roxlap_core/

dda_sprite.rs

//! Clean-room KV6 sprite raycaster for the DDA backend (Substage
//! DDA.8).
//!
//! Renders KV6 sprites by **per-pixel ray casting**: for every screen
//! pixel the sprite covers, transform the camera ray into the sprite's
//! local voxel space, 3D-DDA through the KV6, and depth-composite the
//! first solid voxel against the shared z-buffer. Clean-room (no voxlap
//! code), the sprite counterpart to the terrain renderer in
//! [`crate::dda`].
//!
//! **Depth parity.** Transforming the ray by the inverse sprite basis
//! leaves the ray parameter unchanged in world units — a hit at local
//! parameter `t` is at world point `cam.pos + dir·t` — so the
//! perpendicular depth is `t · (dir·forward)`, exactly the convention
//! [`crate::dda`] writes for terrain. Sprites therefore occlude and are
//! occluded by DDA terrain correctly.
//!
//! Shading reads the KV6 voxel's baked brightness byte (high byte of
//! the packed colour) via `crate::dda::shade` — the clean-room
//! brightness model, not voxlap's `dir`-LUT reflection shading.

use roxlap_formats::kv6::Kv6;
use roxlap_formats::material::{material_for_color, BlendMode, MaterialTable};
use roxlap_formats::sprite::{
    Sprite, SPRITE_FLAG_INVISIBLE, SPRITE_FLAG_LIGHT_AMBIENT_ONLY, SPRITE_FLAG_LIGHT_WORLD_UP,
    SPRITE_FLAG_NO_Z,
};
use roxlap_formats::voxel_clip::{DecodedClip, VoxelFrame};
use roxlap_formats::Rgb;

use std::sync::Arc;

use crate::camera_math::CameraState;
use crate::dda::{
    dda_setup, intersect_aabb, min_axis, pixel_ray, shade, shade_dynamic, CpuLights, ShadowTester,
    WorldOccluder, WorldShadow, WorldShadowCtx,
};
use crate::opticast::OpticastSettings;
use crate::raster_target::RasterTarget;

/// Near-plane parameter: voxels nearer than this (camera-forward) are
/// dropped, keeping the pinhole divide finite.
const NEAR_Z: f32 = 1.0;

/// Force a packed voxel colour to full brightness for the flat-lit
/// clean-room sprite path. KV6 / voxel-clip colours carry voxlap's
/// `dir`/shading slot in the high byte (some `0x80`, some `0x00`), not
/// the 0..128 brightness [`shade`] expects, so a raw value can render
/// black; we render every sprite voxel at its authored RGB.
#[inline]
fn full_bright(col: u32) -> u32 {
    (col & 0x00ff_ffff) | 0x8000_0000
}

/// Dense occupancy + colour grid for one sprite frame, plus its pivot —
/// the decoded form the per-pixel raycaster marches. Built once from a
/// [`Kv6`] ([`SpriteDense::from_kv6`]) or a voxel-clip [`VoxelFrame`]
/// ([`SpriteDense::from_voxel_frame`]); the latter lets an animated clip
/// cache every frame's grid up front instead of rebuilding per frame.
///
/// Both sources store only **surface** voxels (a from-air ray's first
/// hit is the visible surface), so the grid is the visible hull.
#[derive(Clone)]
pub struct SpriteDense {
    dims: [i32; 3],
    occ: Vec<bool>,
    col: Vec<u32>,
    /// Per-voxel material id (TV stage), parallel to [`col`](Self::col) /
    /// [`occ`](Self::occ) (same dense index). **Empty** means every voxel
    /// uses the draw-time uniform material (the TV.1 path); a non-empty
    /// array gives mixed-material models (opaque frame + glass, TV.3). Only
    /// consulted on the [`draw_sprite_dense_shaded`] accumulate path.
    mat: Vec<u8>,
    pivot: [f32; 3],
    /// World units per voxel (the clip's `voxel_world_size`; `1.0`
    /// for kv6 models). The draw + occluder entries scale the
    /// instance basis by it, mirroring the GPU shader's
    /// `(p - pivot) * voxel_world_size` — CPU/GPU parity for scaled
    /// clips.
    voxel_world_size: f32,
}

