreference::PreferGpu`] a GPU failure is not an
/// error (it logs a `warn` and falls back to CPU); `try_new` fails
/// when the last-resort CPU surface can't be created, or when
/// [`BackendPreference::RequireGpu`] couldn't get its GPU.
#[derive(Debug)]
pub enum RenderError {
    /// The CPU backend couldn't bind its software present surface
    /// (softbuffer context/surface creation failed — e.g. a broken or
    /// unsupported display connection).
    CpuSurface(String),
    /// [`BackendPreference::RequireGpu`] and WGPU init failed (QE.7b).
    GpuInit(GpuInitError),
}

impl std::fmt::Display for RenderError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::CpuSurface(msg) => write!(f, "CPU present surface init failed: {msg}"),
            Self::GpuInit(e) => write!(f, "GPU init failed (RequireGpu, no fallback): {e}"),
        }
    }
}

impl std::error::Error for RenderError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::GpuInit(e) => Some(e),
            Self::CpuSurface(_) => None,
        }
    }
}

/// Construction-time options for [`SceneRenderer::new`].
///
/// # Environment overrides (QE-C6)
///
/// Each GPU tuning knob below has a `ROXLAP_*` escape hatch, so a
/// shipped binary can be re-tuned from the shell. All of them are
/// read **once**, at `SceneRenderer` construction (never per frame),
/// the env value wins over the programmatic one, and unparseable
/// values are dropped with a `log::warn!`. On wasm there is no
/// process environment and the field values apply as-is.
///
/// | variable | overrides | values |
/// |---|---|---|
/// | `ROXLAP_GPU_POWER` | [`gpu`](Self::gpu)`.power_preference` | `low` / `high` |
/// | `ROXLAP_GPU_SPRITE_LOD_PX` | [`gpu_sprite_lod_px`](Self::gpu_sprite_lod_px) | `f32`, screen px |
/// | `ROXLAP_GPU_MIP_SCAN_DIST` | [`gpu_mip_scan_dist`](Self::gpu_mip_scan_dist) | `f32`, world units |
/// | `ROXLAP_GPU_CHUNK_BUDGET` | [`gpu_chunk_upload_budget`](Self::gpu_chunk_upload_budget) | `u32`, `0` = unbounded |
/// | `ROXLAP_GPU_CLIP_BUDGET` | [`gpu_clip_upload_budget`](Self::gpu_clip_upload_budget) | `u32`, `0` = unbounded |
///
/// Other `ROXLAP_*` variables you may have seen (`ROXLAP_GPU`,
/// `ROXLAP_SCENE`, `ROXLAP_CAPTURE`, …) belong to the demo hosts, not
/// the library — backend selection is [`backend`](Self::backend),
/// decided by *your* code.
#[derive(Clone)]
pub struct RenderOptions {
    /// Which backend to build (QE.7b — replaces `want_gpu: bool`;
    /// migrate `want_gpu: true` → [`BackendPreference::PreferGpu`],
    /// `false` → [`BackendPreference::Cpu`]).
    pub backend: BackendPreference,
    /// Settings forwarded to [`roxlap_gpu::GpuRenderer`] when the GPU
    /// backend is selected.
    pub gpu: GpuRendererSettings,
    /// Packed `0x00RRGGBB` (alpha ignored) the empty/clear frame fills
    /// with until a scene render lands. Also the CPU sky-miss colour
    /// default if a frame supplies none.
    pub clear_sky: Rgb,
    /// GPU scene-grid LOD scan distance in world units (GPU.11.1):
    /// rays farther than this from the camera sample coarser chunk
    /// mips. QE.2a — construction-time here (was a per-frame
    /// `FrameParams` field); the `ROXLAP_GPU_MIP_SCAN_DIST` env var
    /// still overrides it as a user-side escape hatch. Ignored by the
    /// CPU backend (its mip ladder comes from
    /// [`OpticastSettings::mip_scan_dist`]).
    pub gpu_mip_scan_dist: f32,
    /// GPU per-frame dirty-chunk install budget (QE.2c — was env-only
    /// `ROXLAP_GPU_CHUNK_BUDGET`, which still overrides). Each chunk
    /// install issues several `queue.write_buffer` calls; a big batch
    /// can exhaust the device staging pool (seen as egui-wgpu panics on
    /// Mesa/NVK while flying fast). Small budget = bounded per-frame
    /// writes, more terrain pop-in when moving quickly. `0` =
    /// unbounded.
    pub gpu_chunk_upload_budget: u32,
    /// GPU frames-per-staging-flush while registering a flipbook clip
    /// (QE.2c — was env-only `ROXLAP_GPU_CLIP_BUDGET`, which still
    /// overrides). `0` = unbounded.
    pub gpu_clip_upload_budget: u32,
    /// GPU sprite mip-LOD threshold (QE.8; `ROXLAP_GPU_SPRITE_LOD_PX`
    /// overrides): the pass steps to a coarser sprite mip once a
    /// mip-0 voxel would project smaller than this many screen pixels.
    /// `1.0` (the default) is the "don't go sub-pixel" threshold and
    /// keeps GPU sprites visually identical to the CPU backend (which
    /// has no sprite LOD); raise it to trade sprite fidelity for
    /// distant-sprite speed (the pre-QE.8 behaviour was `4.0`, which
    /// visibly densified thin/hollow translucent models at range).
    pub gpu_sprite_lod_px: f32,
    /// Unused. Sized the strip-parallel opticast's per-frame scratch
    /// pool; the per-pixel DDA renderer that replaced it needs no
    /// pre-sizing, so the value is ignored.
    #[deprecated(
        since = "0.22.0",
        note = "ignored — the DDA renderer replaced the strip-parallel \
                opticast and needs no scratch-pool pre-sizing"
    )]
    pub cpu_max_grid_vsid: u32,
    /// Unused. Set the strip-parallel opticast's thread count; the DDA
    /// renderer parallelises internally over the rayon pool (bound it
    /// with `RAYON_NUM_THREADS` if needed), so the value is ignored.
    #[deprecated(
        since = "0.22.0",
        note = "ignored — the DDA renderer parallelises over the rayon \
                pool; bound it with RAYON_NUM_THREADS"
    )]
    pub cpu_render_threads: usize,
}

// The deprecated fields still have to be constructed until they are
// removed for real (QE has a breaking window; see docs/porting/
// PORTING-QUALITY.md).
#[allow(deprecated)]
impl Default for RenderOptions {
    fn default() -> Self {
        Self {
            backend: BackendPreference::Cpu,
            gpu: GpuRendererSettings::default(),
            clear_sky: Rgb(0x0099_b3d9),
            gpu_mip_scan_dist: 64.0,
            gpu_chunk_upload_budget: 2,
            gpu_clip_upload_budget: 8,
            gpu_sprite_lod_px: 1.0,
            cpu_max_grid_vsid: 32 * roxlap_scene::CHUNK_SIZE_XY,
            cpu_render_threads: 4,
        }
    }
}

/// Depth-test slack (same spirit as the backends' `DEPTH_BIAS`) so a
/// [`SceneRenderer::pick_image`] hit on a sprite resting on a coincident
/// voxel face isn't rejected as "occluded".
const PICK_DEPTH_BIAS: f32 = 0.5;

// --- image-sprite geometry helpers (shared by both backends) ---

fn v_sub(a: [f32; 3], b: [f32; 3]) -> [f32; 3] {
    [a[0] - b[0], a[1] - b[1], a[2] - b[2]]
}
fn v_add(a: [f32; 3], b: [f32; 3]) -> [f32; 3] {
    [a[0] + b[0], a[1] + b[1], a[2] + b[2]]
}
fn v_scale(a: [f32; 3], s: f32) -> [f32; 3] {
    [a[0] * s, a[1] * s, a[2] * s]
}
fn v_dot(a: [f32; 3], b: [f32; 3]) -> f32 {
    a[0] * b[0] + a[1] * b[1] + a[2] * b[2]
}
fn v_cross(a: [f32; 3], b: [f32; 3]) -> [f32; 3] {
    [
        a[1] * b[2] - a[2] * b[1],
        a[2] * b[0] - a[0] * b[2],
        a[0] * b[1] - a[1] * b[0],
    ]
}
fn v_norm(a: [f32; 3]) -> [f32; 3] {
    let len = v_dot(a, a).sqrt();
    if len < 1e-12 {
        a
    } else {
        v_scale(a, 1.0 / len)
    }
}

/// Intersect a ray (`origin` + `dir`, `dir` un-normalised) with a quad
/// `[TL, TR, BL, BR]` and return `(uv, t)` for a front/back hit inside
/// the quad — `uv` in `0..=1` (`(0,0)` = `TL`), `t` the ray parameter
/// (`hit = origin + dir·t`). `None` for a parallel ray, a hit behind the
/// origin, a degenerate quad, or a hit outside the `u`/`v` span. Solves
/// affine coords exactly for a (possibly skew) parallelogram. Standalone
/// so the geometry is unit-testable without a renderer.
fn ray_quad_uv(
    origin: [f32; 3],
    dir: [f32; 3],
    corners: &[[f32; 3]; 4],
) -> Option<([f32; 2], f32)> {
    let [tl, tr, bl, _br] = *corners;
    let ue = v_sub(tr, tl); // +u edge (width)
    let ve = v_sub(bl, tl); // +v edge (height)
    let n = v_cross(ue, ve);
    let denom = v_dot(dir, n);
    if denom.abs() < 1e-12 {
        return None; // ray parallel to the quad's plane
    }
    let t = v_dot(v_sub(tl, origin), n) / denom;
    if t <= 1e-6 {
        return None; // behind / at the origin
    }
    let p = v_add(origin, v_scale(dir, t));
    let rel = v_sub(p, tl);
    let guu = v_dot(ue, ue);
    let guv = v_dot(ue, ve);
    let gvv = v_dot(ve, ve);
    let det = guu * gvv - guv * guv;
    if det.abs() < 1e-12 {
        return None; // degenerate quad
    }
    let wu = v_dot(rel, ue);
    let wv = v_dot(rel, ve);
    let a = (gvv * wu - guv * wv) / det;
    let b = (guu * wv - guv * wu) / det;
    if !(0.0..=1.0).contains(&a) || !(0.0..=1.0).contains(&b) {
        return None; // outside the quad
    }
    Some(([a, b], t))
}

/// Resolve an [`ImageSprite`] into its four world corners (`TL, TR, BL,
/// BR`), or `None` when a `double_sided == false` world quad faces away
/// from the camera (back-face cull) or its plane is degenerate. The
/// camera basis is used only for [`ImageFacing::Billboard`] and the cull
/// test.
fn resolve_quad(sprite: &ImageSprite, camera: &Camera) -> Option<QuadDraw> {
    let cam_pos = [
        camera.pos[0] as f32,
        camera.pos[1] as f32,
        camera.pos[2] as f32,
    ];
    let cam_fwd = v_norm([
        camera.forward[0] as f32,
        camera.forward[1] as f32,
        camera.forward[2] as f32,
    ]);

    let (u_hat, v_hat) = match sprite.facing {
        ImageFacing::World { u, v } => (v_norm(u), v_norm(v)),
        ImageFacing::Billboard { up } => {
            // Horizontal axis ⟂ both the view direction and `up`; fall
            // back to the camera right when `up` is parallel to the view.
            let mut u_hat = v_norm(v_cross(up, cam_fwd));
            if v_dot(u_hat, u_hat) < 1e-12 {
                u_hat = v_norm([
                    camera.right[0] as f32,
                    camera.right[1] as f32,
                    camera.right[2] as f32,
                ]);
            }
            // Vertical axis ⟂ both, pointing *down* (rows grow downward)
            // so the top edge ends up toward `up`.
            let mut v_hat = v_norm(v_cross(cam_fwd, u_hat));
            if v_dot(v_hat, up) > 0.0 {
                v_hat = v_scale(v_hat, -1.0);
            }
            (u_hat, v_hat)
        }
    };

    let du = v_scale(u_hat, sprite.size[0]);
    let dv = v_scale(v_hat, sprite.size[1]);
    let tl = sprite.origin;
    let tr = v_add(tl, du);
    let bl = v_add(tl, dv);
    let br = v_add(tr, dv);

    // Back-face cull for fixed world quads (billboards always face us).
    if !sprite.double_sided {
        if let ImageFacing::World { .. } = sprite.facing {
            let normal = v_cross(du, dv);
            // Front-facing when the quad normal points toward the camera.
            if v_dot(normal, v_sub(cam_pos, tl)) <= 0.0 {
                return None;
            }
        }
    }

    Some(QuadDraw {
        corners: [tl, tr, bl, br],
        image: 0, // placeholder — draw_images resolves the real slot
        tint: sprite.tint,
        depth_test: sprite.depth_test,
        alpha_cutoff: sprite.alpha_cutoff,
    })
}

/// Where the per-pixel raycaster actually runs, decoupled from the window
/// size (RP.0). Both backends are per-pixel marchers, so frame cost scales
/// with the pixel count — rendering into a fixed **logical** target and
/// nearest-upscaling it to the window makes FPS independent of window size
/// and creates the seam for the later posterize / SSAA post (RP.1/RP.2).
///
/// The default ([`Native`](RenderResolution::Native)) keeps `logical == window`
/// and is **byte-identical** to the pre-RP straight blit.
#[derive(Clone, Copy, Debug, PartialEq, Default)]
pub enum RenderResolution {
    /// Logical resolution == window. Default. Identical to pre-RP behaviour.
    #[default]
    Native,
    /// Fixed logical grid, independent of the window (the retro pixel grid).
    /// Upscaled to the window with nearest sampling (hard pixels). A logical
    /// aspect ratio different from the window's stretches non-uniformly — a
    /// deliberate, classic fixed-res look (no letterbox in RP.0).
    Fixed {
        /// Logical framebuffer width, pixels.
        w: u32,
        /// Logical framebuffer height, pixels.
        h: u32,
    },
    /// Logical = `round(window * factor)`. `0.5` ⇒ a quarter of the pixels,
    /// aspect preserved. Clamped to `>= 1px` per axis.
    Scale(f32),
}

impl RenderResolution {
    /// Resolve to a concrete logical pixel size given the current window
    /// (native) size. Always `>= 1` per axis.
    #[must_use]
    pub(crate) fn logical_for(self, native: (u32, u32)) -> (u32, u32) {
        let (nw, nh) = (native.0.max(1), native.1.max(1));
        match self {
            Self::Native => (nw, nh),
            Self::Fixed { w, h } => (w.max(1), h.max(1)),
            Self::Scale(f) => {
                let s = f.max(1e-3);
                #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
                let lw = ((nw as f32) * s).round() as u32;
                #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
                let lh = ((nh as f32) * s).round() as u32;
                (lw.max(1), lh.max(1))
            }
        }
    }
}

/// Dither applied before the posterize quantization (RP.2), to break up
/// banding and turn it into a stable retro pattern instead of crawling edges.
/// Indexed by the *logical* pixel, so each hard pixel still resolves to one
/// colour.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum DitherMode {
    /// No dither — plain round-to-nearest quantization (hard banding).
    #[default]
    None,
    /// Classic `4×4` ordered (Bayer) dither — the cross-hatch console look.
    Bayer4x4,
    /// Interleaved-gradient noise — a cheap, texture-free blue-noise-ish
    /// stochastic dither (finer than Bayer, no repeating grid).
    BlueNoise,
}

/// Reduced-palette post (RP.2), applied at the logical resolution in the
/// resolve step (after the SSAA box-downfilter, before the nearest upscale).
/// Each channel is quantized to its own number of levels; `levels <= 1` leaves
/// that channel untouched. `None` posterize ⇒ the RP.0/RP.1 paths verbatim.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct PosterizeConfig {
    /// Quantisation levels for the red channel (≥ 2).
    pub levels_r: u8,
    /// Quantisation levels for the green channel (≥ 2).
    pub levels_g: u8,
    /// Quantisation levels for the blue channel (≥ 2).
    pub levels_b: u8,
    /// Dither pattern applied before quantisation.
    pub dither: DitherMode,
}

impl PosterizeConfig {
    /// Uniform per-channel level count with the given dither.
    #[must_use]
    pub fn uniform(levels: u8, dither: DitherMode) -> Self {
        Self {
            levels_r: levels,
            levels_g: levels,
            levels_b: levels,
            dither,
        }
    }
}

/// Renderer-internal backend; never exposes wgpu or softbuffer types.
/// The GPU variant owns the whole wgpu device/queue/pipelines, so
/// it's boxed to keep the enum small.
enum BackendImpl {
    // Both variants boxed so the enum stays small regardless of which
    // backend's state is larger (clippy::large_enum_variant).
    Cpu(Box<CpuBackend>),
    Gpu(Box<GpuBackend>),
}

/// Unified renderer over the CPU and GPU paths. See the crate docs.
pub struct SceneRenderer {
    inner: BackendImpl,
    /// Handles for dynamically added sprite instances (see
    /// [`Self::add_sprite_instance`]). Reset by [`Self::set_sprites`].
    dyn_map: DynInstanceMap,
    /// Handles for registered sprite models (see [`Self::add_sprite_model`]
    /// and the models returned by [`Self::set_sprites`]). Reset by
    /// [`Self::set_sprites`].
    model_map: EpochSlotMap<SpriteModelId>,
    /// Handles for registered animated voxel clips (see
    /// [`Self::add_voxel_clip`]). Reset by [`Self::set_sprites`].
    clip_map: EpochSlotMap<VoxelClipId>,
    /// Handles for registered animated characters (see
    /// [`Self::add_character`]). Reset by [`Self::set_sprites`].
    char_map: EpochSlotMap<CharacterId>,
    /// Live character runtimes, parallel to `char_map` slots (VCL.6).
    char_instances: Vec<CharInstance>,
    /// Handles for registered streaming clips (see
    /// [`Self::add_streaming_clip`]). Reset by [`Self::set_sprites`].
    streaming_map: EpochSlotMap<StreamingClipId>,
    /// Streaming-clip runtimes (cursor + one re-uploaded model), parallel
    /// to `streaming_map` slots; `None` once removed (#3).
    streaming_clips: Vec<Option<StreamingClipState>>,
    /// Metadata per registered flipbook clip, indexed by the backend clip
    /// index (parallel to `clip_map`). Captured at [`Self::add_voxel_clip`]
    /// so the editor queries ([`Self::clip_metadata`]) + the auto-player
    /// don't have to re-pass / shadow the `DecodedClip`. Reset by
    /// [`Self::set_sprites`].
    clip_meta: Vec<ClipMeta>,
    /// Auto-advancing clip players (#6); ticked by
    /// [`Self::advance_voxel_clips`]. Reset by [`Self::set_sprites`].
    clip_players: Vec<ClipPlayer>,
    /// Camera-facing billboard instances (BB.2): each carries its world
    /// position + mode, re-oriented every [`Self::face_billboards_to`].
    /// Reset by [`Self::set_sprites`].
    billboards: Vec<BillboardRec>,
    /// Handles for high-level directional billboard actors (BB.4). Reset by
    /// [`Self::set_sprites`].
    actor_map: EpochSlotMap<BillboardActorId>,
    /// Live billboard-actor runtimes, parallel to `actor_map` slots; `None`
    /// once removed. Driven by [`Self::update_billboard_actors`].
    billboard_actors: Vec<Option<BillboardActor>>,
    /// QE.3a - the once-per-facade scene bookkeeping both backends
    /// read (materials, terrain map, clip-instance frames).
    state: SceneState,
    /// QE-B6 — generational image-slot guard (see [`ImageSlotMap`]).
    image_map: ImageSlotMap,
}

impl SceneRenderer {
    /// Wrap a constructed backend with the facade's (empty) handle
    /// registries — the shared tail of every constructor.
    fn from_backend(inner: BackendImpl) -> Self {
        Self {
            inner,
            image_map: ImageSlotMap::default(),
            dyn_map: DynInstanceMap::default(),
            model_map: EpochSlotMap::default(),
            clip_map: EpochSlotMap::default(),
            char_map: EpochSlotMap::default(),
            char_instances: Vec::new(),
            streaming_map: EpochSlotMap::default(),
            streaming_clips: Vec::new(),
            clip_meta: Vec::new(),
            clip_players: Vec::new(),
            billboards: Vec::new(),
            actor_map: EpochSlotMap::default(),
            billboard_actors: Vec::new(),
            state: SceneState::default(),
        }
    }

    /// Build a renderer for `window` — any [`raw-window-handle`]
    /// provider (winit, SDL, GLFW, …) in an `Arc`. `size` is the
    /// window's initial physical framebuffer size in pixels; thereafter
    /// the host reports changes via [`Self::resize`]. Passing the size
    /// explicitly keeps the facade decoupled from any one windowing
    /// library's size API.
    ///
    /// Builds the backend [`opts.backend`](RenderOptions::backend)
    /// asks for. Under [`BackendPreference::PreferGpu`] a WGPU-init
    /// failure falls back to the CPU backend with a `warn` through
    /// the [`log`] facade (install a logger like `env_logger` to see
    /// why a machine is software-rendering);
    /// [`BackendPreference::RequireGpu`] turns that failure into an
    /// error instead (QE.7b — headless CI / benchmark rigs must not
    /// silently measure a software render).
    ///
    /// # Errors
    /// [`RenderError::CpuSurface`] — the last-resort softbuffer
    /// context/surface couldn't bind to `window`;
    /// [`RenderError::GpuInit`] — `RequireGpu` and WGPU init failed.
    ///
    /// [`raw-window-handle`]: raw_window_handle
    #[cfg(not(target_arch = "wasm32"))]
    pub fn try_new<W>(
        window: Arc<W>,
        size: (u32, u32),
        opts: &RenderOptions,
    ) -> Result<Self, RenderError>
    where
        W: HasWindowHandle + HasDisplayHandle + Send + Sync + 'static,
    {
        // QE-C6 — resolve `ROXLAP_*` env overrides once, here, and
        // thread plain options into the backends (no env reads below).
        let opts = &env_config::EnvOverrides::from_env().applied(opts);
        if opts.backend != BackendPreference::Cpu {
            match GpuBackend::new(window.clone(), size, opts) {
                Ok(g) => return Ok(Self::from_backend(BackendImpl::Gpu(Box::new(g)))),
                Err(e) if opts.backend == BackendPreference::RequireGpu => {
                    return Err(RenderError::GpuInit(e));
                }
                Err(e) => {
                    log::warn!("GPU init failed ({e}); falling back to the CPU renderer");
                }
            }
        }
        let cpu = CpuBackend::try_new(window, size, opts)?;
        Ok(Self::from_backend(BackendImpl::Cpu(Box::new(cpu))))
    }

    /// Infallible [`Self::try_new`]: **panics** if even the CPU
    /// software surface can't be created — the convenient default for
    /// demos and tools, where "no window surface at all" has no
    /// meaningful recovery. Games that want to show their own error UI
    /// call [`try_new`](Self::try_new).
    ///
    /// # Panics
    /// When [`Self::try_new`] returns [`RenderError::CpuSurface`].
    #[cfg(not(target_arch = "wasm32"))]
    #[must_use]
    pub fn new<W>(window: Arc<W>, size: (u32, u32), opts: &RenderOptions) -> Self
    where
        W: HasWindowHandle + HasDisplayHandle + Send + Sync + 'static,
    {
        match Self::try_new(window, size, opts) {
            Ok(r) => r,
            Err(e) => panic!("SceneRenderer::new: {e} (use try_new to handle this)"),
        }
    }

    /// wasm/WebGPU build-time entry: build a renderer over an HTML
    /// `canvas`. `size` is the canvas's initial framebuffer size in
    /// pixels; the host reports later changes via [`Self::resize`].
    ///
    /// Async because the browser drives wgpu's adapter/device requests
    /// through its event loop — `await` it inside a
    /// `wasm_bindgen_futures::spawn_local` task. Selects the GPU
    /// (WebGPU) backend when `opts.backend` asks for GPU and WebGPU is
    /// available (`RequireGpu` degrades to `PreferGpu` here — no
    /// `Result` channel in the async wasm constructor);
    /// otherwise (no WebGPU, or init failed) it falls back to the CPU
    /// opticast path presented through a WebGL2 blit on the same canvas.
    /// **Never fails** — the message is logged to the browser console.
    #[cfg(target_arch = "wasm32")]
    pub async fn new_from_canvas_async(
        canvas: web_sys::HtmlCanvasElement,
        size: (u32, u32),
        opts: &RenderOptions,
    ) -> Self {
        // QE-C6 mirror of the native path; on wasm every override is
        // `None` (no process environment), so this is a plain copy.
        let opts = &env_config::EnvOverrides::from_env().applied(opts);
        if opts.backend != BackendPreference::Cpu {
            // `SurfaceTarget::Canvas` moves the canvas into wgpu, so the
            // GPU attempt gets a clone — the CPU fallback keeps the
            // original if WebGPU init fails.
            match GpuBackend::new_async(canvas.clone(), size, opts).await {
                Ok(g) => return Self::from_backend(BackendImpl::Gpu(Box::new(g))),
                Err(e) => {
                    web_sys::console::warn_1(
                        &format!("roxlap-render: WebGPU init failed ({e}); using the CPU renderer")
                            .into(),
                    );
                }
            }
        }
        Self::from_backend(BackendImpl::Cpu(Box::new(CpuBackend::new_from_canvas(
            canvas, size, opts,
        ))))
    }

    /// Which backend was selected.
    #[must_use]
    pub fn backend(&self) -> Backend {
        match self.inner {
            BackendImpl::Cpu(_) => Backend::Cpu,
            BackendImpl::Gpu(_) => Backend::Gpu,
        }
    }

    /// Whether the active backend supports `feature` (QE.7a) — the
    /// queryable form of the parity table on [`Feature`]. Methods on
    /// the "❌" side stay callable and degrade to documented no-ops;
    /// use this to pick a strategy up front (e.g. photo mode: capture
    /// vs "not available on this platform").
    #[must_use]
    pub fn supports(&self, feature: Feature) -> bool {
        let gpu = matches!(self.inner, BackendImpl::Gpu(_));
        match feature {
            Feature::Capture => {
                // GPU capture is a blocking readback — native only.
                !gpu || cfg!(not(target_arch = "wasm32"))
            }
            Feature::SkyPanorama | Feature::CarveActiveSprite => gpu,
            // Translucency landed on BOTH backends in the TV stage
            // (TV.3 sprites / TV.6 terrain, GPU visually verified);
            // QE.7a shipped this table stale — saying `!gpu` — and the
            // author re-confirmed the GPU paths before this fix.
            Feature::TranslucentSpriteMaterials | Feature::TranslucentTerrain => true,
            Feature::FreePickDepth => !gpu,
        }
    }

    /// The GPU adapter description when on the GPU backend, else
    /// `None`.
    #[must_use]
    pub fn adapter_info(&self) -> Option<&str> {
        match &self.inner {
            BackendImpl::Gpu(g) => Some(g.adapter_info()),
            BackendImpl::Cpu(_) => None,
        }
    }

    /// `true` when this renderer runs on modest hardware: the CPU
    /// backend, or a GPU adapter that is not a discrete card
    /// (integrated / software / virtual). A hint for hosts to pick a
    /// lighter default render resolution — never consulted internally.
    #[must_use]
    pub fn is_low_power(&self) -> bool {
        match &self.inner {
            BackendImpl::Cpu(_) => true,
            BackendImpl::Gpu(g) => g.low_power(),
        }
    }

    /// Upload an equirectangular sky panorama (RGBA8, `w×h`) for the
    /// GPU marcher's sky sampling. No-op on the CPU backend, which
    /// samples the [`Sky`] passed in each [`FrameParams`] instead.
    pub fn set_sky_panorama(&mut self, rgba: &[u8], w: u32, h: u32) {
        if let BackendImpl::Gpu(g) = &mut self.inner {
            g.set_sky_panorama(rgba, w, h);
        }
    }

    /// Follow a window resize. CPU resizes its framebuffer lazily, so
    /// this only matters to the GPU swapchain — but it's safe to call
    /// for both.
    pub fn resize(&mut self, width: u32, height: u32) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.resize(width, height),
            BackendImpl::Gpu(g) => g.resize(width, height),
        }
    }

    /// Set the logical (fixed) render resolution (RP.0). The scene marches at
    /// the resolved logical size and is nearest-upscaled to the window, so the
    /// raycaster's cost — and thus FPS — stops depending on the window size.
    /// [`RenderResolution::Native`] (the default) keeps `logical == window`
    /// and is byte-identical to pre-RP rendering. Takes effect from the next
    /// [`render`](Self::render).
    pub fn set_render_resolution(&mut self, res: RenderResolution) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_render_resolution(res),
            BackendImpl::Gpu(g) => g.set_render_resolution(res),
        }
    }

    /// Set the supersampling factor (RP.1). `1` = off; `2` marches `2×2`
    /// samples per logical pixel and box-downfilters back before the upscale,
    /// anti-aliasing the retro grid. Clamped to `1..=4`. The marcher then runs
    /// at `logical_dims × factor` — predictable cost, independent of the window
    /// size. Takes effect from the next [`render`](Self::render).
    pub fn set_ssaa(&mut self, factor: u8) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_ssaa(factor),
            BackendImpl::Gpu(g) => g.set_ssaa(factor),
        }
    }

    /// The resolution the raycaster actually runs at this frame —
    /// `logical_dims × ssaa` (RP.1). Reflects the most recent window size,
    /// [`RenderResolution`], and SSAA factor.
    #[must_use]
    pub fn render_dims(&self) -> (u32, u32) {
        match &self.inner {
            BackendImpl::Cpu(c) => c.render_dims(),
            BackendImpl::Gpu(g) => g.render_dims(),
        }
    }

    /// Set the reduced-palette posterize post (RP.2), or `None` to disable it
    /// (the default — RP.0/RP.1 paths verbatim). Quantization runs at the
    /// logical resolution in the resolve step, after the SSAA downfilter and
    /// before the nearest upscale, with the configured dither. Takes effect
    /// from the next [`render`](Self::render).
    pub fn set_posterize(&mut self, cfg: Option<PosterizeConfig>) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_posterize(cfg),
            BackendImpl::Gpu(g) => g.set_posterize(cfg),
        }
    }

    /// The logical (fixed) render-target size resolved against the current
    /// window size, per the active [`RenderResolution`].
    #[must_use]
    pub fn logical_dims(&self) -> (u32, u32) {
        match &self.inner {
            BackendImpl::Cpu(c) => c.logical_dims(),
            BackendImpl::Gpu(g) => g.logical_dims(),
        }
    }

    /// One-call per-frame animation tick (QE.1b): drives **every**
    /// facade-owned animated collection, replacing the multi-call
    /// protocol hosts previously had to know (and could silently get
    /// wrong — a missed call meant frozen clips or unfaced
    /// billboards). Call once per frame before [`render`](Self::render):
    ///
    /// 1. [`advance_voxel_clips`](Self::advance_voxel_clips) —
    ///    auto-playing flipbook + streaming clip players;
    /// 2. every live character — skeleton + clip attachments (the
    ///    all-characters sweep of
    ///    [`advance_character`](Self::advance_character));
    /// 3. [`update_billboard_actors`](Self::update_billboard_actors) —
    ///    actor direction/state clips + camera facing;
    /// 4. [`face_billboards_to`](Self::face_billboards_to) — plain
    ///    billboard instances.
    ///
    /// The fine-grained methods stay public for hosts that need a
    /// custom per-entity `dt` (slow-motion on one character) or their
    /// own ordering — `tick` is the "do the right thing" default and
    /// is exactly equivalent to calling them in the order above.
    /// KFA sprites driven via
    /// [`update_kfa_poses`](Self::update_kfa_poses) remain a separate
    /// call (they mutate host-owned skeletons).
    pub fn tick(&mut self, camera: &Camera, dt: f64) {
        self.advance_voxel_clips(dt);
        let live: Vec<usize> = self.char_map.live_indices().collect();
        for idx in live {
            self.advance_character_at(idx, dt);
        }
        self.update_billboard_actors(camera, dt);
        self.face_billboards_to(camera);
    }

    /// Composite `scene` from `camera` with `frame` params into the
    /// backend's frame buffer — **without presenting**. The CPU backend
    /// fills sky + runs the opticast compositor into an owned buffer;
    /// the GPU backend uploads/refreshes the scene, runs the compute
    /// marcher + sprite pass, and acquires (but does not present) the
    /// swapchain frame.
    ///
    /// Finish the frame with exactly one of [`present`](Self::present)
    /// (no overlay) or [`paint_egui`](Self::paint_egui) (UI overlay).
    /// Calling `render` again without finishing drops the pending frame.
    pub fn render(&mut self, scene: &mut Scene, camera: &Camera, frame: &FrameParams) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.render(scene, camera, frame, &self.state),
            BackendImpl::Gpu(g) => g.render(scene, camera, frame, &self.state),
        }
        // The GPU mirrored its device palettes this frame if it needed
        // to; the CPU reads the table live. Either way the change flag
        // is consumed.
        self.state.materials_dirty = false;
    }

    /// Draw world-space [`Line3`] segments over the frame
    /// [`render`](Self::render) composited, using that frame's camera +
    /// projection + depth buffer. Call **after** [`render`](Self::render)
    /// and **before** [`present`](Self::present) /
    /// [`paint_egui`](Self::paint_egui) — the lines land in the
    /// framebuffer, so a subsequent `paint_egui` still draws its panels
    /// on top.
    ///
    /// `camera` must be the one the last frame rendered with (the
    /// projection is taken from that frame). Depth-tested segments
    /// (`Line3::depth_test`) are occluded by nearer rendered geometry;
    /// always-on-top segments ignore depth. See [`Line3`] for colour /
    /// width / blend semantics.
    pub fn draw_lines(&mut self, camera: &Camera, lines: &[Line3]) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.draw_lines(camera, lines),
            BackendImpl::Gpu(g) => g.draw_lines(camera, lines),
        }
    }

    /// Upload (or replace) an RGBA8 image and return a stable [`ImageId`]
    /// to reference it in [`draw_images`](Self::draw_images). `rgba` is
    /// row-major, `width * height * 4` bytes, **straight** (un-premultiplied)
    /// alpha. The texture is retained until [`drop_image`](Self::drop_image),
    /// so the per-frame draw call stays cheap. Sampling is
    /// nearest-neighbour (pixel-art friendly — no blurring).
    ///
    /// Returns `None` for malformed input — a wrong byte count
    /// (`!= width·height·4`) or a zero dimension — so a bad upload can't be
    /// confused with the first valid id (`ImageId(0)`).
    pub fn upload_image(&mut self, rgba: &[u8], width: u32, height: u32) -> Option<ImageId> {
        if width == 0 || height == 0 || rgba.len() != (width as usize) * (height as usize) * 4 {
            return None;
        }
        let slot = match &mut self.inner {
            BackendImpl::Cpu(c) => c.upload_image(rgba, width, height),
            BackendImpl::Gpu(g) => g.upload_image(rgba, width, height),
        };
        Some(self.image_map.alloc(slot))
    }

    /// Release a texture uploaded with [`upload_image`](Self::upload_image).
    /// Stale handles (already dropped, or aliasing a reused slot) are a
    /// safe no-op — ids are generational since QE-B6.
    pub fn drop_image(&mut self, id: ImageId) {
        let Some(slot) = self.image_map.index(id) else {
            return;
        };
        self.image_map.remove(id);
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.drop_image(slot),
            BackendImpl::Gpu(g) => g.drop_image(slot),
        }
    }

    /// Draw 2D [`ImageSprite`]s over the frame [`render`](Self::render)
    /// composited — flat textured quads placed in world space, using that
    /// frame's camera + projection + depth buffer. Same contract as
    /// [`draw_lines`](Self::draw_lines): call **after** [`render`](Self::render)
    /// and **before** [`present`](Self::present) / [`paint_egui`](Self::paint_egui).
    ///
    /// UVs are perspective-correct (no affine warp on an obliquely-viewed
    /// quad). Depth-tested sprites are occluded by nearer rendered
    /// geometry (with a bias to avoid z-fighting on a coincident face);
    /// the texture's straight alpha + the [`ImageSprite::tint`] composite
    /// over the scene. `camera` must be the one the last frame rendered.
    pub fn draw_images(&mut self, camera: &Camera, images: &[ImageSprite]) {
        if images.is_empty() {
            return;
        }
        let quads: Vec<QuadDraw> = images
            .iter()
            .filter_map(|s| {
                // QE-B6 — stale image handles draw nothing (vs aliasing
                // whatever texture reused the slot).
                let slot = self.image_map.index(s.image)?;
                let mut q = resolve_quad(s, camera)?;
                q.image = slot;
                Some(q)
            })
            .collect();
        if quads.is_empty() {
            return;
        }
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.draw_images(camera, &quads),
            BackendImpl::Gpu(g) => g.draw_images(camera, &quads),
        }
    }

    /// Project a world point to window pixel coordinates `(x, y)` under
    /// the projection the **last frame** rendered with — the backend-correct
    /// `world → screen` inverse of [`view_ray`](Self::view_ray). `None`
    /// before the first frame or for a point at/behind the camera near
    /// plane.
    ///
    /// Both backends honour their own projection (CPU `setcamera`
    /// `hx/hy/hz`, GPU vertical-FOV pinhole), so hosts never reconstruct
    /// it themselves. The returned `(x, y)` may fall outside `[0, w) ×
    /// [0, h)` for points off-screen but in front of the camera.
    #[must_use]
    pub fn project_point(&self, camera: &Camera, world: [f32; 3]) -> Option<(f32, f32)> {
        match &self.inner {
            BackendImpl::Cpu(c) => c.project_point(camera, world),
            BackendImpl::Gpu(g) => g.project_point(camera, world),
        }
    }