impl SpriteDense {
    /// Decode a [`Kv6`]'s surface-voxel run tables into a dense grid.
    #[must_use]
    #[allow(clippy::cast_possible_wrap)]
    pub fn from_kv6(kv6: &Kv6) -> Self {
        let dims = [kv6.xsiz as i32, kv6.ysiz as i32, kv6.zsiz as i32];
        let n = (dims[0].max(0) * dims[1].max(0) * dims[2].max(0)) as usize;
        let mut occ = vec![false; n];
        let mut col = vec![0u32; n];
        let mut vi = 0usize;
        for x in 0..kv6.xsiz as usize {
            for y in 0..kv6.ysiz as usize {
                let cnt = usize::from(kv6.ylen[x][y]);
                for _ in 0..cnt {
                    let v = kv6.voxels[vi];
                    vi += 1;
                    let z = i32::from(v.z);
                    if z >= 0 && z < dims[2] {
                        let idx = ((x as i32 * dims[1] + y as i32) * dims[2] + z) as usize;
                        occ[idx] = true;
                        col[idx] = full_bright(v.col);
                    }
                }
            }
        }
        Self {
            dims,
            occ,
            col,
            mat: Vec::new(),
            pivot: [kv6.xpiv, kv6.ypiv, kv6.zpiv],
            voxel_world_size: 1.0,
        }
    }

    /// Like [`from_kv6`](Self::from_kv6) but classifies each voxel into a
    /// material id by colour (TV.3 mixed models) via `material_map`
    /// (`(rgb, material_id)` pairs; see
    /// [`material_for_color`]).
    /// The resulting per-voxel `mat` array is consulted by the
    /// [`draw_sprite_dense_shaded`] accumulate path. An empty map yields the
    /// same all-opaque (uniform) result as `from_kv6`.
    #[must_use]
    #[allow(clippy::cast_possible_wrap)]
    pub fn from_kv6_with_materials(kv6: &Kv6, material_map: &[(Rgb, u8)]) -> Self {
        let mut dense = Self::from_kv6(kv6);
        if !material_map.is_empty() {
            let n = dense.col.len();
            let mut mat = vec![0u8; n];
            for (idx, slot) in mat.iter_mut().enumerate() {
                if dense.occ[idx] {
                    *slot = material_for_color(material_map, dense.col[idx]);
                }
            }
            dense.mat = mat;
        }
        dense
    }

    /// Decode a voxel-clip [`VoxelFrame`] (dense-column layout) into the
    /// dense grid, given the clip's `dims` + `pivot`. The frame's columns
    /// are `col = x + y*dims[0]`, each a per-column occupancy bitmask with
    /// an ascending-z colour run — walked here into the raycaster's
    /// `(x·my + y)·mz + z` grid.
    #[must_use]
    #[allow(clippy::cast_possible_wrap)]
    pub fn from_voxel_frame(frame: &VoxelFrame, dims: [u32; 3], pivot: [f32; 3]) -> Self {
        let (mx, my, mz) = (dims[0], dims[1], dims[2]);
        let owpc = mz.div_ceil(32).max(1) as usize;
        let n = (mx * my * mz) as usize;
        let mut occ = vec![false; n];
        let mut col = vec![0u32; n];
        for col_idx in 0..(mx * my) as usize {
            let x = col_idx as u32 % mx;
            let y = col_idx as u32 / mx;
            let run_start = frame.color_offsets[col_idx] as usize;
            let mut k = 0usize;
            for z in 0..mz {
                let word = frame.occupancy[col_idx * owpc + (z >> 5) as usize];
                if (word >> (z & 31)) & 1 != 0 {
                    let idx = (((x * my + y) * mz) + z) as usize;
                    occ[idx] = true;
                    col[idx] = full_bright(frame.colors[run_start + k]);
                    k += 1;
                }
            }
        }
        Self {
            dims: [mx as i32, my as i32, mz as i32],
            occ,
            col,
            mat: Vec::new(),
            pivot,
            voxel_world_size: 1.0,
        }
    }