    /// Screen→sprite pick: the nearest [`ImageSprite`] hit under window
    /// pixel `(x, y)`, resolving which texel was clicked. `sprites` is the
    /// same list passed to [`draw_images`](Self::draw_images) (image
    /// sprites are immediate-mode, so the caller owns the set). `None` for
    /// a miss.
    ///
    /// The ray is intersected with each quad's plane and mapped to its
    /// `uv` / source texel. A texel whose alpha is below the sprite's
    /// [`ImageSprite::alpha_cutoff`] (and any fully-transparent texel) is
    /// **see-through** — the pick passes through it to a sprite behind.
    /// For [`depth_test`](ImageSprite::depth_test) sprites the hit is
    /// rejected when nearer scene geometry occludes that pixel (shares the
    /// depth convention + bias of [`pick`](Self::pick); on the GPU backend
    /// the occlusion test costs a click-time depth readback).
    #[must_use]
    pub fn pick_image(
        &self,
        camera: &Camera,
        x: f64,
        y: f64,
        sprites: &[ImageSprite],
    ) -> Option<ImagePickHit> {
        if sprites.is_empty() {
            return None;
        }
        let dir = self.pixel_ray(camera, x, y)?;
        let dir = [dir[0] as f32, dir[1] as f32, dir[2] as f32];
        let dir_len = v_dot(dir, dir).sqrt();
        if dir_len < 1e-9 {
            return None;
        }
        let origin = [
            camera.pos[0] as f32,
            camera.pos[1] as f32,
            camera.pos[2] as f32,
        ];
        // Scene surface distance under this pixel (sky / no-hit → None);
        // used to occlude depth-tested sprites. Same metric as `pick`.
        let scene_t = self.pick_depth(x as u32, y as u32);

        let mut best: Option<ImagePickHit> = None;
        for sprite in sprites {
            // Reuse the render-path resolve (back-face cull included), so
            // a single-sided quad that isn't drawn also can't be picked.
            let Some(q) = resolve_quad(sprite, camera) else {
                continue;
            };
            let Some(([a, b], t)) = ray_quad_uv(origin, dir, &q.corners) else {
                continue; // miss / parallel / behind
            };
            let d_eucl = t * dir_len;
            if best.is_some_and(|cur| d_eucl >= cur.t) {
                continue; // a nearer sprite already won
            }
            let p = v_add(origin, v_scale(dir, t));

            let Some((iw, ih)) = self.image_dims(sprite.image) else {
                continue; // dropped / unknown image
            };
            let tx = ((a * iw as f32) as i32).clamp(0, iw as i32 - 1) as u32;
            let ty = ((b * ih as f32) as i32).clamp(0, ih as i32 - 1) as u32;

            // See-through test: a texel is solid when its alpha clears the
            // cutoff (and a fully-transparent texel is never solid).
            let cutoff_u8 = (sprite.alpha_cutoff.clamp(0.0, 1.0) * 255.0) as u32;
            let solid_thresh = cutoff_u8.max(1);
            if u32::from(self.image_alpha_at(sprite.image, tx, ty)) < solid_thresh {
                continue;
            }

            // Occlusion: a depth-tested sprite behind nearer geometry loses.
            if sprite.depth_test {
                if let Some(st) = scene_t {
                    if d_eucl > st + PICK_DEPTH_BIAS {
                        continue;
                    }
                }
            }

            best = Some(ImagePickHit {
                image: sprite.image,
                uv: [a, b],
                texel: (tx, ty),
                world: p,
                t: d_eucl,
            });
        }
        best
    }

    /// Source dimensions of an uploaded image, or `None` if the id was
    /// dropped / never uploaded. Internal helper for [`Self::pick_image`].
    fn image_dims(&self, id: ImageId) -> Option<(u32, u32)> {
        let slot = self.image_map.index(id)?;
        match &self.inner {
            BackendImpl::Cpu(c) => c.image_dims(slot),
            BackendImpl::Gpu(g) => g.image_dims(slot),
        }
    }

    /// Alpha byte of texel `(tx, ty)` in an uploaded image (`0` for an
    /// unknown id / out-of-range texel). Internal helper for
    /// [`Self::pick_image`].
    fn image_alpha_at(&self, id: ImageId, tx: u32, ty: u32) -> u8 {
        let Some(slot) = self.image_map.index(id) else {
            return 0;
        };
        match &self.inner {
            BackendImpl::Cpu(c) => c.image_alpha_at(slot, tx, ty),
            BackendImpl::Gpu(g) => g.image_alpha_at(slot, tx, ty),
        }
    }

    /// Mirror the rendered 3D scene horizontally before display. The flip is
    /// applied *before* any egui overlay, so the UI stays upright while the
    /// viewport un-mirrors — a fix for the engine's left-handed render.
    /// Supported on both backends (CPU reverses the framebuffer rows; GPU
    /// mirrors the scene blit + line/image overlays). Picking/projection are
    /// unchanged, so a host that flips must mirror its cursor X (`width - x`)
    /// for ray casts.
    pub fn set_flip_x(&mut self, flip: bool) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_flip_x(flip),
            BackendImpl::Gpu(g) => g.set_flip_x(flip),
        }
    }

    /// Present the frame [`render`](Self::render) composited, with no UI
    /// overlay. Pairs with `render`; use [`paint_egui`](Self::paint_egui)
    /// instead to overlay an egui UI before presenting.
    pub fn present(&mut self) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.present(),
            BackendImpl::Gpu(g) => g.present(),
        }
    }

    /// Block until the active backend has finished all in-flight work, ready
    /// for a clean teardown. On the GPU backend this drains the device queue
    /// and releases any acquired-but-unpresented swapchain frame; on the CPU
    /// backend it is a no-op (nothing is in flight).
    ///
    /// Call this at shutdown **before dropping the renderer and its window**,
    /// so the GPU device/surface tear down with no commands queued and no
    /// half-presented frame. Skipping it (or dropping the window first) can
    /// leave the driver/compositor showing stale buffers after an exit — the
    /// "leftover triangles / flicker" symptom of an unclean shutdown.
    pub fn wait_idle(&mut self) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.wait_idle(),
            BackendImpl::Gpu(g) => g.wait_idle(),
        }
    }

    /// Composite `scene` and return a [`Frame`] guard — the type-state
    /// form of the `render → overlays → present`/`paint_egui` protocol
    /// (QE-B6), which the split calls enforce by documentation only.
    /// With the guard, misuse is unrepresentable: overlays can only be
    /// drawn on a live frame (with the camera it was rendered with —
    /// the guard holds it), presenting consumes the guard so a double
    /// present can't compile, and *dropping* an unfinished frame
    /// presents it — a bare `renderer.frame(..);` shows the frame.
    ///
    /// ```ignore
    /// renderer
    ///     .frame(&mut scene, &camera, &params)
    ///     .draw_lines(&gizmo)
    ///     .present(); // or .paint_egui(..); or just drop it
    /// ```
    ///
    /// The split [`render`](Self::render) / [`present`](Self::present)
    /// calls remain available for hosts that need custom control flow
    /// between the stages.
    pub fn frame(&mut self, scene: &mut Scene, camera: &Camera, params: &FrameParams) -> Frame<'_> {
        self.render(scene, camera, params);
        Frame {
            renderer: self,
            camera: *camera,
            finished: false,
        }
    }

    /// Overlay an egui UI on the frame [`render`](Self::render)
    /// composited, then present it (`hud` feature). The host runs egui
    /// itself (e.g. `egui` + `egui-winit`) and passes the tessellated
    /// `jobs` ([`egui::Context::tessellate`]) and the per-frame
    /// `textures` delta from [`egui::FullOutput`]; `pixels_per_point` is
    /// the UI scale (`ctx.pixels_per_point()`).
    ///
    /// The GPU backend paints via `egui-wgpu`; the CPU backend
    /// software-rasterises the tessellation into its framebuffer. Use
    /// this **instead of** [`present`](Self::present) — both finish the
    /// frame.
    #[cfg(feature = "hud")]
    pub fn paint_egui(
        &mut self,
        jobs: &[egui::ClippedPrimitive],
        textures: &egui::TexturesDelta,
        pixels_per_point: f32,
    ) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.paint_egui(jobs, textures, pixels_per_point),
            BackendImpl::Gpu(g) => g.paint_egui(jobs, textures, pixels_per_point),
        }
    }

    /// Reset the whole sprite world: every model, dynamic instance,
    /// voxel clip, streaming clip, character, billboard and actor is
    /// dropped, and **every outstanding handle from any of those
    /// families goes stale** (resolving to safe no-ops — the maps are
    /// generational). The explicit scene-switch verb (QE-B6): what the
    /// demo host does between scenes, without the "register an empty
    /// set" idiom spelling it.
    pub fn clear_sprites(&mut self) {
        self.set_sprites(&SpriteSet {
            models: Vec::new(),
            instances: Vec::new(),
            carve_model: None,
        });
    }

    /// Register sprite models + instances — **replacing the whole
    /// sprite world**. This is a bulk *set*, not an append: every
    /// handle family (models, dynamic instances, clips, streaming
    /// clips, characters, billboards, actors) is reset and previously
    /// issued handles go stale (safe no-ops). Call once at setup, or
    /// again to replace everything; to build up incrementally instead,
    /// use [`add_sprite_model`](Self::add_sprite_model) /
    /// [`add_sprite_instance`](Self::add_sprite_instance) &c., and to
    /// drop everything use [`clear_sprites`](Self::clear_sprites).
    pub fn set_sprites(&mut self, set: &SpriteSet) -> Vec<SpriteModelId> {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_sprites(set),
            BackendImpl::Gpu(g) => g.set_sprites(set),
        }
        // A fresh sprite set replaces the instance world, so any
        // previously added dynamic instances + models are gone — drop their
        // handles and re-seat the model slotmap with `set.models.len()`
        // live ids `0..n` (model index = chain id on both backends).
        self.dyn_map = DynInstanceMap::default();
        self.model_map.reset_live(set.models.len());
        // A full sprite rebuild drops the dynamic + clip layers on both
        // backends (the GPU registry is replaced), so reset the clip +
        // character maps too.
        self.clip_map.reset();
        self.char_map.reset();
        self.char_instances.clear();
        self.streaming_map.reset();
        self.streaming_clips.clear();
        self.clip_meta.clear();
        self.clip_players.clear();
        self.billboards.clear();
        self.actor_map.reset();
        self.billboard_actors.clear();
        self.state.dyn_clip.clear();
        (0..set.models.len() as u32)
            .map(|slot| self.model_map.minted(slot))
            .collect()
    }

    /// Re-register one sprite model's geometry after you've edited its
    /// content (a carve or recolour of its `kv6`). `model` is the
    /// [`SpriteModelId`] handed back by [`set_sprites`](Self::set_sprites);
    /// `kv6` is the model's **new** geometry — the caller owns the source
    /// of truth (e.g. a dense carve grid the surface-only `kv6` can't
    /// represent) and supplies the refreshed mesh here.
    ///
    /// This is a **backend-agnostic content refresh**, not a GPU upload:
    /// the renderer brings its stored model up to date however its active
    /// backend needs to. The instance set is left untouched (an edit never
    /// moves or adds an instance), so on the GPU backend only that one
    /// model's voxel data is re-uploaded — through a slack-backed
    /// suballocator, one model's bytes rather than the whole registry —
    /// while the CPU backend swaps the cached `kv6` into each instance of
    /// the model. Use [`set_sprites`](Self::set_sprites) to add/remove
    /// models or change the instance set.
    pub fn refresh_sprite_model(&mut self, model: SpriteModelId, kv6: &Kv6) {
        let Some(idx) = self.model_map.index(model) else {
            return; // stale / removed handle → no-op
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.update_sprite_model(idx, kv6),
            BackendImpl::Gpu(g) => g.update_sprite_model(idx, kv6),
        }
    }

    /// Like [`refresh_sprite_model`](Self::refresh_sprite_model) but also
    /// re-classifies the refreshed voxels into per-voxel material ids by
    /// colour (TV.3) via `material_map` — used by the material-aware streaming
    /// clip path so a re-uploaded frame keeps its per-voxel materials. An
    /// empty map matches `refresh_sprite_model`.
    pub fn refresh_sprite_model_with_materials(
        &mut self,
        model: SpriteModelId,
        kv6: &Kv6,
        material_map: &[(Rgb, u8)],
    ) {
        let Some(idx) = self.model_map.index(model) else {
            return; // stale / removed handle → no-op
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => {
                c.update_sprite_model_with_materials(idx, kv6, Some(material_map));
            }
            BackendImpl::Gpu(g) => g.update_sprite_model_with_materials(idx, kv6, material_map),
        }
    }

    /// Add one sprite instance of an already-registered `model` at world
    /// `pos`, **incrementally** — the cheap streaming-spawn path that both
    /// backends now share (GPU: append to the instance buffer, growing by
    /// powers of two; CPU: push one pre-posed [`Sprite`]). Returns a
    /// stable [`SpriteInstanceId`] for later removal.
    ///
    /// `model` must be a [`SpriteModelId`] from the current
    /// [`set_sprites`](Self::set_sprites) (a model registered there, even
    /// with zero initial instances). Dynamic instances live *after* the
    /// static set + any KFA limbs, so register those first.
    ///
    /// Returns `None` — spawning nothing — on a stale/removed `model`
    /// handle (QE.1c; previously a silent sentinel id).
    pub fn add_sprite_instance(
        &mut self,
        model: SpriteModelId,
        pos: [f32; 3],
    ) -> Option<SpriteInstanceId> {
        self.add_sprite_instance_posed(
            model,
            DynSpriteTransform {
                pos,
                ..DynSpriteTransform::default()
            },
        )
    }

    /// Add one sprite instance of an already-registered `model`,
    /// pre-posed with the orientation in `xf` — the streaming-spawn path
    /// for objects that appear mid-flight already rotated (so there's no
    /// one-frame axis-aligned flash before the first
    /// [`set_sprite_instance_transform`](Self::set_sprite_instance_transform)).
    /// Otherwise identical to
    /// [`add_sprite_instance`](Self::add_sprite_instance) (which is just
    /// this with the identity basis). Returns a stable
    /// [`SpriteInstanceId`].
    ///
    /// Returns `None` — spawning nothing — on a stale/removed `model`
    /// handle (QE.1c; previously a silent sentinel id). `xf`'s basis
    /// must be non-singular; a degenerate one makes the instance
    /// silently skip drawing (see [`DynSpriteTransform`]).
    pub fn add_sprite_instance_posed(
        &mut self,
        model: SpriteModelId,
        xf: DynSpriteTransform,
    ) -> Option<SpriteInstanceId> {
        let idx = self.model_map.index(model)?;
        let dyn_index = match &mut self.inner {
            BackendImpl::Cpu(c) => c.add_dyn_instance_posed(idx, xf),
            BackendImpl::Gpu(g) => g.add_dyn_instance_posed(idx, xf),
        }?;
        self.state.dyn_clip.push(None); // a plain model instance
        Some(self.dyn_map.alloc(dyn_index as u32))
    }

    /// Remove a dynamic sprite instance added by
    /// [`add_sprite_instance`](Self::add_sprite_instance). O(1) on both
    /// backends (swap-remove); other dynamic handles stay valid. Returns
    /// `false` if the handle is stale / already removed.
    pub fn remove_sprite_instance(&mut self, id: SpriteInstanceId) -> bool {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return false;
        };
        let moved = match &mut self.inner {
            BackendImpl::Cpu(c) => c.remove_dyn_instance(dyn_index as usize),
            BackendImpl::Gpu(g) => g.remove_dyn_instance(dyn_index as usize),
        };
        // Mirror the backend's swap-remove on the facade's parallel
        // clip bookkeeping (QE.3a).
        self.state.dyn_clip.swap_remove(dyn_index as usize);
        self.dyn_map.remove(id, dyn_index, moved.map(|m| m as u32));
        true
    }

    /// Number of live dynamic sprite instances (those added via
    /// [`add_sprite_instance`](Self::add_sprite_instance)).
    #[must_use]
    pub fn dynamic_sprite_count(&self) -> usize {
        self.dyn_map.order.len()
    }

    /// Define a global voxel **material** (TV stage): the opacity + blend
    /// mode that a per-voxel material id resolves to. The renderer owns one
    /// 256-entry palette shared by every model and grid.
    ///
    /// Id `0` is permanently [`Material::OPAQUE`] — the value every voxel
    /// without explicit material data resolves to — and **cannot** be
    /// redefined; passing `id == 0` is a no-op that returns `false`. Any
    /// other id returns `true`.
    ///
    /// While no translucent material is defined the renderer stays on the
    /// fully-opaque fast path, so this is inert until first called. See
    /// `PORTING-TRANSPARENCY.md`.
    pub fn define_material(&mut self, id: u8, mat: Material) -> bool {
        let changed = self.state.materials.set(id, mat);
        if changed {
            self.state.materials_dirty = true;
        }
        changed
    }

    /// The [`Material`] currently at palette `id` ([`Material::OPAQUE`] for
    /// any id never passed to [`define_material`](Self::define_material)).
    #[must_use]
    pub fn material(&self, id: u8) -> Material {
        self.state.materials.get(id)
    }

    /// Set the **terrain** colour→material map (TV.4): pairs of `(rgb,
    /// material_id)` that make matching-colour world (grid) voxels translucent
    /// — glass walls, water pools. The materials themselves are defined via
    /// [`define_material`](Self::define_material). An empty map (the default)
    /// keeps all terrain opaque. The CPU backend composites these today; the
    /// GPU backend renders them once the TV.6 device path lands.
    pub fn set_terrain_materials(&mut self, map: &[(Rgb, u8)]) {
        if self.state.terrain_materials != map {
            self.state.terrain_materials = map.to_vec();
            self.state.materials_dirty = true;
        }
    }

    /// Register one new sprite **model** incrementally from `kv6`,
    /// **without** rebuilding the existing model set — the streaming-in
    /// counterpart to [`add_sprite_instance`](Self::add_sprite_instance)
    /// for unique generated geometry (procedural asteroids, debris).
    /// Returns a stable [`SpriteModelId`] usable immediately with
    /// [`add_sprite_instance`](Self::add_sprite_instance) /
    /// [`add_sprite_instance_posed`](Self::add_sprite_instance_posed).
    ///
    /// Works before any [`set_sprites`](Self::set_sprites) (it establishes
    /// residency on the GPU backend's first model). The GPU backend
    /// appends one LOD chain to the resident registry (amortised O(model
    /// voxels)); the CPU backend pushes an axis-aligned template.
    pub fn add_sprite_model(&mut self, kv6: &Kv6) -> SpriteModelId {
        let model_index = match &mut self.inner {
            BackendImpl::Cpu(c) => c.add_model(kv6),
            BackendImpl::Gpu(g) => g.add_model(kv6),
        };
        self.model_map.alloc(model_index as u32)
    }

    /// Register a **mixed-material** sprite model (TV.3): `material_map` pairs
    /// a voxel RGB colour (`0xRRGGBB`) with a material id (defined via
    /// [`define_material`](Self::define_material)), so a single model can mix
    /// opaque and translucent voxels — an opaque window frame around glass, a
    /// bottle around a translucent potion. Voxels whose colour isn't in the
    /// map are opaque (material 0). Like [`add_sprite_model`](Self::add_sprite_model)
    /// otherwise.
    ///
    /// The CPU backend composites per-voxel materials today; the GPU backend
    /// carries the data and renders per-voxel materials once the TV.3b device
    /// path lands (until then it uses the instance's uniform material).
    pub fn add_sprite_model_with_materials(
        &mut self,
        kv6: &Kv6,
        material_map: &[(Rgb, u8)],
    ) -> SpriteModelId {
        let model_index = match &mut self.inner {
            BackendImpl::Cpu(c) => c.add_model_with_materials(kv6, material_map),
            BackendImpl::Gpu(g) => g.add_model_with_materials(kv6, material_map),
        };
        self.model_map.alloc(model_index as u32)
    }

    /// Remove a registered sprite model, freeing its voxel data. Returns
    /// `false` if `id` is stale / already removed.
    ///
    /// The model's slot is tombstoned **in place**: its id is never
    /// reused, so every other [`SpriteModelId`] stays valid (no remap).
    /// Existing instances of the removed model are **not** dropped here —
    /// they linger but draw as nothing on the GPU backend (the CPU
    /// backend keeps each instance's own kv6 clone, so they keep drawing
    /// until removed via
    /// [`remove_sprite_instance`](Self::remove_sprite_instance)); remove
    /// them when convenient. Call
    /// [`compact_sprite_models`](Self::compact_sprite_models) afterwards
    /// to reclaim the GPU buffer holes.
    pub fn remove_sprite_model(&mut self, id: SpriteModelId) -> bool {
        let Some(idx) = self.model_map.index(id) else {
            return false;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.remove_model(idx),
            BackendImpl::Gpu(g) => g.remove_model(idx),
        }
        self.model_map.remove(id)
    }

    /// Reclaim the GPU buffer space left by
    /// [`remove_sprite_model`](Self::remove_sprite_model) by repacking the
    /// resident registry to its live models only. Model ids are preserved
    /// (no remap). O(live voxel volume) — call it when many models have
    /// been removed, not every frame. No-op on the CPU backend (which
    /// keeps cheap empty placeholders) and when nothing was removed.
    pub fn compact_sprite_models(&mut self) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.compact_models(),
            BackendImpl::Gpu(g) => g.compact_models(),
        }
    }

    /// Update one dynamic instance's full pose (position + orientation)
    /// for this frame. `id` is from
    /// [`add_sprite_instance`](Self::add_sprite_instance) /
    /// [`add_sprite_instance_posed`](Self::add_sprite_instance_posed). A
    /// stale / removed handle is a no-op.
    ///
    /// For many instances per frame prefer
    /// [`set_sprite_instance_transforms`](Self::set_sprite_instance_transforms):
    /// the GPU backend flushes all pending pose changes to the device
    /// once per [`render`](Self::render), so a per-instance call here is
    /// still O(1) device work, but the batch variant avoids re-walking
    /// the slotmap.
    pub fn set_sprite_instance_transform(&mut self, id: SpriteInstanceId, xf: DynSpriteTransform) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_dyn_instance_transform(dyn_index as usize, xf),
            BackendImpl::Gpu(g) => g.set_dyn_instance_transform(dyn_index as usize, xf),
        }
    }

    /// Batch form of
    /// [`set_sprite_instance_transform`](Self::set_sprite_instance_transform)
    /// — apply many `(instance, pose)` updates in one call. Stale handles
    /// in `updates` are skipped. On the GPU backend this marks the
    /// instance buffer dirty once and uploads the new poses a single time
    /// at the next [`render`](Self::render), so spinning a whole cluster
    /// of instances per frame is one device upload, not one per instance.
    pub fn set_sprite_instance_transforms(
        &mut self,
        updates: &[(SpriteInstanceId, DynSpriteTransform)],
    ) {
        for &(id, xf) in updates {
            let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
                continue;
            };
            match &mut self.inner {
                BackendImpl::Cpu(c) => c.set_dyn_instance_transform(dyn_index as usize, xf),
                BackendImpl::Gpu(g) => g.set_dyn_instance_transform(dyn_index as usize, xf),
            }
        }
    }

    /// Set sprite instance `id`'s voxel-material id (TV stage) — indexes the
    /// global palette defined via [`define_material`](Self::define_material)
    /// for this whole instance's opacity + blend mode. `0` (the default) is
    /// opaque. Stale handles are ignored.
    ///
    /// Only the CPU backend composites translucent sprites today; the GPU
    /// backend retains the value for the forthcoming device-side path (see
    /// `PORTING-TRANSPARENCY.md`).
    pub fn set_sprite_instance_material(&mut self, id: SpriteInstanceId, material: u8) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_dyn_instance_material(dyn_index as usize, material),
            BackendImpl::Gpu(g) => g.set_dyn_instance_material(dyn_index as usize, material),
        }
    }

    /// Set sprite instance `id`'s per-instance alpha multiplier (TV stage),
    /// `0..=255` (`255` = unscaled). Scales the material's opacity so an
    /// effect can fade out by cheap per-frame updates without re-uploading
    /// its volume. Stale handles are ignored.
    pub fn set_sprite_instance_alpha(&mut self, id: SpriteInstanceId, alpha_mul: u8) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_dyn_instance_alpha(dyn_index as usize, alpha_mul),
            BackendImpl::Gpu(g) => g.set_dyn_instance_alpha(dyn_index as usize, alpha_mul),
        }
    }

    /// Set sprite instance `id`'s per-instance **RGB tint**, packed
    /// `0x00RRGGBB`: every rendered voxel's colour is multiplied by it (per
    /// channel), so instances of one model can be recoloured cheaply per frame.
    /// `0x00FF_FFFF` (white, the default) is a no-op. Works on both backends;
    /// stale handles are ignored. Tint is colour only — for transparency, use a
    /// translucent material with
    /// [`set_sprite_instance_alpha`](Self::set_sprite_instance_alpha).
    pub fn set_sprite_instance_tint(&mut self, id: SpriteInstanceId, tint: Rgb) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_dyn_instance_tint(dyn_index as usize, tint.0),
            BackendImpl::Gpu(g) => g.set_dyn_instance_tint(dyn_index as usize, tint.0),
        }
    }

    /// Toggle a sprite/clip instance's shadow participation **live**
    /// (XS.4 flags, BB.3). [`ShadowFlags::default`] (both on) is what
    /// every spawn starts with. The per-instance counterpart to the
    /// template-level `Sprite::with_casts_shadow` /
    /// `with_receives_shadow` — e.g. a flat additive glow billboard
    /// that should not cast, or a UI marker that ignores shadows.
    /// Other flag bits are preserved. No-op on a stale id.
    ///
    /// QE.7b — takes [`ShadowFlags`] instead of the old unreadable
    /// `(id, true, false)` bool pair; migrate with
    /// `ShadowFlags { casts, receives }`.
    pub fn set_sprite_instance_shadow_flags(&mut self, id: SpriteInstanceId, shadows: ShadowFlags) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => {
                c.set_dyn_instance_shadow_flags(
                    dyn_index as usize,
                    shadows.casts,
                    shadows.receives,
                );
            }
            BackendImpl::Gpu(g) => {
                g.set_dyn_instance_shadow_flags(
                    dyn_index as usize,
                    shadows.casts,
                    shadows.receives,
                );
            }
        }
    }

    /// Set a sprite/clip instance's **lighting mode** live (BB.2b): how its
    /// shading normal is derived ([`BillboardLighting`]). Useful for
    /// camera-facing billboards whose face normal would otherwise track the
    /// camera. Other flag bits are preserved; only affects the dynamic
    /// lighting path. No-op on a stale id.
    pub fn set_sprite_instance_lighting(&mut self, id: SpriteInstanceId, mode: BillboardLighting) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_dyn_instance_lighting(dyn_index as usize, mode),
            BackendImpl::Gpu(g) => g.set_dyn_instance_lighting(dyn_index as usize, mode),
        }
    }

    // ---- animated voxel clips (VCL.4) ------------------------------------

    /// Register an animated voxel clip ("GIF/MP4 for voxels"): decode all
    /// its frames and upload the flipbook to the active backend (GPU: one
    /// LOD chain per frame; CPU: a cached dense grid per frame). Returns a
    /// [`VoxelClipId`] to spawn instances of it via
    /// [`add_clip_instance_posed`](Self::add_clip_instance_posed).
    ///
    /// Build the [`DecodedClip`] from a `.rvc` via
    /// [`VoxelClip::decode`](roxlap_formats::voxel_clip::VoxelClip::decode).
    /// Like [`add_sprite_model`](Self::add_sprite_model), this works before
    /// any [`set_sprites`](Self::set_sprites); a later `set_sprites`
    /// **drops** all registered clips (re-register afterwards).
    pub fn add_voxel_clip(&mut self, clip: &DecodedClip) -> VoxelClipId {
        self.add_voxel_clip_with_materials(clip, &[])
    }

    /// Register a **mixed-material** animated voxel clip (TV.3): the clip
    /// analogue of
    /// [`add_sprite_model_with_materials`](Self::add_sprite_model_with_materials).
    /// `material_map` pairs a voxel RGB colour (`0xRRGGBB`) with a material id
    /// (defined via [`define_material`](Self::define_material)), classifying
    /// every frame's voxels so an animated clip can mix opaque and translucent
    /// voxels — an opaque torch handle around an additive flame, a spinning
    /// glass orb. Voxels whose colour isn't in the map stay opaque
    /// (material 0). Like [`add_voxel_clip`](Self::add_voxel_clip) otherwise.
    pub fn add_voxel_clip_with_materials(
        &mut self,
        clip: &DecodedClip,
        material_map: &[(Rgb, u8)],
    ) -> VoxelClipId {
        let clip_index = match &mut self.inner {
            BackendImpl::Cpu(c) => c.add_voxel_clip_with_materials(clip, material_map),
            BackendImpl::Gpu(g) => g.add_voxel_clip_with_materials(clip, material_map),
        };
        // Capture metadata for editor queries + #6 auto-play; clip indices
        // are sequential and parallel to `clip_meta`.
        debug_assert_eq!(clip_index, self.clip_meta.len());
        self.clip_meta.push(ClipMeta {
            dims: clip.dims,
            pivot: clip.pivot,
            voxel_world_size: clip.voxel_world_size,
            durations: clip.durations.clone(),
            loop_mode: clip.loop_mode,
            material_map: material_map.to_vec(),
        });
        self.clip_map.alloc(clip_index as u32)
    }

    /// Remove a registered clip, freeing its per-frame volumes. Instances
    /// of it linger but draw nothing until removed via
    /// [`remove_sprite_instance`](Self::remove_sprite_instance). Returns
    /// `false` if `id` is stale / already removed.
    pub fn remove_voxel_clip(&mut self, id: VoxelClipId) -> bool {
        let Some(clip_index) = self.clip_map.index(id) else {
            return false;
        };
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.remove_voxel_clip(clip_index),
            BackendImpl::Gpu(g) => g.remove_voxel_clip(clip_index),
        }
        // Detach instances that were playing this clip so they stop
        // being treated as clip instances (QE.3a — facade bookkeeping;
        // was duplicated in both backends).
        for slot in &mut self.state.dyn_clip {
            if matches!(slot, Some((c, _)) if *c == clip_index) {
                *slot = None;
            }
        }
        self.clip_map.remove(id)
    }

    /// Spawn an instance of clip `clip`, posed by `xf`, starting on frame
    /// 0. Returns a [`SpriteInstanceId`] — a clip instance is a dynamic
    /// sprite instance, so move it with
    /// [`set_sprite_instance_transform`](Self::set_sprite_instance_transform),
    /// advance its frame with
    /// [`set_clip_instance_frame`](Self::set_clip_instance_frame), and drop
    /// it with [`remove_sprite_instance`](Self::remove_sprite_instance).
    /// Returns `None` — spawning nothing — on a stale/removed `clip`
    /// handle (QE.1c; previously a silent sentinel id).
    ///
    /// This instance has **no playback clock**: drive its frame yourself via
    /// [`set_clip_instance_frame`](Self::set_clip_instance_frame) (frame-based
    /// scrubbing). For *clock*-based control — auto-advance, play/pause, or
    /// [`set_clip_instance_clock_ms`](Self::set_clip_instance_clock_ms)
    /// scrubbing — spawn with
    /// [`add_clip_instance_playing`](Self::add_clip_instance_playing) instead
    /// (the player-control methods no-op on an instance with no player).
    pub fn add_clip_instance_posed(
        &mut self,
        clip: VoxelClipId,
        xf: DynSpriteTransform,
    ) -> Option<SpriteInstanceId> {
        let clip_index = self.clip_map.index(clip)?;
        let dyn_index = match &mut self.inner {
            BackendImpl::Cpu(c) => c.add_clip_instance(clip_index, xf),
            BackendImpl::Gpu(g) => g.add_clip_instance(clip_index, xf),
        }?;
        self.state.dyn_clip.push(Some((clip_index, 0)));
        Some(self.dyn_map.alloc(dyn_index as u32))
    }

    /// Select which frame a clip instance shows — the per-frame playback
    /// step. Cheap on both backends (GPU: swap the instance's model id;
    /// CPU: select the cached frame grid), with no volume re-upload. Drive
    /// it from a playback clock via
    /// [`DecodedClip::frame_at`](roxlap_formats::voxel_clip::DecodedClip::frame_at).
    /// No-op on a stale id or a non-clip instance.
    pub fn set_clip_instance_frame(&mut self, id: SpriteInstanceId, frame: u32) {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return;
        };
        let frame = frame as usize;
        let Some(Some((clip_index, cur_frame))) = self.state.dyn_clip.get_mut(dyn_index as usize)
        else {
            return; // not a clip instance
        };
        // PF.5 — same-frame guard: a player ticking at render rate
        // re-applies the same frame most ticks; skip the bookkeeping +
        // GPU writes (QE.3a: the guard now covers both backends).
        if *cur_frame == frame {
            return;
        }
        let clip_index = *clip_index;
        *cur_frame = frame;
        // The CPU render reads the frame from `state.dyn_clip`; only
        // the GPU keeps device-side state (the instance's model id).
        if let BackendImpl::Gpu(g) = &mut self.inner {
            g.apply_clip_frame(dyn_index as usize, clip_index, frame);
        }
    }

    /// Retarget a live clip instance onto a **different** registered clip,
    /// restarting it at frame 0 while keeping its world transform and any
    /// auto-playback clock *policy* (speed / paused). The per-frame primitive
    /// for directional ("8-way") billboards and animation-state changes
    /// (idle → walk → attack): far cheaper than `remove_sprite_instance` +
    /// `add_clip_instance_*`, reusing the instance's existing GPU residency
    /// (just a model-id swap, no volume re-upload).
    ///
    /// If the instance has a playback clock
    /// ([`add_clip_instance_playing`](Self::add_clip_instance_playing)), its
    /// timeline is retargeted to the new clip (durations + loop mode) and the
    /// clock restarts at 0; the speed and paused state carry over.
    ///
    /// Returns `false` (a safe no-op) on a stale instance id, a stale `clip`,
    /// or a non-clip instance.
    pub fn set_clip_instance_clip(&mut self, id: SpriteInstanceId, clip: VoxelClipId) -> bool {
        let Some(dyn_index) = self.dyn_map.dyn_index(id) else {
            return false;
        };
        let Some(clip_index) = self.clip_map.index(clip) else {
            return false;
        };
        if !matches!(self.state.dyn_clip.get(dyn_index as usize), Some(Some(_))) {
            return false; // not a clip instance
        }
        // Only the GPU has device-side state to retarget (the model
        // id); it reports whether the new clip actually has frames.
        let ok = match &mut self.inner {
            BackendImpl::Cpu(_) => true,
            BackendImpl::Gpu(g) => g.apply_clip_retarget(dyn_index as usize, clip_index),
        };
        if ok {
            self.state.dyn_clip[dyn_index as usize] = Some((clip_index, 0));
            // Retarget the auto-player's timeline to the new clip (different
            // frame count / durations / loop), restart its clock, keep the
            // playback policy (speed + paused). Clone metadata first so the
            // immutable borrow ends before the mutable player borrow.
            let durations = self.clip_meta[clip_index].durations.clone();
            let loop_mode = self.clip_meta[clip_index].loop_mode;
            if let Some(player) = self.flipbook_player_mut(id) {
                player.clock.retarget(&durations, loop_mode);
            }
        }
        ok
    }

    // ---- billboards (BB.2) -----------------------------------------------

    /// Spawn a clip instance that auto-orients toward the camera every
    /// [`face_billboards_to`](Self::face_billboards_to) — a Doom/Build-style
    /// billboard. `pos` is its world position (the clip pivot maps here);
    /// `mode` chooses cylindrical (the Doom default) or spherical facing.
    /// Drive its animation through the clip player
    /// ([`advance_voxel_clips`](Self::advance_voxel_clips)) and swap
    /// animations with [`set_clip_instance_clip`](Self::set_clip_instance_clip).
    ///
    /// The instance starts axis-aligned until the first `face_billboards_to`,
    /// so call that (with the frame's camera) before `render` — like
    /// `advance_voxel_clips(dt)`. Returns a stale id on a stale `clip` (no
    /// billboard recorded).
    pub fn add_billboard_instance(
        &mut self,
        clip: VoxelClipId,
        pos: [f32; 3],
        mode: BillboardMode,
    ) -> Option<SpriteInstanceId> {
        let xf = DynSpriteTransform {
            pos,
            ..Default::default()
        };
        let id = self.add_clip_instance_posed(clip, xf)?;
        self.billboards.push(BillboardRec { id, pos, mode });
        Some(id)
    }

    /// Change a billboard instance's facing mode. No-op on a non-billboard id.
    pub fn set_billboard_mode(&mut self, id: SpriteInstanceId, mode: BillboardMode) {
        if let Some(b) = self.billboards.iter_mut().find(|b| b.id == id) {
            b.mode = mode;
        }
    }

    /// Move a billboard instance. Its auto-orientation is preserved; the new
    /// position takes effect on the next
    /// [`face_billboards_to`](Self::face_billboards_to). No-op on a
    /// non-billboard id.
    pub fn set_billboard_position(&mut self, id: SpriteInstanceId, pos: [f32; 3]) {
        if let Some(b) = self.billboards.iter_mut().find(|b| b.id == id) {
            b.pos = pos;
        }
    }

    /// Re-orient every billboard instance to face `camera` — one batched
    /// transform flush (BB.2). Call once per frame before `render`, after
    /// moving billboards / the camera (the billboard analogue of
    /// [`advance_voxel_clips`](Self::advance_voxel_clips)). Billboards whose
    /// instance was removed are pruned; a degenerate pose (camera on the
    /// sprite's vertical axis) is skipped for that frame.
    pub fn face_billboards_to(&mut self, camera: &Camera) {
        let cam = camera.pos;
        let dyn_map = &self.dyn_map;
        let mut updates: Vec<(SpriteInstanceId, DynSpriteTransform)> = Vec::new();
        self.billboards.retain(|b| {
            if dyn_map.dyn_index(b.id).is_none() {
                return false; // the instance was removed → drop the record
            }
            if let Some(xf) = billboard_transform(b.pos, cam, b.mode) {
                updates.push((b.id, xf));
            }
            true
        });
        self.set_sprite_instance_transforms(&updates);
    }