    /// Set the render scale (world units per voxel) — the clip's
    /// `voxel_world_size`. Chainable; the default is `1.0`.
    #[must_use]
    pub fn with_voxel_world_size(mut self, vws: f32) -> Self {
        self.voxel_world_size = vws;
        self
    }

    /// Like [`from_voxel_frame`](Self::from_voxel_frame) but classifies each
    /// voxel into a material id by colour (TV.3 mixed models) via
    /// `material_map` — the clip analogue of
    /// [`from_kv6_with_materials`](Self::from_kv6_with_materials). An empty
    /// map yields the same all-opaque (uniform) result as `from_voxel_frame`.
    #[must_use]
    pub fn from_voxel_frame_with_materials(
        frame: &VoxelFrame,
        dims: [u32; 3],
        pivot: [f32; 3],
        material_map: &[(Rgb, u8)],
    ) -> Self {
        let mut dense = Self::from_voxel_frame(frame, dims, pivot);
        if !material_map.is_empty() {
            let n = dense.col.len();
            let mut mat = vec![0u8; n];
            for (idx, slot) in mat.iter_mut().enumerate() {
                if dense.occ[idx] {
                    *slot = material_for_color(material_map, dense.col[idx]);
                }
            }
            dense.mat = mat;
        }
        dense
    }

    #[inline]
    #[allow(clippy::cast_sign_loss)]
    fn idx_of(&self, c: [i32; 3]) -> usize {
        ((c[0] * self.dims[1] + c[1]) * self.dims[2] + c[2]) as usize
    }

    #[inline]
    fn at(&self, c: [i32; 3]) -> Option<u32> {
        let idx = self.idx_of(c);
        self.occ[idx].then(|| self.col[idx])
    }
}

/// Inverse of the column-matrix `[s | h | f]` (the sprite basis), or
/// `None` if degenerate. Maps a world delta into local voxel space.
fn invert_basis(s: [f32; 3], h: [f32; 3], f: [f32; 3]) -> Option<[[f32; 3]; 3]> {
    let det = s[0] * (h[1] * f[2] - f[1] * h[2]) - h[0] * (s[1] * f[2] - f[1] * s[2])
        + f[0] * (s[1] * h[2] - h[1] * s[2]);
    if det.abs() < 1e-12 {
        return None;
    }
    let inv = 1.0 / det;
    Some([
        [
            (h[1] * f[2] - f[1] * h[2]) * inv,
            -(h[0] * f[2] - f[0] * h[2]) * inv,
            (h[0] * f[1] - f[0] * h[1]) * inv,
        ],
        [
            -(s[1] * f[2] - f[1] * s[2]) * inv,
            (s[0] * f[2] - f[0] * s[2]) * inv,
            -(s[0] * f[1] - f[0] * s[1]) * inv,
        ],
        [
            (s[1] * h[2] - h[1] * s[2]) * inv,
            -(s[0] * h[2] - h[0] * s[2]) * inv,
            (s[0] * h[1] - h[0] * s[1]) * inv,
        ],
    ])
}

#[inline]
fn mat_apply(m: &[[f32; 3]; 3], v: [f32; 3]) -> [f32; 3] {
    [
        m[0][0] * v[0] + m[0][1] * v[1] + m[0][2] * v[2],
        m[1][0] * v[0] + m[1][1] * v[1] + m[1][2] * v[2],
        m[2][0] * v[0] + m[2][1] * v[1] + m[2][2] * v[2],
    ]
}