    // ---- billboard actors (BB.4) -----------------------------------------

    /// Build a [`ClipClock`] seeded from `clip`'s timeline (durations + loop
    /// mode), or an empty/looping clock if `clip` is `None`/stale.
    fn clock_for_clip(&self, clip: Option<VoxelClipId>, speed: f32) -> ClipClock {
        let (durations, loop_mode) = clip.and_then(|c| self.clip_map.index(c)).map_or_else(
            || (Vec::new(), LoopMode::Loop),
            |ci| {
                (
                    self.clip_meta[ci].durations.clone(),
                    self.clip_meta[ci].loop_mode,
                )
            },
        );
        ClipClock::new(&durations, loop_mode, speed_to_q8(speed), 0.0)
    }

    /// Register a high-level **directional billboard actor** (BB.4): the
    /// renderer owns one clip instance and, every
    /// [`update_billboard_actors`](Self::update_billboard_actors), picks the
    /// directional clip from the view angle, faces it to the camera, and
    /// advances its state animation. The convenience layer over
    /// [`add_billboard_instance`](Self::add_billboard_instance) +
    /// [`set_clip_instance_clip`](Self::set_clip_instance_clip) + the clip
    /// clock for Doom-style monsters.
    ///
    /// `pos` is the actor's world position; `facing_yaw` is the world yaw it
    /// faces (radians; the dir picker compares the camera's bearing to it).
    /// Returns `None` — spawning nothing — if `def` has no states / a
    /// state with no dirs, or the initial clip is stale (QE.1c;
    /// previously a silent sentinel id).
    pub fn add_billboard_actor(
        &mut self,
        def: BillboardActorDef,
        pos: [f32; 3],
        facing_yaw: f64,
    ) -> Option<BillboardActorId> {
        if def.states.is_empty() || def.states.iter().any(|s| s.dirs.is_empty()) {
            return None;
        }
        let init_clip = def.states[0].dirs[0];
        let xf = DynSpriteTransform {
            pos,
            ..Default::default()
        };
        let inst = self.add_clip_instance_posed(init_clip, xf)?;
        self.set_sprite_instance_shadow_flags(inst, def.shadows);
        self.set_sprite_instance_lighting(inst, def.lighting);
        let clock = self.clock_for_clip(Some(init_clip), def.speed);
        let actor = BillboardActor {
            scale: def.scale,
            inst,
            states: def.states,
            cur_state: 0,
            pos,
            facing_yaw,
            mode: def.mode,
            clock,
            showing: None,
            speed: def.speed,
        };
        let index = self.billboard_actors.len() as u32;
        self.billboard_actors.push(Some(actor));
        Some(self.actor_map.alloc(index))
    }

    /// Switch an actor to a named animation state, restarting its clock (the
    /// directional clip is reselected on the next
    /// [`update_billboard_actors`](Self::update_billboard_actors)). No-op on a
    /// stale id or an unknown state name.
    pub fn set_actor_state(&mut self, id: BillboardActorId, state: &str) -> bool {
        let Some(idx) = self.actor_map.index(id) else {
            return false;
        };
        let Some(a) = self.billboard_actors[idx].as_ref() else {
            return false;
        };
        let Some(state_idx) = a.states.iter().position(|s| s.name == state) else {
            return false;
        };
        let rep = a.states[state_idx].dirs.first().copied();
        let speed = a.speed;
        let clock = self.clock_for_clip(rep, speed);
        let a = self.billboard_actors[idx].as_mut().unwrap();
        a.cur_state = state_idx;
        a.clock = clock;
        a.showing = None; // force a clip reselect next update
        true
    }

    /// Move/turn an actor. Its orientation + directional clip update on the
    /// next [`update_billboard_actors`](Self::update_billboard_actors). No-op
    /// on a stale id.
    pub fn set_actor_transform(&mut self, id: BillboardActorId, pos: [f32; 3], facing_yaw: f64) {
        let Some(idx) = self.actor_map.index(id) else {
            return;
        };
        if let Some(a) = self.billboard_actors[idx].as_mut() {
            a.pos = pos;
            a.facing_yaw = facing_yaw;
        }
    }

    /// Change an actor's lighting mode at runtime (BB.2b) — the per-actor
    /// counterpart to [`BillboardActorDef::lighting`], routed to its clip
    /// instance via [`set_sprite_instance_lighting`](Self::set_sprite_instance_lighting).
    /// Returns `false` on a stale id.
    pub fn set_actor_lighting(&mut self, id: BillboardActorId, mode: BillboardLighting) -> bool {
        let Some(idx) = self.actor_map.index(id) else {
            return false;
        };
        let Some(inst) = self.billboard_actors[idx].as_ref().map(|a| a.inst) else {
            return false;
        };
        self.set_sprite_instance_lighting(inst, mode);
        true
    }

    /// Tint an actor at runtime — the per-actor counterpart to
    /// [`set_sprite_instance_tint`](Self::set_sprite_instance_tint), routed to
    /// its clip instance. `tint` is an `0x00RR_GGBB` colour multiply
    /// (`0x00FF_FFFF` = white = no-op). Returns `false` on a stale id.
    pub fn set_actor_tint(&mut self, id: BillboardActorId, tint: Rgb) -> bool {
        let Some(idx) = self.actor_map.index(id) else {
            return false;
        };
        let Some(inst) = self.billboard_actors[idx].as_ref().map(|a| a.inst) else {
            return false;
        };
        self.set_sprite_instance_tint(inst, tint);
        true
    }

    /// Remove an actor and its clip instance. Returns `false` on a stale id.
    pub fn remove_billboard_actor(&mut self, id: BillboardActorId) -> bool {
        let Some(idx) = self.actor_map.index(id) else {
            return false;
        };
        if let Some(a) = self.billboard_actors[idx].take() {
            self.remove_sprite_instance(a.inst);
        }
        self.actor_map.remove(id)
    }

    /// Drive every billboard actor by `dt` seconds (BB.4): for each, pick the
    /// directional clip from the camera bearing (swapping clips only on
    /// change), advance its state-animation clock, and face it to the camera.
    /// Call once per frame before `render` (the actor analogue of
    /// [`advance_voxel_clips`](Self::advance_voxel_clips) +
    /// [`face_billboards_to`](Self::face_billboards_to)). Actors whose
    /// instance was removed are pruned.
    pub fn update_billboard_actors(&mut self, camera: &Camera, dt: f64) {
        struct Action {
            inst: SpriteInstanceId,
            set_clip: Option<VoxelClipId>,
            frame: u32,
            xf: Option<DynSpriteTransform>,
        }
        let cam = camera.pos;
        let dyn_map = &self.dyn_map;
        let mut actions: Vec<Action> = Vec::new();
        for slot in &mut self.billboard_actors {
            let Some(a) = slot.as_mut() else {
                continue;
            };
            if dyn_map.dyn_index(a.inst).is_none() {
                *slot = None; // instance gone → drop the actor
                continue;
            }
            let dir = a.pick_dir(cam);
            let desired = a.states[a.cur_state].dirs[dir];
            let set_clip = (a.showing != Some(desired)).then(|| {
                a.showing = Some(desired);
                desired
            });
            let frame = a.clock.tick(dt);
            let xf = billboard_transform(a.pos, cam, a.mode).map(|mut xf| {
                for axis in [&mut xf.right, &mut xf.up, &mut xf.forward] {
                    for c in axis.iter_mut() {
                        *c *= a.scale;
                    }
                }
                xf
            });
            actions.push(Action {
                inst: a.inst,
                set_clip,
                frame,
                xf,
            });
        }
        // Apply (each call borrows self mutably; disjoint from the loop above).
        let mut xforms: Vec<(SpriteInstanceId, DynSpriteTransform)> = Vec::new();
        for act in actions {
            if let Some(clip) = act.set_clip {
                self.set_clip_instance_clip(act.inst, clip);
            }
            // After a clip swap the backend reset the frame to 0; set the
            // clock's frame so the walk cycle stays continuous across turns.
            self.set_clip_instance_frame(act.inst, act.frame);
            if let Some(xf) = act.xf {
                xforms.push((act.inst, xf));
            }
        }
        self.set_sprite_instance_transforms(&xforms);
    }

    // ---- clip queries (editor inspector) ---------------------------------

    /// Frame count of a registered flipbook clip, or `None` if `id` is
    /// stale. (Same as `clip_metadata(id)?.frame_count`, without the clone.)
    #[must_use]
    pub fn clip_frame_count(&self, id: VoxelClipId) -> Option<usize> {
        let idx = self.clip_map.index(id)?;
        Some(self.clip_meta[idx].durations.len())
    }

    /// Inspector metadata (dims / pivot / scale / loop mode / per-frame
    /// durations) of a registered flipbook clip, or `None` if `id` is stale
    /// — so an editor needn't shadow the source [`DecodedClip`].
    #[must_use]
    pub fn clip_metadata(&self, id: VoxelClipId) -> Option<ClipMetadata> {
        let idx = self.clip_map.index(id)?;
        let m = &self.clip_meta[idx];
        Some(ClipMetadata {
            dims: m.dims,
            pivot: m.pivot,
            voxel_world_size: m.voxel_world_size,
            loop_mode: m.loop_mode,
            frame_count: m.durations.len(),
            durations: m.durations.clone(),
            total_ms: m
                .durations
                .iter()
                .fold(0u32, |acc, &d| acc.saturating_add(d)),
        })
    }

    /// Which frame a clip instance is currently showing (the timeline
    /// scrubber's read-back), or `None` if `id` isn't a live clip instance.
    #[must_use]
    pub fn clip_instance_frame(&self, id: SpriteInstanceId) -> Option<u32> {
        let dyn_index = self.dyn_map.dyn_index(id)? as usize;
        let (_, frame) = (*self.state.dyn_clip.get(dyn_index)?)?;
        u32::try_from(frame).ok()
    }

    /// QE.7b — renamed: this was the only `get_`-prefixed method in
    /// the crate. Forwarding shim for one minor release.
    #[deprecated(since = "0.22.0", note = "renamed to `clip_instance_frame`")]
    #[must_use]
    pub fn get_clip_instance_frame(&self, id: SpriteInstanceId) -> Option<u32> {
        self.clip_instance_frame(id)
    }

    /// Re-upload a **single** `frame` of registered clip `id` in place — the
    /// editor's one-voxel paint, O(1 frame) instead of `remove_voxel_clip` +
    /// `add_voxel_clip` (which rebuilds all N volumes). `vf` must fit the
    /// clip's fixed `dims`. Returns `false` on a stale `id`, an out-of-range
    /// `frame`, or a frame that fails the clip's layout (so it can't corrupt
    /// the flipbook).
    pub fn update_clip_frame(&mut self, id: VoxelClipId, frame: u32, vf: &VoxelFrame) -> bool {
        let Some(clip_index) = self.clip_map.index(id) else {
            return false;
        };
        let m = &self.clip_meta[clip_index];
        let (dims, pivot, vws) = (m.dims, m.pivot, m.voxel_world_size);
        if vf.validate(dims).is_err() {
            return false;
        }
        // Re-classify with the clip's registered colour→material map (TV.3) so
        // an in-place frame edit keeps the clip's per-voxel materials.
        let material_map = m.material_map.clone();
        let frame = frame as usize;
        match &mut self.inner {
            BackendImpl::Cpu(c) => {
                c.update_clip_frame(clip_index, frame, vf, dims, pivot, vws, &material_map)
            }
            BackendImpl::Gpu(g) => {
                g.update_clip_frame(clip_index, frame, vf, dims, pivot, vws, &material_map)
            }
        }
    }

    // ---- streaming voxel clips (#3) --------------------------------------

    /// Register a **streaming** voxel clip — `O(1-frame)` memory (one sprite
    /// model + the compact encoded stream) rather than the N-volume flipbook
    /// [`add_voxel_clip`](Self::add_voxel_clip) builds, for huge clips where
    /// N frames are too costly to hold resident. Builds the model from frame
    /// 0; advance it with
    /// [`set_streaming_clip_frame`](Self::set_streaming_clip_frame). Spawn
    /// instances with
    /// [`add_streaming_clip_instance`](Self::add_streaming_clip_instance) —
    /// note that, unlike a flipbook, **all** instances of a streaming clip
    /// share its one model and so always show the same (current) frame.
    ///
    /// Takes the *encoded* [`VoxelClip`] (not a [`DecodedClip`]) — the whole
    /// point is to avoid materialising every frame.
    ///
    /// # Errors
    /// [`DecodeError`] if the clip's frame stream is empty or doesn't begin
    /// with a keyframe.
    pub fn add_streaming_clip(&mut self, clip: &VoxelClip) -> Result<StreamingClipId, DecodeError> {
        self.add_streaming_clip_with_materials(clip, &[])
    }

    /// Register a **mixed-material** streaming voxel clip (TV.3): the streaming
    /// analogue of
    /// [`add_voxel_clip_with_materials`](Self::add_voxel_clip_with_materials).
    /// `material_map` pairs a voxel RGB colour with a material id (defined via
    /// [`define_material`](Self::define_material)); it is re-applied on every
    /// per-frame re-upload, so the single streamed model keeps its per-voxel
    /// materials as the clip advances. An empty map is identical to
    /// [`add_streaming_clip`](Self::add_streaming_clip).
    ///
    /// # Errors
    /// As [`add_streaming_clip`](Self::add_streaming_clip).
    pub fn add_streaming_clip_with_materials(
        &mut self,
        clip: &VoxelClip,
        material_map: &[(Rgb, u8)],
    ) -> Result<StreamingClipId, DecodeError> {
        let cursor = StreamingClip::new(clip)?;
        let dims = cursor.dims();
        let pivot = cursor.pivot();
        let kv6 = cursor.current_frame().to_kv6(dims, pivot);
        let resident_frame = cursor.current_index();
        let model = self.add_sprite_model_with_materials(&kv6, material_map);
        let index = self.streaming_clips.len() as u32;
        self.streaming_clips.push(Some(StreamingClipState {
            cursor,
            model,
            dims,
            pivot,
            material_map: material_map.to_vec(),
            // The model was just built from the cursor's current frame.
            last_applied: Some(resident_frame),
        }));
        Ok(self.streaming_map.alloc(index))
    }

    /// Spawn an instance of streaming clip `id`, posed by `xf`. Returns a
    /// [`SpriteInstanceId`] — move it with
    /// [`set_sprite_instance_transform`](Self::set_sprite_instance_transform)
    /// and drop it with
    /// [`remove_sprite_instance`](Self::remove_sprite_instance), like any
    /// dynamic instance. All instances of one streaming clip share its single
    /// model. Returns `None` — spawning nothing — on a stale/removed
    /// `id` (QE.1c; previously a silent sentinel handle).
    pub fn add_streaming_clip_instance(
        &mut self,
        id: StreamingClipId,
        xf: DynSpriteTransform,
    ) -> Option<StreamingInstanceId> {
        let model = self
            .streaming_map
            .index(id)
            .and_then(|idx| self.streaming_clips[idx].as_ref())
            .map(|s| s.model)?;
        let inst = self.add_sprite_instance_posed(model, xf)?;
        Some(StreamingInstanceId(inst))
    }

    /// Re-pose a streaming-clip instance (world transform). No-op on a stale
    /// handle.
    pub fn set_streaming_instance_transform(
        &mut self,
        id: StreamingInstanceId,
        xf: DynSpriteTransform,
    ) {
        self.set_sprite_instance_transform(id.0, xf);
    }

    /// Remove a streaming-clip instance. Returns `false` if `id` is stale.
    pub fn remove_streaming_instance(&mut self, id: StreamingInstanceId) -> bool {
        self.remove_sprite_instance(id.0)
    }

    /// Advance a streaming clip to `frame`: seek the cursor and re-upload its
    /// single model — the per-frame streaming step (one volume re-upload,
    /// vs the flipbook's cheap model-select). Updates **every** instance of
    /// the clip at once. Drive it from a clock via
    /// [`frame_at`](roxlap_formats::voxel_clip::frame_at). No-op on a stale
    /// id; `frame` is clamped to the last.
    pub fn set_streaming_clip_frame(&mut self, id: StreamingClipId, frame: u32) {
        let Some(idx) = self.streaming_map.index(id) else {
            return;
        };
        let Some((model, kv6, material_map)) = self.streaming_clips[idx].as_mut().and_then(|s| {
            s.cursor.seek(frame as usize).ok()?;
            // PF.5 — same-frame guard on the RESOLVED (clamped) index: skip
            // the dense rebuild + re-upload when this frame is already
            // resident on the model.
            let resolved = s.cursor.current_index();
            if s.last_applied == Some(resolved) {
                return None;
            }
            s.last_applied = Some(resolved);
            let vf = s.cursor.current_frame();
            Some((s.model, vf.to_kv6(s.dims, s.pivot), s.material_map.clone()))
        }) else {
            return;
        };
        self.refresh_sprite_model_with_materials(model, &kv6, &material_map);
    }

    /// Remove a streaming clip: free its model and drop the cursor (the
    /// memory win for huge clips). Instances linger but draw nothing until
    /// removed. Returns `false` if `id` is stale / already removed.
    pub fn remove_streaming_clip(&mut self, id: StreamingClipId) -> bool {
        let Some(idx) = self.streaming_map.index(id) else {
            return false;
        };
        let model = self.streaming_clips[idx].as_ref().map(|s| s.model);
        self.streaming_clips[idx] = None;
        if let Some(model) = model {
            self.remove_sprite_model(model);
        }
        self.streaming_map.remove(id)
    }

    // ---- auto-advancing clip players (#6) --------------------------------

    /// Spawn a flipbook-clip instance that **plays itself**: like
    /// [`add_clip_instance_posed`](Self::add_clip_instance_posed), but the
    /// facade tracks a playback clock so a single
    /// [`advance_voxel_clips`](Self::advance_voxel_clips) call advances every
    /// such instance — no per-frame `frame_at` + `set_clip_instance_frame`
    /// bookkeeping in the host. `speed` is the playback rate (`1.0` =
    /// authored speed, negative = reverse; QE.7b - was Q8, migrate with
    /// `speed_q8 as f32 / 256.0`); `start_phase_ms` offsets the clock
    /// (stagger copies of one clip). Returns `None` — spawning nothing,
    /// registering no player — on a stale/removed `clip` (QE.1c;
    /// previously a silent sentinel id).
    pub fn add_clip_instance_playing(
        &mut self,
        clip: VoxelClipId,
        xf: DynSpriteTransform,
        speed: f32,
        start_phase_ms: u32,
    ) -> Option<SpriteInstanceId> {
        let clip_index = self.clip_map.index(clip)?;
        let meta = &self.clip_meta[clip_index];
        let clock = ClipClock::new(
            &meta.durations,
            meta.loop_mode,
            speed_to_q8(speed),
            f64::from(start_phase_ms),
        );
        let inst = self.add_clip_instance_posed(clip, xf)?;
        self.clip_players.push(ClipPlayer {
            target: PlayerTarget::Flipbook(inst),
            clock,
            paused: false,
        });
        Some(inst)
    }

    /// Give a streaming clip ([`add_streaming_clip`](Self::add_streaming_clip))
    /// its own playback clock, advanced by
    /// [`advance_voxel_clips`](Self::advance_voxel_clips). A streaming clip's
    /// frame is per-clip (all its instances share one model), so this is
    /// keyed on the clip, not an instance — register instances separately
    /// with
    /// [`add_streaming_clip_instance`](Self::add_streaming_clip_instance).
    /// No-op on a stale `clip`.
    ///
    /// Control the player (play/pause/scrub) via
    /// [`set_streaming_clip_paused`](Self::set_streaming_clip_paused) /
    /// [`set_streaming_clip_speed`](Self::set_streaming_clip_speed) /
    /// [`set_streaming_clip_clock_ms`](Self::set_streaming_clip_clock_ms), the
    /// per-clip analogues of the flipbook `set_clip_instance_*` methods.
    /// `speed` is the playback rate (`1.0` = authored; QE.7b - was Q8).
    pub fn play_streaming_clip(&mut self, clip: StreamingClipId, speed: f32, start_phase_ms: u32) {
        let Some(idx) = self.streaming_map.index(clip) else {
            return;
        };
        let Some(state) = self.streaming_clips[idx].as_ref() else {
            return;
        };
        let clock = ClipClock::new(
            state.cursor.durations(),
            state.cursor.loop_mode(),
            speed_to_q8(speed),
            f64::from(start_phase_ms),
        );
        self.clip_players.push(ClipPlayer {
            target: PlayerTarget::Streaming(clip),
            clock,
            paused: false,
        });
    }

    /// Advance every auto-playing clip ([`add_clip_instance_playing`] /
    /// [`play_streaming_clip`]) by `dt` seconds: tick each clock, resolve its
    /// frame via [`frame_at`](roxlap_formats::voxel_clip::frame_at), and
    /// apply it. Players whose instance / clip was removed are pruned. Call
    /// once per frame.
    ///
    /// [`add_clip_instance_playing`]: Self::add_clip_instance_playing
    /// [`play_streaming_clip`]: Self::play_streaming_clip
    pub fn advance_voxel_clips(&mut self, dt: f64) {
        // Phase 1: tick clocks → (target, frame), pruning dead players.
        // Borrow only the maps (disjoint from `clip_players`).
        let dyn_map = &self.dyn_map;
        let streaming_map = &self.streaming_map;
        let mut updates: Vec<(PlayerTarget, u32)> = Vec::new();
        self.clip_players.retain_mut(|p| {
            let alive = match p.target {
                PlayerTarget::Flipbook(inst) => dyn_map.dyn_index(inst).is_some(),
                PlayerTarget::Streaming(clip) => streaming_map.index(clip).is_some(),
            };
            if !alive {
                return false;
            }
            // A paused player keeps its clock + frame (the editor's pause).
            if !p.paused {
                updates.push((p.target, p.clock.tick(dt)));
            }
            true
        });
        // Phase 2: apply (borrows self mutably, disjoint from the above).
        for (target, frame) in updates {
            self.apply_player_frame(target, frame);
        }
    }

    /// Apply a resolved frame to a player's target (flipbook instance vs.
    /// streaming clip).
    fn apply_player_frame(&mut self, target: PlayerTarget, frame: u32) {
        match target {
            PlayerTarget::Flipbook(inst) => self.set_clip_instance_frame(inst, frame),
            PlayerTarget::Streaming(clip) => self.set_streaming_clip_frame(clip, frame),
        }
    }

    /// Find the auto-player driving flipbook instance `inst`, if any.
    fn flipbook_player_mut(&mut self, inst: SpriteInstanceId) -> Option<&mut ClipPlayer> {
        self.clip_players
            .iter_mut()
            .find(|p| matches!(p.target, PlayerTarget::Flipbook(i) if i == inst))
    }

    /// Pause / resume the auto-player driving clip instance `id` (the
    /// editor's play/pause). No-op if `id` has no player.
    pub fn set_clip_instance_paused(&mut self, id: SpriteInstanceId, paused: bool) {
        if let Some(p) = self.flipbook_player_mut(id) {
            p.paused = paused;
        }
    }

    /// Whether clip instance `id`'s auto-player is paused, or `None` if it
    /// has no player.
    #[must_use]
    pub fn is_clip_instance_paused(&self, id: SpriteInstanceId) -> Option<bool> {
        self.clip_players
            .iter()
            .find(|p| matches!(p.target, PlayerTarget::Flipbook(i) if i == id))
            .map(|p| p.paused)
    }

    /// Set the playback speed (`1.0` = authored rate, negative =
    /// reverse; QE.7b — was Q8) of clip instance `id`'s auto-player.
    /// No-op if `id` has no player.
    pub fn set_clip_instance_speed(&mut self, id: SpriteInstanceId, speed: f32) {
        if let Some(p) = self.flipbook_player_mut(id) {
            p.clock.speed_q8 = speed_to_q8(speed);
        }
    }

    /// **Scrub**: set clip instance `id`'s playback clock to `clock_ms` and
    /// immediately show the matching frame (works while paused). No-op if
    /// `id` has no player.
    pub fn set_clip_instance_clock_ms(&mut self, id: SpriteInstanceId, clock_ms: f64) {
        let Some((target, frame)) = self.flipbook_player_mut(id).map(|p| {
            p.clock.clock_ms = clock_ms;
            #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
            let frame = p.clock.frame_for(clock_ms.max(0.0) as u32);
            (p.target, frame)
        }) else {
            return;
        };
        self.apply_player_frame(target, frame);
    }

    /// Clip instance `id`'s current playback-clock position (ms), or `None`
    /// if it has no player — the scrubber's read-back.
    #[must_use]
    pub fn clip_instance_clock_ms(&self, id: SpriteInstanceId) -> Option<f64> {
        self.clip_players
            .iter()
            .find(|p| matches!(p.target, PlayerTarget::Flipbook(i) if i == id))
            .map(|p| p.clock.clock_ms)
    }

    /// Find the auto-player driving streaming clip `clip`, if any (a player
    /// registered via [`play_streaming_clip`](Self::play_streaming_clip)).
    fn streaming_player_mut(&mut self, clip: StreamingClipId) -> Option<&mut ClipPlayer> {
        self.clip_players
            .iter_mut()
            .find(|p| matches!(p.target, PlayerTarget::Streaming(c) if c == clip))
    }

    /// Pause / resume a streaming clip's auto-player
    /// ([`play_streaming_clip`](Self::play_streaming_clip)). No-op if `clip`
    /// has no player.
    pub fn set_streaming_clip_paused(&mut self, clip: StreamingClipId, paused: bool) {
        if let Some(p) = self.streaming_player_mut(clip) {
            p.paused = paused;
        }
    }

    /// Whether streaming clip `clip`'s auto-player is paused, or `None` if it
    /// has no player.
    #[must_use]
    pub fn is_streaming_clip_paused(&self, clip: StreamingClipId) -> Option<bool> {
        self.clip_players
            .iter()
            .find(|p| matches!(p.target, PlayerTarget::Streaming(c) if c == clip))
            .map(|p| p.paused)
    }

    /// Set the playback speed (`1.0` = authored rate, negative =
    /// reverse; QE.7b — was Q8) of streaming clip `clip`'s
    /// auto-player. No-op if `clip` has no player.
    pub fn set_streaming_clip_speed(&mut self, clip: StreamingClipId, speed: f32) {
        if let Some(p) = self.streaming_player_mut(clip) {
            p.clock.speed_q8 = speed_to_q8(speed);
        }
    }

    /// **Scrub** a streaming clip: set its auto-player's clock to `clock_ms`
    /// and immediately show the matching frame (works while paused). No-op if
    /// `clip` has no player.
    pub fn set_streaming_clip_clock_ms(&mut self, clip: StreamingClipId, clock_ms: f64) {
        let Some((target, frame)) = self.streaming_player_mut(clip).map(|p| {
            p.clock.clock_ms = clock_ms;
            #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
            let frame = p.clock.frame_for(clock_ms.max(0.0) as u32);
            (p.target, frame)
        }) else {
            return;
        };
        self.apply_player_frame(target, frame);
    }

    /// Streaming clip `clip`'s current playback-clock position (ms), or
    /// `None` if it has no player — the scrubber's read-back.
    #[must_use]
    pub fn streaming_clip_clock_ms(&self, clip: StreamingClipId) -> Option<f64> {
        self.clip_players
            .iter()
            .find(|p| matches!(p.target, PlayerTarget::Streaming(c) if c == clip))
            .map(|p| p.clock.clock_ms)
    }

    // ---- animated characters (VCL.6) -------------------------------------

    /// Register an animated character (RKC v3): upload its meshes as sprite
    /// models + its embedded voxel clips as flipbooks, then spawn one
    /// renderer instance **per bone attachment** — a static mesh sits at
    /// its bone, a clip attachment plays back on its own clock. `clip`
    /// selects a skeletal animation clip to drive the bones (`None` =
    /// rest pose). Returns a [`CharacterId`]; advance it each frame with
    /// [`advance_character`](Self::advance_character).
    ///
    /// Like clips, this works before any [`set_sprites`](Self::set_sprites);
    /// a later `set_sprites` drops all registered characters.
    pub fn add_character(&mut self, ch: &Character, clip: Option<usize>) -> CharacterId {
        // 1. Meshes → sprite models.
        let model_ids: Vec<SpriteModelId> =
            ch.meshes.iter().map(|m| self.add_sprite_model(m)).collect();
        // 2. Voxel clips → flipbooks; keep each one's timing for the clocks.
        let clip_regs: Vec<Option<(VoxelClipId, Vec<u32>, LoopMode)>> = ch
            .voxel_clips
            .iter()
            .map(|vc| {
                vc.decode().ok().map(|d| {
                    let id = self.add_voxel_clip(&d);
                    (id, d.durations, d.loop_mode)
                })
            })
            .collect();
        // 3. Build + solve the skeleton (rest pose → bone transforms).
        let mut skeleton = ch.to_kfa_sprite(clip);
        solve_kfa_limbs(&mut skeleton);
        // 4. One instance per attachment, posed by bone × local_offset.
        let mut attaches = Vec::new();
        for (bi, bone) in ch.bones.iter().enumerate() {
            let limb = &skeleton.limbs[bi];
            for att in &bone.attachments {
                let (s, h, f, p) =
                    compose_attachment(limb.s, limb.h, limb.f, limb.p, &att.local_offset);
                let xf = DynSpriteTransform {
                    pos: p,
                    right: s,
                    up: h,
                    forward: f,
                };
                match att.target {
                    MeshRef::Static(mi) => {
                        // The model was registered a few lines up, so the
                        // spawn can only fail on an out-of-range mesh index.
                        if let Some(inst) = model_ids
                            .get(mi)
                            .and_then(|&mid| self.add_sprite_instance_posed(mid, xf))
                        {
                            attaches.push(AttachInst {
                                bone: bi,
                                local_offset: att.local_offset,
                                inst,
                                clip: None,
                            });
                        }
                    }
                    MeshRef::Clip(ci) => {
                        if let Some(Some((cid, durations, loop_mode))) = clip_regs.get(ci) {
                            let Some(inst) = self.add_clip_instance_posed(*cid, xf) else {
                                continue;
                            };
                            attaches.push(AttachInst {
                                bone: bi,
                                local_offset: att.local_offset,
                                inst,
                                clip: Some(ClipClock::new(
                                    durations,
                                    *loop_mode,
                                    att.playback.speed_q8,
                                    f64::from(att.playback.start_phase_ms),
                                )),
                            });
                        }
                    }
                }
            }
        }
        let clips: Vec<VoxelClipId> = clip_regs
            .iter()
            .filter_map(|r| r.as_ref().map(|(cid, _, _)| *cid))
            .collect();
        let idx = self.char_instances.len();
        self.char_instances.push(CharInstance {
            skeleton,
            attaches,
            models: model_ids,
            clips,
        });
        self.char_map.alloc(idx as u32)
    }

    /// Advance a character by `dt` seconds: tick its skeletal animation +
    /// each clip attachment's clock, then re-pose every attachment
    /// (bone × local_offset) and select each clip's current frame. No-op on
    /// a stale id.
    pub fn advance_character(&mut self, id: CharacterId, dt: f64) {
        let Some(idx) = self.char_map.index(id) else {
            return;
        };
        self.advance_character_at(idx, dt);
    }

    /// [`advance_character`](Self::advance_character)'s resolved-index
    /// core, shared with [`tick`](Self::tick)'s all-characters sweep.
    #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
    fn advance_character_at(&mut self, idx: usize, dt: f64) {
        // Phase 1: solve the skeleton + compute each attachment's update,
        // borrowing only `char_instances[idx]`.
        let updates: Vec<(SpriteInstanceId, DynSpriteTransform, Option<u32>)> = {
            let CharInstance {
                skeleton, attaches, ..
            } = &mut self.char_instances[idx];
            skeleton.animsprite((dt * 1000.0) as i32);
            solve_kfa_limbs(skeleton);
            attaches
                .iter_mut()
                .map(|a| {
                    let limb = &skeleton.limbs[a.bone];
                    let (s, h, f, p) =
                        compose_attachment(limb.s, limb.h, limb.f, limb.p, &a.local_offset);
                    let xf = DynSpriteTransform {
                        pos: p,
                        right: s,
                        up: h,
                        forward: f,
                    };
                    let frame = a.clip.as_mut().map(|c| c.tick(dt));
                    (a.inst, xf, frame)
                })
                .collect()
        };
        // Phase 2: apply via the facade primitives (disjoint from
        // `char_instances`).
        for (inst, xf, frame) in updates {
            self.set_sprite_instance_transform(inst, xf);
            if let Some(f) = frame {
                self.set_clip_instance_frame(inst, f);
            }
        }
    }

    /// Move/re-orient a character to a new world transform `xf` (the root
    /// limb's world pose) **without** ticking its animation or clip clocks —
    /// a teleport that holds the current animation frame (e.g. dragging a
    /// paused character in an editor). Re-solves the skeleton from the new
    /// root + re-poses every attachment; clip frames are left as-is. No-op on
    /// a stale id.
    pub fn set_character_world_transform(&mut self, id: CharacterId, xf: DynSpriteTransform) {
        let Some(idx) = self.char_map.index(id) else {
            return;
        };
        // Phase 1: set the root pose + re-solve (no animsprite), then compute
        // each attachment's new transform — borrowing only `char_instances`.
        let updates: Vec<(SpriteInstanceId, DynSpriteTransform)> = {
            let CharInstance {
                skeleton, attaches, ..
            } = &mut self.char_instances[idx];
            skeleton.p = xf.pos;
            skeleton.s = xf.right;
            skeleton.h = xf.up;
            skeleton.f = xf.forward;
            solve_kfa_limbs(skeleton);
            attaches
                .iter()
                .map(|a| {
                    let limb = &skeleton.limbs[a.bone];
                    let (s, h, f, p) =
                        compose_attachment(limb.s, limb.h, limb.f, limb.p, &a.local_offset);
                    (
                        a.inst,
                        DynSpriteTransform {
                            pos: p,
                            right: s,
                            up: h,
                            forward: f,
                        },
                    )
                })
                .collect()
        };
        // Phase 2: apply (clip frames untouched — clocks didn't tick).
        for (inst, t) in updates {
            self.set_sprite_instance_transform(inst, t);
        }
    }

    /// Remove a character, dropping all its attachment instances **and**
    /// freeing the sprite models + voxel clips it registered. Returns
    /// `false` if `id` is stale.
    pub fn remove_character(&mut self, id: CharacterId) -> bool {
        let Some(idx) = self.char_map.index(id) else {
            return false;
        };
        let insts: Vec<SpriteInstanceId> = self.char_instances[idx]
            .attaches
            .iter()
            .map(|a| a.inst)
            .collect();
        for inst in insts {
            self.remove_sprite_instance(inst);
        }
        self.char_instances[idx].attaches.clear();
        // Free the models + clips this character registered (else they leak
        // until a `set_sprites` — costly for an editor hot-swapping all
        // session). `mem::take` so the per-id frees can borrow `self`.
        let models = std::mem::take(&mut self.char_instances[idx].models);
        let clips = std::mem::take(&mut self.char_instances[idx].clips);
        for model in models {
            self.remove_sprite_model(model);
        }
        for clip in clips {
            self.remove_voxel_clip(clip);
        }
        self.char_map.remove(id)
    }

    /// Register animated KFA sprites (one or more bone hierarchies).
    /// The GPU backend uploads each limb's kv6 as an instanced model
    /// **once** (appended to the sprite registry) and seeds the limb
    /// instances at their current pose; the CPU backend caches the
    /// posed limbs for drawing. Call once at setup, after
    /// [`set_sprites`](Self::set_sprites), then drive motion per frame
    /// with [`update_kfa_poses`](Self::update_kfa_poses).
    ///
    /// Limbs are posed from the sprites' current
    /// [`kfaval`](roxlap_formats::kfa::KfaSprite::kfaval) (advance
    /// [`animsprite`](roxlap_formats::kfa::KfaSprite::animsprite) first
    /// if using a baked curve), so `kfas` is taken `&mut`.
    pub fn set_kfa_sprites(&mut self, kfas: &mut [KfaSprite]) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.set_kfa_sprites(kfas),
            BackendImpl::Gpu(g) => g.set_kfa_sprites(kfas),
        }
    }

    /// Re-pose the registered KFA sprites from their current
    /// `kfaval[]`. Call each frame after advancing the animation
    /// (`kfa.animsprite(dt_ms)` or poking `kfaval[]`). The GPU backend
    /// takes the cheap transform-only update (no model-volume
    /// re-upload); the CPU backend re-solves limb transforms for the
    /// next [`render`](Self::render). Must follow a
    /// [`set_kfa_sprites`](Self::set_kfa_sprites) with the same sprites.
    pub fn update_kfa_poses(&mut self, kfas: &mut [KfaSprite]) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.update_kfa_poses(kfas),
            BackendImpl::Gpu(g) => g.update_kfa_poses(kfas),
        }
    }

    /// Carve the next z-layer off the [`SpriteSet::carve_model`] and
    /// re-upload (the demo's `G` hotkey + GPU.12 copy-on-modify). GPU
    /// only; a no-op on the CPU backend. Returns the voxels removed.
    pub fn carve_active_sprite(&mut self) -> u32 {
        match &mut self.inner {
            BackendImpl::Cpu(_) => 0,
            BackendImpl::Gpu(g) => g.carve_active_sprite(),
        }
    }