/// Cast one ray (already in the sprite's local voxel space) into the
/// dense KV6 and return `(colour, t)` of the first solid voxel — `t` is
/// the world-units ray parameter (shared with the world ray).
#[allow(clippy::cast_possible_truncation)]
/// First solid voxel along the local ray. Returns `(color, t, normal_local,
/// cell)`: `normal_local` is the **model-local** face normal of the hit
/// (points back toward the ray; zero for the entry voxel, no face crossed) —
/// the caller rotates it to world for dynamic lighting (DL.7); `cell` is the
/// hit voxel for the flat-per-voxel world centre.
fn cast_local(
    dense: &SpriteDense,
    origin: [f32; 3],
    dir: [f32; 3],
) -> Option<(u32, f32, [f32; 3], [i32; 3])> {
    #[allow(clippy::cast_precision_loss)]
    let hi = [
        dense.dims[0] as f32,
        dense.dims[1] as f32,
        dense.dims[2] as f32,
    ];
    let (t0, t1) = intersect_aabb(origin, dir, [0.0; 3], hi)?;
    let start = t0 + 1e-4;
    let p = [
        origin[0] + dir[0] * start,
        origin[1] + dir[1] * start,
        origin[2] + dir[2] * start,
    ];
    let mut cell = [
        (p[0].floor() as i32).clamp(0, dense.dims[0] - 1),
        (p[1].floor() as i32).clamp(0, dense.dims[1] - 1),
        (p[2].floor() as i32).clamp(0, dense.dims[2] - 1),
    ];
    let (step, mut t_max, t_delta) = dda_setup(origin, dir, cell, 1.0);
    let mut t_curr = t0;
    // Face crossed to reach the current cell (model-local normal). The entry
    // voxel (solid at t0, no step yet) has none → zero normal.
    let mut normal = [0.0f32; 3];
    let max_steps = (dense.dims[0] + dense.dims[1] + dense.dims[2]) as usize + 8;
    for _ in 0..max_steps {
        if cell[0] < 0
            || cell[0] >= dense.dims[0]
            || cell[1] < 0
            || cell[1] >= dense.dims[1]
            || cell[2] < 0
            || cell[2] >= dense.dims[2]
            || t_curr > t1
        {
            return None;
        }
        if let Some(color) = dense.at(cell) {
            return Some((color, t_curr, normal, cell));
        }
        let axis = min_axis(t_max);
        t_curr = t_max[axis];
        cell[axis] += step[axis];
        t_max[axis] += t_delta[axis];
        normal = [0.0; 3];
        normal[axis] = -(step[axis] as f32);
    }
    None
}

/// XS.2 — one sprite volume in the scene shadow occluder: its decoded dense
/// voxels + world pose, with the cached inverse instance basis for
/// world→sprite-local transforms. PF.8 — the dense grid is shared
/// (`Arc`) with the draw path's per-model cache instead of deep-cloned
/// per occluder rebuild.
struct SpriteOccEntry {
    dense: Arc<SpriteDense>,
    pos: [f32; 3],
    pivot: [f32; 3],
    minv: [[f32; 3]; 3],
}

/// XS.2 — a [`WorldOccluder`] over sprite volumes, so **sprites cast** hard
/// shadows onto terrain and each other (and so a sprite-receive query also
/// sees other sprites). Owns the decoded [`SpriteDense`] grids; populate with
/// [`Self::push`].
///
/// A world-space shadow ray is transformed into each sprite's local frame and
/// the dense occupancy is DDA-marched. Assumes orthonormal unit instance bases
/// (as the sprite draw does); a non-uniform scale would skew the `max_t`
/// distance bound. Empty ⇒ casts nothing.
#[derive(Default)]
pub struct SpriteOccluder {
    entries: Vec<SpriteOccEntry>,
}

impl SpriteOccluder {
    /// An empty occluder (casts nothing). Add sprite volumes with
    /// [`push`](Self::push).
    #[must_use]
    pub fn new() -> Self {
        Self::default()
    }