    /// Request that a frame be captured for
    /// [`take_capture`](Self::take_capture) — screenshots, photo
    /// modes, golden tests. Works on **both** backends since QE.7a
    /// (previously a GPU no-op — screenshots were impossible on the
    /// backend most games run). CPU: the next
    /// [`render`](Self::render) copies its composited framebuffer.
    /// GPU: arms [`take_capture`](Self::take_capture) to read back the
    /// most recent frame.
    pub fn request_capture(&mut self) {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.request_capture(),
            BackendImpl::Gpu(g) => g.request_capture(),
        }
    }

    /// Take the captured frame as packed `0x00RRGGBB` pixels +
    /// dimensions (the **logical** resolution on GPU —
    /// post-SSAA/posterize, pre-upscale), or `None` if no capture was
    /// requested / nothing rendered yet. The GPU readback blocks like
    /// [`pick_depth`](Self::pick_depth) — a hotkey path, not
    /// per-frame — and returns `None` on wasm (WebGPU can't block).
    pub fn take_capture(&mut self) -> Option<(Vec<u32>, u32, u32)> {
        match &mut self.inner {
            BackendImpl::Cpu(c) => c.take_capture(),
            BackendImpl::Gpu(g) => g.take_capture(),
        }
    }

    /// Screen→world picking input: the world-space hit distance `t` at
    /// window pixel `(x, y)` from the **last rendered frame**, or `None`
    /// for out-of-bounds pixels and sky / no-hit. The host reconstructs
    /// the world hit point as `cam.pos + t * normalize(ray_dir)`, where
    /// `ray_dir` is the same per-pixel ray the frame was rendered with
    /// (see the backend's projection).
    ///
    /// `t` is the distance to the nearest **scene-grid** surface
    /// (terrain + grids); sprites do not occlude it (the sprite pass
    /// reads depth read-only), so a cursor sprite under the pointer is
    /// transparent to the pick.
    ///
    /// Cost: the CPU backend reads its in-memory z-buffer (free); the
    /// GPU backend stages the depth buffer and blocks on a device poll
    /// (cheap at click time — do not call every frame). The GPU path
    /// only has depth when the last frame drew sprites (`write_depth`).
    #[must_use]
    pub fn pick_depth(&self, x: u32, y: u32) -> Option<f32> {
        match &self.inner {
            BackendImpl::Cpu(c) => c.pick_depth(x, y),
            BackendImpl::Gpu(g) => g.pick_depth(x, y),
        }
    }

    /// World-space view-ray direction (un-normalised) for window pixel
    /// `(x, y)`, under the projection the **last frame** rendered with.
    /// The backends differ (CPU `setcamera` vs GPU vertical-FOV
    /// pinhole), so this hides which one is active. `None` before the
    /// first frame. Intersect it with a plane for tile picking, or feed
    /// it to [`Self::pick`] for a voxel.
    #[must_use]
    pub fn pixel_ray(&self, camera: &Camera, x: f64, y: f64) -> Option<[f64; 3]> {
        match &self.inner {
            BackendImpl::Cpu(c) => c.pixel_ray(camera, x, y),
            BackendImpl::Gpu(g) => g.pixel_ray(camera, x, y),
        }
    }

    /// Canonical screen→world unproject: the full view [`Ray`]
    /// (`camera.pos` origin + unit direction) for window pixel
    /// `(x, y)`, under whichever projection the last frame used. The
    /// one entry point both backends honour — hosts never reconstruct
    /// the projection. `None` before the first frame or for a
    /// degenerate ray.
    ///
    /// Compose with [`roxlap_scene::Scene::raycast`] for depth-free
    /// picking that's identical on CPU and GPU:
    /// `renderer.view_ray(cam, x, y).and_then(|r| scene.raycast(r.origin, r.dir, max))`.
    #[must_use]
    pub fn view_ray(&self, camera: &Camera, x: f64, y: f64) -> Option<Ray> {
        let d = self.pixel_ray(camera, x, y)?;
        let len = (d[0] * d[0] + d[1] * d[1] + d[2] * d[2]).sqrt();
        if len < 1e-12 {
            return None;
        }
        Some(Ray {
            origin: glam::DVec3::from_array([camera.pos[0], camera.pos[1], camera.pos[2]]),
            dir: glam::DVec3::new(d[0] / len, d[1] / len, d[2] / len),
        })
    }

    /// One-call screen→world voxel pick: unproject pixel `(x, y)` with
    /// the active backend's projection, read the last frame's depth
    /// there, reconstruct the world hit, and resolve it to the owning
    /// grid + grid-local voxel via [`Scene::resolve_voxel`]. `None` on
    /// sky / no-hit, or when no grid claims the surface.
    ///
    /// `scene` and `camera` must be the ones the last frame rendered;
    /// the projection (size + FOV / `hx,hy,hz`) is taken from that
    /// frame. Cheap on CPU (in-memory z-buffer); on GPU it stages the
    /// depth buffer (a click-time device poll — not per frame).
    #[must_use]
    pub fn pick(&self, scene: &Scene, camera: &Camera, x: u32, y: u32) -> Option<PickHit> {
        let dir = self.pixel_ray(camera, f64::from(x), f64::from(y))?;
        let t = f64::from(self.pick_depth(x, y)?);
        let len = (dir[0] * dir[0] + dir[1] * dir[1] + dir[2] * dir[2]).sqrt();
        if len < 1e-9 {
            return None;
        }
        let s = t / len; // world = cam.pos + t · (dir / |dir|)
        let world = glam::DVec3::new(
            camera.pos[0] + dir[0] * s,
            camera.pos[1] + dir[1] * s,
            camera.pos[2] + dir[2] * s,
        );
        let (grid, voxel) = scene.resolve_voxel(world, glam::DVec3::from_array(dir))?;
        #[allow(clippy::cast_possible_truncation)]
        let world_f32 = [world.x as f32, world.y as f32, world.z as f32];
        Some(PickHit {
            world: world_f32,
            grid,
            voxel,
        })
    }
}

/// A frame in flight (QE-B6) — returned by [`SceneRenderer::frame`].
/// Overlays draw into the composited frame with the camera it was
/// rendered with; exactly one present happens, by [`Self::present`] /
/// [`Self::paint_egui`] or on drop.
pub struct Frame<'r> {
    renderer: &'r mut SceneRenderer,
    /// The camera the frame composited with — overlays must use it.
    camera: Camera,
    finished: bool,
}

impl Frame<'_> {
    /// Draw world-space overlay lines (see
    /// [`SceneRenderer::draw_lines`]); chainable.
    #[must_use = "the frame must still be presented (or dropped)"]
    pub fn draw_lines(self, lines: &[Line3]) -> Self {
        let cam = self.camera;
        self.renderer.draw_lines(&cam, lines);
        self
    }

    /// Draw 2D image-sprite quads (see
    /// [`SceneRenderer::draw_images`]); chainable.
    #[must_use = "the frame must still be presented (or dropped)"]
    pub fn draw_images(self, images: &[ImageSprite]) -> Self {
        let cam = self.camera;
        self.renderer.draw_images(&cam, images);
        self
    }

    /// Present with no UI overlay. Consuming — a second present
    /// cannot compile.
    pub fn present(mut self) {
        self.finish();
    }

    /// Overlay a tessellated egui UI, then present (`hud` feature) —
    /// see [`SceneRenderer::paint_egui`] for the arguments. Consuming.
    #[cfg(feature = "hud")]
    pub fn paint_egui(
        mut self,
        jobs: &[egui::ClippedPrimitive],
        textures: &egui::TexturesDelta,
        pixels_per_point: f32,
    ) {
        self.finished = true;
        self.renderer.paint_egui(jobs, textures, pixels_per_point);
    }

    fn finish(&mut self) {
        if !self.finished {
            self.finished = true;
            self.renderer.present();
        }
    }
}

impl Drop for Frame<'_> {
    /// An unfinished frame presents itself — forgetting to present is
    /// not representable.
    fn drop(&mut self) {
        self.finish();
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// RP.0 — `Native` resolves to the window size verbatim (the byte-identical
    /// gate), `Fixed` ignores the window, `Scale` scales + clamps, and every
    /// result is `>= 1` per axis.
    #[test]
    fn render_resolution_logical_for() {
        let win = (1920, 1080);
        assert_eq!(RenderResolution::Native.logical_for(win), win);
        assert_eq!(
            RenderResolution::Fixed { w: 860, h: 520 }.logical_for(win),
            (860, 520)
        );
        // Fixed is independent of the window.
        assert_eq!(
            RenderResolution::Fixed { w: 860, h: 520 }.logical_for((640, 480)),
            (860, 520)
        );
        assert_eq!(RenderResolution::Scale(0.5).logical_for(win), (960, 540));
        // Scale rounds, not truncates: 801 * 0.5 = 400.5 → 401.
        assert_eq!(
            RenderResolution::Scale(0.5).logical_for((801, 601)),
            (401, 301)
        );
        // Degenerate inputs never produce a zero axis.
        assert_eq!(RenderResolution::Scale(0.001).logical_for((1, 1)), (1, 1));
        assert_eq!(
            RenderResolution::Fixed { w: 0, h: 0 }.logical_for(win),
            (1, 1)
        );
        assert_eq!(RenderResolution::Native.logical_for((0, 0)), (1, 1));
    }

    /// The handle map must survive the backends' swap-remove indexing:
    /// drive a model `DynInstanceMap` against a `Vec` "backend" that
    /// swap-removes, and check every live handle keeps resolving to its
    /// own payload through a sequence of adds + removes.
    #[test]
    fn dyn_instance_map_survives_swap_removes() {
        let mut map = DynInstanceMap::default();
        // The "backend": payload per dynamic index; swap_remove mirrors
        // both backends' remove_dyn_instance.
        let mut backend: Vec<u32> = Vec::new();
        // Our bookkeeping: handle -> the payload we expect it to address.
        let mut expect: Vec<(SpriteInstanceId, u32)> = Vec::new();

        let add = |map: &mut DynInstanceMap,
                   backend: &mut Vec<u32>,
                   expect: &mut Vec<(SpriteInstanceId, u32)>,
                   payload: u32| {
            let dyn_index = backend.len() as u32;
            backend.push(payload);
            let id = map.alloc(dyn_index);
            expect.push((id, payload));
        };

        for p in 0..6 {
            add(&mut map, &mut backend, &mut expect, p);
        }

        // Remove a middle handle (payload 2) and a later one (payload 4),
        // plus the current last — covering swap and no-swap paths.
        for victim_payload in [2u32, 4, 5] {
            let pos = expect
                .iter()
                .position(|&(_, p)| p == victim_payload)
                .unwrap();
            let (id, _) = expect.remove(pos);
            let dyn_index = map.dyn_index(id).expect("live handle resolves");
            // Backend swap-remove + report moved index (old last), exactly
            // like remove_dyn_instance on both backends.
            let last = backend.len() - 1;
            backend.swap_remove(dyn_index as usize);
            let moved = (dyn_index as usize != last).then_some(last as u32);
            map.remove(id, dyn_index, moved);
            // The removed handle is now stale.
            assert!(map.dyn_index(id).is_none(), "removed handle is stale");
        }

        // Every surviving handle still resolves to its own payload.
        for &(id, payload) in &expect {
            let idx = map.dyn_index(id).expect("survivor resolves");
            assert_eq!(
                backend[idx as usize], payload,
                "handle addresses its payload"
            );
        }
        assert_eq!(map.order.len(), backend.len());
        assert_eq!(backend.len(), expect.len());
    }

    /// The model slotmap mints stable ids, resolves only live handles,
    /// and never reuses a slot — so a removed model's id stays dead and
    /// every other id survives the remove.
    #[test]
    fn dyn_model_map_lifecycle() {
        let mut map = EpochSlotMap::default();
        // `set_sprites(3 models)` seeds ids 0..3, all live.
        map.reset_live(3);
        let ids: Vec<SpriteModelId> = (0..3).map(|s| SpriteModelId { slot: s, gen: 0 }).collect();
        for (i, &id) in ids.iter().enumerate() {
            assert_eq!(map.index(id), Some(i));
        }

        // Incrementally add a fourth model.
        let extra = map.alloc(3);
        assert_eq!(extra, SpriteModelId { slot: 3, gen: 0 });
        assert_eq!(map.index(extra), Some(3));

        // Remove model 1: its handle goes stale, the rest stay valid.
        assert!(map.remove(ids[1]));
        assert_eq!(map.index(ids[1]), None);
        assert_eq!(map.index(ids[0]), Some(0));
        assert_eq!(map.index(ids[2]), Some(2));
        assert_eq!(map.index(extra), Some(3));

        // Double remove / stale removal is a no-op returning false.
        assert!(!map.remove(ids[1]));

        // A bogus / out-of-range handle resolves to nothing, no panic.
        let bogus = SpriteModelId { slot: 999, gen: 0 };
        assert_eq!(map.index(bogus), None);
        assert!(!map.remove(bogus));

        // A handle with a mismatched generation never resolves (guards a
        // future compacting registry).
        let wrong_gen = SpriteModelId { slot: 0, gen: 7 };
        assert_eq!(map.index(wrong_gen), None);
    }

    /// The voxel-clip slotmap (VCL.4) mints stable ids, resolves only live
    /// handles, tombstones in place, and `reset` clears it — mirroring the
    /// model slotmap, since clips register append-only too.
    #[test]
    fn dyn_clip_map_lifecycle() {
        let mut map = EpochSlotMap::default();
        // Two clips registered incrementally (indices 0, 1).
        let c0 = map.alloc(0);
        let c1 = map.alloc(1);
        assert_eq!(c0, VoxelClipId { slot: 0, gen: 0 });
        assert_eq!(map.index(c0), Some(0));
        assert_eq!(map.index(c1), Some(1));

        // Remove clip 0: stale handle, clip 1 stays valid; slot not reused.
        assert!(map.remove(c0));
        assert_eq!(map.index(c0), None);
        assert_eq!(map.index(c1), Some(1));
        // Double / stale / out-of-range removes are false, no panic.
        assert!(!map.remove(c0));
        assert!(!map.remove(VoxelClipId { slot: 99, gen: 0 }));
        // Mismatched generation never resolves.
        assert_eq!(map.index(VoxelClipId { slot: 1, gen: 5 }), None);

        // `set_sprites` resets the clip layer → ids restart at slot 0, but
        // the epoch bumps so old handles don't alias the new clips.
        map.reset();
        assert_eq!(map.index(c1), None, "reset invalidates old handles");
        let again = map.alloc(0); // re-takes slot 0 under the new epoch
        assert_eq!(again, VoxelClipId { slot: 0, gen: 1 });
        assert_eq!(map.index(again), Some(0));
        // The footgun fix: c0 (slot 0, old epoch) must NOT resolve to the new
        // clip now occupying slot 0.
        assert_eq!(
            map.index(c0),
            None,
            "a pre-reset handle must not alias a new clip on the same slot"
        );
    }

    /// The character slotmap (VCL.6) mints stable ids, resolves only live
    /// handles, tombstones in place, and `reset` clears it.
    #[test]
    fn char_map_lifecycle() {
        let mut map = EpochSlotMap::default();
        let a = map.alloc(0);
        let b = map.alloc(1);
        assert_eq!(a, CharacterId { slot: 0, gen: 0 });
        assert_eq!(map.index(a), Some(0));
        assert_eq!(map.index(b), Some(1));

        assert!(map.remove(a));
        assert_eq!(map.index(a), None);
        assert_eq!(map.index(b), Some(1));
        assert!(!map.remove(a)); // double remove is a no-op
        assert!(!map.remove(CharacterId { slot: 9, gen: 0 }));
        assert_eq!(map.index(CharacterId { slot: 1, gen: 7 }), None);

        map.reset();
        assert_eq!(map.index(b), None);
        assert_eq!(map.alloc(0), CharacterId { slot: 0, gen: 1 });
        assert_eq!(map.index(a), None, "pre-reset handle must not alias slot 0");
    }

    /// The streaming-clip slotmap (#3) mints stable ids, resolves only live
    /// handles, tombstones in place, and `reset` clears it.
    #[test]
    fn streaming_clip_map_lifecycle() {
        let mut map = EpochSlotMap::default();
        let a = map.alloc(0);
        let b = map.alloc(1);
        assert_eq!(a, StreamingClipId { slot: 0, gen: 0 });
        assert_eq!(map.index(a), Some(0));
        assert_eq!(map.index(b), Some(1));

        assert!(map.remove(a));
        assert_eq!(map.index(a), None);
        assert_eq!(map.index(b), Some(1));
        assert!(!map.remove(a)); // double remove is a no-op
        assert!(!map.remove(StreamingClipId { slot: 9, gen: 0 }));
        assert_eq!(map.index(StreamingClipId { slot: 1, gen: 7 }), None);

        map.reset();
        assert_eq!(map.index(b), None);
        assert_eq!(map.alloc(0), StreamingClipId { slot: 0, gen: 1 });
        assert_eq!(map.index(a), None, "pre-reset handle must not alias slot 0");
    }

    /// The shared clip-playback clock (#6 / VCL.6): `tick` accumulates time
    /// at its Q8 speed, resolves the frame, honours `start_phase`, and reads
    /// a rewound (negative) clock as frame 0.
    #[test]
    fn clip_clock_tick_advances_and_resolves_frames() {
        // 3 frames, 100 ms each → total 300 ms, looping.
        let mut c = ClipClock::new(&[100, 100, 100], LoopMode::Loop, 256, 0.0); // 1×
        assert_eq!(c.tick(0.0), 0); // t=0 → frame 0
        assert_eq!(c.tick(0.10), 1); // t=100 → frame 1 (100 is not < 100)
        assert_eq!(c.clock_ms as u32, 100);
        assert_eq!(c.tick(0.15), 2); // t=250 → frame 2
        assert_eq!(c.tick(0.10), 0); // t=350 → 350%300=50 → frame 0
                                     // 0.5× speed advances half as fast.
        let mut slow = ClipClock::new(&[100, 100], LoopMode::Once, 128, 0.0); // 0.5×
        assert_eq!(slow.tick(0.20), 1); // 200ms wall → 100ms clock → frame 1
        assert!((slow.clock_ms - 100.0).abs() < 1e-6);
        // start_phase seeds the clock; negative clock reads as frame 0.
        // rewind at -1×, clock seeded mid frame 1
        let mut phased = ClipClock::new(&[50, 50, 50], LoopMode::Loop, -256, 50.0);
        assert_eq!(phased.tick(0.10), 0); // 50 - 100 = -50 → max(0)=0 → frame 0
        assert!(phased.clock_ms < 0.0); // kept signed
    }

    #[test]
    fn clip_clock_retarget_swaps_timeline_restarts_keeps_speed() {
        // BB.1: swapping a billboard's animation retargets the player's
        // timeline (durations + loop) and restarts the clock, but keeps the
        // playback rate (the clock policy).
        let mut c = ClipClock::new(&[100, 100, 100], LoopMode::Loop, 512, 250.0); // 2×
        c.retarget(&[50, 50], LoopMode::Once);
        assert_eq!(c.prefix, vec![50, 100]); // new clip's timeline (prefix sums)
        assert_eq!(c.loop_mode, LoopMode::Once); // new clip's loop mode
        assert!((c.clock_ms - 0.0).abs() < 1e-9); // restarted at frame 0
        assert_eq!(c.speed_q8, 512); // playback rate preserved
                                     // After retarget, ticking advances on the *new* timeline.
        assert_eq!(c.tick(0.0), 0);
        assert_eq!(c.tick(0.025), 1); // 25ms wall × 2× = 50ms → frame 1
    }

    fn dot(a: [f32; 3], b: [f32; 3]) -> f32 {
        a[0] * b[0] + a[1] * b[1] + a[2] * b[2]
    }
    fn unit(v: [f32; 3]) -> bool {
        (dot(v, v) - 1.0).abs() < 1e-5
    }

    #[test]
    fn billboard_cylindrical_faces_camera_upright_and_ignores_height() {
        // Camera due +x of the sprite. Cylindrical normal (local +y) points
        // at the camera horizontally; image vertical (local +z) is world up.
        let xf = billboard_transform(
            [0.0, 0.0, 0.0],
            [10.0, 0.0, 0.0],
            BillboardMode::Cylindrical,
        )
        .expect("non-degenerate");
        assert_eq!(xf.up, [1.0, 0.0, 0.0]); // normal → toward camera
        assert_eq!(xf.forward, BILLBOARD_UP); // image vertical → world up (-z)
        assert_eq!(xf.right, [0.0, -1.0, 0.0]); // image horizontal = screen-right
                                                // Cylindrical ignores camera height: a camera at a different z gives
                                                // the same (vertical) basis.
        let high = billboard_transform(
            [0.0, 0.0, 0.0],
            [10.0, 0.0, -50.0],
            BillboardMode::Cylindrical,
        )
        .unwrap();
        assert_eq!(high.up, xf.up);
        assert_eq!(high.forward, xf.forward);
        // Orthonormal basis.
        for v in [xf.right, xf.up, xf.forward] {
            assert!(unit(v));
        }
        assert!(dot(xf.right, xf.up).abs() < 1e-5);
        assert!(dot(xf.up, xf.forward).abs() < 1e-5);
        assert!(dot(xf.right, xf.forward).abs() < 1e-5);
    }

    #[test]
    fn billboard_spherical_tilts_with_view_and_normal_points_at_camera() {
        // Camera above (-z) and in front (+x): the normal tilts up; the
        // image vertical gains an up-tilt too (unlike cylindrical).
        let cam = [10.0, 0.0, -10.0];
        let xf = billboard_transform([0.0, 0.0, 0.0], cam, BillboardMode::Spherical).unwrap();
        // Normal (local +y) = normalized direction to the camera.
        let n = bb_norm([cam[0] as f32, cam[1] as f32, cam[2] as f32]).unwrap();
        for (u, ni) in xf.up.iter().zip(n.iter()) {
            assert!((u - ni).abs() < 1e-5);
        }
        // Not vertical-locked: image vertical tilts off world up.
        assert!(xf.forward != BILLBOARD_UP);
        for v in [xf.right, xf.up, xf.forward] {
            assert!(unit(v));
        }
        assert!(dot(xf.right, xf.up).abs() < 1e-5);
        assert!(dot(xf.up, xf.forward).abs() < 1e-5);
        assert!(dot(xf.right, xf.forward).abs() < 1e-5);
    }

    #[test]
    fn dir_index_bins_view_angle_front_ccw() {
        let o = [0.0, 0.0, 0.0];
        // N == 1 (non-directional) is always 0, regardless of camera.
        assert_eq!(dir_index(o, 0.0, [5.0, 3.0, 0.0], 1), 0);
        // 8-way, actor facing +x (yaw 0). Camera in front (+x) = front = 0.
        assert_eq!(dir_index(o, 0.0, [10.0, 0.0, 0.0], 8), 0);
        // Camera at +y (90° CCW from facing) → sector 2 (90° / 45°).
        assert_eq!(dir_index(o, 0.0, [0.0, 10.0, 0.0], 8), 2);
        // Camera behind (−x, 180°) → sector 4.
        assert_eq!(dir_index(o, 0.0, [-10.0, 0.0, 0.0], 8), 4);
        // Camera at −y (270°) → sector 6.
        assert_eq!(dir_index(o, 0.0, [0.0, -10.0, 0.0], 8), 6);
        // Rotating the actor's facing rotates the picked sector: facing +y
        // (yaw 90°), camera at +y is now "front" → 0.
        let fy = std::f64::consts::FRAC_PI_2;
        assert_eq!(dir_index(o, fy, [0.0, 10.0, 0.0], 8), 0);
        // Camera straight overhead (no horizontal bearing) → 0.
        assert_eq!(dir_index(o, 0.0, [0.0, 0.0, -10.0], 8), 0);
        // 4-way still bins front/left/back/right.
        assert_eq!(dir_index(o, 0.0, [10.0, 0.0, 0.0], 4), 0);
        assert_eq!(dir_index(o, 0.0, [0.0, 10.0, 0.0], 4), 1);
    }

    #[test]
    fn apply_shadow_flags_toggles_bits_and_preserves_others() {
        use roxlap_formats::sprite::{SPRITE_FLAG_NO_SHADOW_CAST, SPRITE_FLAG_NO_SHADOW_RECEIVE};
        let other = 1u32 << 2; // an unrelated flag bit must survive every call
        let mut f = other;
        apply_shadow_flags(&mut f, true, true); // both on ⇒ no NO_* bits
        assert_eq!(f & SPRITE_FLAG_NO_SHADOW_CAST, 0);
        assert_eq!(f & SPRITE_FLAG_NO_SHADOW_RECEIVE, 0);
        apply_shadow_flags(&mut f, false, true); // no cast
        assert_ne!(f & SPRITE_FLAG_NO_SHADOW_CAST, 0);
        assert_eq!(f & SPRITE_FLAG_NO_SHADOW_RECEIVE, 0);
        apply_shadow_flags(&mut f, true, false); // no receive
        assert_eq!(f & SPRITE_FLAG_NO_SHADOW_CAST, 0);
        assert_ne!(f & SPRITE_FLAG_NO_SHADOW_RECEIVE, 0);
        apply_shadow_flags(&mut f, false, false); // neither
        assert_ne!(f & SPRITE_FLAG_NO_SHADOW_CAST, 0);
        assert_ne!(f & SPRITE_FLAG_NO_SHADOW_RECEIVE, 0);
        assert_eq!(f & other, other, "unrelated bit preserved throughout");
    }

    #[test]
    fn apply_lighting_flags_sets_exclusive_mode_and_preserves_others() {
        use roxlap_formats::sprite::{
            SPRITE_FLAG_LIGHT_AMBIENT_ONLY, SPRITE_FLAG_LIGHT_WORLD_UP, SPRITE_FLAG_NO_SHADOW_CAST,
        };
        let other = SPRITE_FLAG_NO_SHADOW_CAST; // a shadow bit must survive
        let mut f = other;
        apply_lighting_flags(&mut f, BillboardLighting::WorldUp);
        assert_ne!(f & SPRITE_FLAG_LIGHT_WORLD_UP, 0);
        assert_eq!(f & SPRITE_FLAG_LIGHT_AMBIENT_ONLY, 0);
        apply_lighting_flags(&mut f, BillboardLighting::AmbientOnly);
        assert_eq!(f & SPRITE_FLAG_LIGHT_WORLD_UP, 0, "modes are exclusive");
        assert_ne!(f & SPRITE_FLAG_LIGHT_AMBIENT_ONLY, 0);
        apply_lighting_flags(&mut f, BillboardLighting::FullBright);
        assert_ne!(
            f & SPRITE_FLAG_LIGHT_WORLD_UP,
            0,
            "full-bright sets both bits"
        );
        assert_ne!(f & SPRITE_FLAG_LIGHT_AMBIENT_ONLY, 0);
        apply_lighting_flags(&mut f, BillboardLighting::FaceNormal);
        assert_eq!(
            f & (SPRITE_FLAG_LIGHT_WORLD_UP | SPRITE_FLAG_LIGHT_AMBIENT_ONLY),
            0
        );
        assert_eq!(f & other, other, "unrelated bit preserved throughout");
    }

    #[test]
    fn billboard_degenerate_keeps_position_none_stays_none() {
        // Degenerate view axes (camera straight overhead) must still
        // yield a transform — a fixed fallback facing — because
        // dropping it also drops the POSITION update and the actor
        // freezes at its last pose (the CC.4 third-person bug: a
        // camera at the actor's own xy column degenerated every
        // frame). The card is edge-on right then, so the facing
        // itself is cosmetically irrelevant.
        let xf = billboard_transform(
            [1.0, 2.0, 0.0],
            [1.0, 2.0, -10.0],
            BillboardMode::Cylindrical,
        )
        .expect("degenerate cylindrical falls back, not drops");
        assert_eq!(xf.pos, [1.0, 2.0, 0.0], "position always lands");
        let xf = billboard_transform([1.0, 2.0, 0.0], [1.0, 2.0, -10.0], BillboardMode::Spherical)
            .expect("degenerate spherical falls back, not drops");
        assert_eq!(xf.pos, [1.0, 2.0, 0.0]);
        // None mode is never auto-oriented.
        assert!(
            billboard_transform([0.0, 0.0, 0.0], [10.0, 0.0, 0.0], BillboardMode::None).is_none()
        );
    }

    #[test]
    fn dyn_sprite_transform_default_is_identity_and_applies() {
        let xf = DynSpriteTransform::default();
        assert_eq!(xf.pos, [0.0, 0.0, 0.0]);
        assert_eq!(xf.right, [1.0, 0.0, 0.0]);
        assert_eq!(xf.up, [0.0, 1.0, 0.0]);
        assert_eq!(xf.forward, [0.0, 0.0, 1.0]);

        let mut s = Sprite::axis_aligned(
            roxlap_formats::kv6::Kv6::solid_cube(2, roxlap_formats::VoxColor(0x80_FF_FF_FF)),
            [9.0, 9.0, 9.0],
        );
        let posed = DynSpriteTransform {
            pos: [1.0, 2.0, 3.0],
            right: [0.0, 0.0, 1.0],
            up: [0.0, 1.0, 0.0],
            forward: [1.0, 0.0, 0.0],
        };
        posed.apply_to(&mut s);
        assert_eq!(s.p, [1.0, 2.0, 3.0]);
        assert_eq!(s.s, [0.0, 0.0, 1.0]);
        assert_eq!(s.h, [0.0, 1.0, 0.0]);
        assert_eq!(s.f, [1.0, 0.0, 0.0]);
    }

    #[test]
    fn options_default_is_cpu_intent() {
        let o = RenderOptions::default();
        assert_eq!(o.backend, BackendPreference::Cpu);
        assert_eq!(o.clear_sky.0 & 0xFF00_0000, 0, "clear_sky is 0x00RRGGBB");
    }

    /// A camera at the origin looking down +Y (voxlap z-down world): right
    /// = +X, down = +Z, forward = +Y. Handedness `right × down == forward`.
    fn cam_looking_y() -> Camera {
        Camera {
            pos: [0.0, 0.0, 0.0],
            right: [1.0, 0.0, 0.0],
            down: [0.0, 0.0, 1.0],
            forward: [0.0, 1.0, 0.0],
        }
    }

    #[test]
    fn world_quad_corner_layout() {
        // Top-left at (-5, 10, -5); u = +X (width), v = +Z (down). A
        // 10×10 quad facing the camera (its +Y normal points back at us).
        let sprite = ImageSprite {
            image: ImageId { slot: 0, gen: 0 },
            origin: [-5.0, 10.0, -5.0],
            facing: ImageFacing::World {
                u: [1.0, 0.0, 0.0],
                v: [0.0, 0.0, 1.0],
            },
            size: [10.0, 10.0],
            tint: OverlayColor(0xFFFF_FFFF),
            alpha_cutoff: 0.0,
            depth_test: true,
            double_sided: true,
        };
        let q = resolve_quad(&sprite, &cam_looking_y()).expect("front-facing");
        assert_eq!(q.corners[0], [-5.0, 10.0, -5.0], "TL = origin");
        assert_eq!(q.corners[1], [5.0, 10.0, -5.0], "TR = origin + u·size");
        assert_eq!(q.corners[2], [-5.0, 10.0, 5.0], "BL = origin + v·size");
        assert_eq!(q.corners[3], [5.0, 10.0, 5.0], "BR = origin + u + v");
    }

    #[test]
    fn world_quad_backface_culls_when_single_sided() {
        // Same plane but spanned so its normal (u × v) points *away* from
        // the camera: swap u/v so the winding flips.
        let sprite = ImageSprite {
            image: ImageId { slot: 0, gen: 0 },
            origin: [-5.0, 10.0, -5.0],
            facing: ImageFacing::World {
                u: [0.0, 0.0, 1.0], // v-ish
                v: [1.0, 0.0, 0.0], // u-ish → normal flips to -Y... toward camera?
            },
            size: [10.0, 10.0],
            tint: OverlayColor(0xFFFF_FFFF),
            alpha_cutoff: 0.0,
            depth_test: true,
            double_sided: false,
        };
        // With double_sided=false one of the two windings must cull; the
        // opposite winding must draw. Exactly one of the two resolves.
        let a = resolve_quad(&sprite, &cam_looking_y()).is_some();
        let mut flipped = sprite;
        flipped.facing = ImageFacing::World {
            u: [1.0, 0.0, 0.0],
            v: [0.0, 0.0, 1.0],
        };
        let b = resolve_quad(&flipped, &cam_looking_y()).is_some();
        assert!(a ^ b, "exactly one winding is front-facing");
    }

    #[test]
    fn double_sided_never_culls() {
        let mut sprite = ImageSprite {
            image: ImageId { slot: 0, gen: 0 },
            origin: [-5.0, 10.0, -5.0],
            facing: ImageFacing::World {
                u: [0.0, 0.0, 1.0],
                v: [1.0, 0.0, 0.0],
            },
            size: [10.0, 10.0],
            tint: OverlayColor(0xFFFF_FFFF),
            alpha_cutoff: 0.0,
            depth_test: true,
            double_sided: true,
        };
        assert!(resolve_quad(&sprite, &cam_looking_y()).is_some());
        sprite.facing = ImageFacing::World {
            u: [1.0, 0.0, 0.0],
            v: [0.0, 0.0, 1.0],
        };
        assert!(resolve_quad(&sprite, &cam_looking_y()).is_some());
    }

    #[test]
    fn ray_quad_uv_center_and_corners() {
        // 10×10 quad on the y=10 plane: TL(-5,10,-5) u=+X v=+Z. Camera at
        // origin looking +Y. A ray straight at the quad centre → uv (.5,.5).
        let corners = [
            [-5.0, 10.0, -5.0], // TL
            [5.0, 10.0, -5.0],  // TR
            [-5.0, 10.0, 5.0],  // BL
            [5.0, 10.0, 5.0],   // BR
        ];
        let (uv, t) = ray_quad_uv([0.0, 0.0, 0.0], [0.0, 1.0, 0.0], &corners).expect("center hit");
        assert!(
            (uv[0] - 0.5).abs() < 1e-5 && (uv[1] - 0.5).abs() < 1e-5,
            "centre → (.5,.5)"
        );
        assert!((t - 10.0).abs() < 1e-4, "t = plane distance");
        // Ray toward the TL corner texel region (−x, +y, −z) → uv near (0,0).
        let (uv_tl, _) = ray_quad_uv([0.0, 0.0, 0.0], [-4.0, 10.0, -4.0], &corners).unwrap();
        assert!(uv_tl[0] < 0.2 && uv_tl[1] < 0.2, "toward TL → small uv");
    }

    #[test]
    fn ray_quad_uv_misses_outside_and_behind() {
        let corners = [
            [-5.0, 10.0, -5.0],
            [5.0, 10.0, -5.0],
            [-5.0, 10.0, 5.0],
            [5.0, 10.0, 5.0],
        ];
        // Ray pointing away (−Y) never reaches the +Y plane in front.
        assert!(ray_quad_uv([0.0, 0.0, 0.0], [0.0, -1.0, 0.0], &corners).is_none());
        // Ray parallel to the quad plane (in +X) → no intersection.
        assert!(ray_quad_uv([0.0, 0.0, 0.0], [1.0, 0.0, 0.0], &corners).is_none());
        // Ray hitting the plane far outside the quad → outside uv.
        assert!(ray_quad_uv([100.0, 0.0, 0.0], [0.0, 1.0, 0.0], &corners).is_none());
    }

    #[test]
    fn billboard_axes_orthogonal_and_top_toward_up() {
        // World up = -Z (z-down world). The billboard's v (top→bottom)
        // must point away from `up`, and u/v must be ⟂ the view direction.
        let up = [0.0, 0.0, -1.0];
        let sprite = ImageSprite {
            image: ImageId { slot: 0, gen: 0 },
            origin: [0.0, 50.0, 0.0],
            facing: ImageFacing::Billboard { up },
            size: [4.0, 4.0],
            tint: OverlayColor(0xFFFF_FFFF),
            alpha_cutoff: 0.0,
            depth_test: false,
            double_sided: false, // billboards must NEVER cull
        };
        let q = resolve_quad(&sprite, &cam_looking_y()).expect("billboard always faces camera");
        let u = v_sub(q.corners[1], q.corners[0]); // TR - TL = u·size
        let v = v_sub(q.corners[2], q.corners[0]); // BL - TL = v·size
        let fwd = [0.0, 1.0, 0.0];
        assert!(v_dot(u, fwd).abs() < 1e-5, "u ⟂ view");
        assert!(v_dot(v, fwd).abs() < 1e-5, "v ⟂ view");
        assert!(v_dot(u, v).abs() < 1e-5, "u ⟂ v");
        assert!(
            v_dot(v, up) < 0.0,
            "rows grow away from `up` (top edge toward up)"
        );
    }
}

#[cfg(test)]
mod image_map_tests {
    use super::{ImageId, ImageSlotMap};

    /// QE-B6 regression — image slots are reused by the backends, so
    /// a stale handle must resolve to nothing rather than alias the
    /// texture that re-took its slot (the pre-B6 behaviour).
    #[test]
    fn image_ids_are_generational() {
        let mut m = ImageSlotMap::default();
        let a = m.alloc(0);
        assert_eq!(m.index(a), Some(0));
        assert!(m.remove(a));
        assert_eq!(m.index(a), None);

        // The backend hands slot 0 out again: a fresh generation.
        let b = m.alloc(0);
        assert_ne!(a, b, "reused slot must mint a distinct handle");
        assert_eq!(m.index(b), Some(0));
        assert_eq!(m.index(a), None, "stale handle stays stale");
        assert!(!m.remove(a), "stale drop is refused…");
        assert_eq!(m.index(b), Some(0), "…and cannot hurt the newcomer");

        // Appending past the end still works.
        let c = m.alloc(1);
        assert_eq!(m.index(c), Some(1));
        let _ = ImageId { slot: 9, gen: 9 };
        assert_eq!(m.index(ImageId { slot: 9, gen: 9 }), None);
    }
}