    /// Whether the occluder holds any sprite volumes.
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }

    /// Add a decoded sprite volume at a world pose (`pos` = world pivot;
    /// `s`/`h`/`f` = model→world basis columns, the same pose the draw uses).
    /// A degenerate (non-invertible) basis is skipped. PF.8 — takes the
    /// dense grid by `Arc` (an occluder rebuild shares the draw path's
    /// cached decodes instead of re-densifying every caster per frame).
    pub fn push(
        &mut self,
        dense: Arc<SpriteDense>,
        pos: [f32; 3],
        s: [f32; 3],
        h: [f32; 3],
        f: [f32; 3],
    ) {
        // Same clip render scale as the draw entry (see
        // `draw_sprite_dense_shaded`) — shadows must match the pixels.
        let sc = dense.voxel_world_size;
        let (s, h, f) = (bb_scale3(s, sc), bb_scale3(h, sc), bb_scale3(f, sc));
        let Some(minv) = invert_basis(s, h, f) else {
            return;
        };
        let pivot = dense.pivot;
        self.entries.push(SpriteOccEntry {
            dense,
            pos,
            pivot,
            minv,
        });
    }
}

impl WorldOccluder for SpriteOccluder {
    fn occluded_world(&self, origin: [f64; 3], dir: [f32; 3], max_t: f32) -> bool {
        self.entries
            .iter()
            .any(|e| sprite_entry_occluded(e, origin, dir, max_t))
    }
}

/// March one sprite entry's dense occupancy along a world-space ray; `true` if
/// a solid voxel blocks it within `max_t` world units.
#[allow(clippy::cast_possible_truncation, clippy::cast_precision_loss)]
/// Scale a basis column by the clip render scale.
fn bb_scale3(v: [f32; 3], k: f32) -> [f32; 3] {
    [v[0] * k, v[1] * k, v[2] * k]
}

fn sprite_entry_occluded(e: &SpriteOccEntry, ow: [f64; 3], dw: [f32; 3], max_t: f32) -> bool {
    // World → sprite-local voxel space (same transform as the draw). SC.2 —
    // the origin subtraction runs in f64 (both are large world coords, so this
    // avoids catastrophic cancellation) then narrows to f32 for the local
    // basis apply; the sprite frame's coords are small.
    #[allow(clippy::cast_possible_truncation)]
    let rel = [
        (ow[0] - f64::from(e.pos[0])) as f32,
        (ow[1] - f64::from(e.pos[1])) as f32,
        (ow[2] - f64::from(e.pos[2])) as f32,
    ];
    let ol = mat_apply(&e.minv, rel);
    let origin = [ol[0] + e.pivot[0], ol[1] + e.pivot[1], ol[2] + e.pivot[2]];
    let dir = mat_apply(&e.minv, dw);

    let hi = [
        e.dense.dims[0] as f32,
        e.dense.dims[1] as f32,
        e.dense.dims[2] as f32,
    ];
    let Some((t0, t1)) = intersect_aabb(origin, dir, [0.0; 3], hi) else {
        return false;
    };
    let t_enter = t0.max(0.0);
    let t_exit = t1.min(max_t);
    if t_enter > t_exit {
        return false;
    }
    let start = t_enter + 1e-4;
    let p = [
        origin[0] + dir[0] * start,
        origin[1] + dir[1] * start,
        origin[2] + dir[2] * start,
    ];
    let mut cell = [
        (p[0].floor() as i32).clamp(0, e.dense.dims[0] - 1),
        (p[1].floor() as i32).clamp(0, e.dense.dims[1] - 1),
        (p[2].floor() as i32).clamp(0, e.dense.dims[2] - 1),
    ];
    let (step, mut t_max, t_delta) = dda_setup(origin, dir, cell, 1.0);
    let mut t_curr = t_enter;
    let max_steps = (e.dense.dims[0] + e.dense.dims[1] + e.dense.dims[2]) as usize + 8;
    for _ in 0..max_steps {
        if cell[0] < 0
            || cell[0] >= e.dense.dims[0]
            || cell[1] < 0
            || cell[1] >= e.dense.dims[1]
            || cell[2] < 0
            || cell[2] >= e.dense.dims[2]
            || t_curr > t_exit
        {
            return false;
        }
        if e.dense.occ[e.dense.idx_of(cell)] {
            return true;
        }
        let a = min_axis(t_max);
        t_curr = t_max[a];
        cell[a] += step[a];
        t_max[a] += t_delta[a];
    }
    false
}

/// Material context for a translucent sprite draw (TV stage): the global
/// [`MaterialTable`] plus this instance's uniform material id and per-frame
/// alpha multiplier. Passed (as `Some`) to [`draw_sprite_dense_shaded`] /
/// [`ClipFlipbook::draw_frame_shaded`] to enable front-to-back
/// accumulate-and-continue compositing; `None` (or an all-opaque effective
/// material) takes the existing first-hit opaque path byte-for-byte.
#[derive(Clone, Copy)]
pub struct SpriteShade<'a> {
    /// Global voxel-material palette (per-voxel id → opacity + blend mode).
    pub materials: &'a MaterialTable,
    /// Uniform material id for every voxel of this sprite whose dense
    /// per-voxel `mat` array is empty (the TV.1 whole-sprite material).
    pub material: u8,
    /// Per-instance opacity multiplier (`255` = unscaled), so an effect can
    /// fade out by cheap per-frame updates without re-uploading the volume.
    pub alpha_mul: u8,
    /// Per-instance RGB colour tint, packed `0x00RRGGBB` — each rendered
    /// voxel's colour is multiplied by it. `0x00FF_FFFF` (white) is a no-op.
    pub tint: u32,
    /// DL.7 — world-space dynamic lights. When `enabled`, the opaque hit is
    /// lit (sun + point lights + cel + ramp, flat per voxel) instead of the
    /// baked `shade`. `CpuLights::default()` (disabled) ⇒ unchanged.
    pub lights: CpuLights<'a>,
    /// XS.2 — world-space scene occluder for **sprites receiving** hard
    /// shadows: a lit sprite voxel marches a shadow ray (world space) against
    /// this and is darkened where terrain / other sprites block the caster.
    /// `None` (the default) ⇒ unshadowed sprites (the pre-XS.2 look).
    pub shadow: Option<&'a dyn WorldOccluder>,
}

/// Accumulated front-to-back composite for one ray through a sprite.
struct LayerAccum {
    /// Premultiplied accumulated colour, channels in `0..=~1` (additive may
    /// exceed 1; clamped at pack time).
    rgb: [f32; 3],
    /// Remaining transmittance (starts 1.0, decays through `AlphaBlend`).
    trans: f32,
    /// The opaque/background hit that terminated the march, if any: its
    /// already-shaded packed colour + world-ray parameter `t`. `None` if the
    /// ray exited (or fully attenuated) without an opaque voxel — then the
    /// background is whatever the framebuffer already holds (terrain/sky).
    opaque: Option<(u32, f32)>,
}

/// Per-instance RGB tint: multiply `color`'s RGB by `tint`'s (both packed,
/// `tint`'s channels normalised by 255), preserving `color`'s high byte. White
/// tint (`0x00FF_FFFF`) returns `color` unchanged. Mirrors the GPU `apply_tint`.
#[inline]
fn tint_packed(color: u32, tint: u32) -> u32 {
    if tint & 0x00FF_FFFF == 0x00FF_FFFF {
        return color;
    }
    let mul = |shift: u32| {
        let c = (color >> shift) & 0xff;
        let t = (tint >> shift) & 0xff;
        ((c * t) / 255) & 0xff
    };
    (color & 0xff00_0000) | (mul(16) << 16) | (mul(8) << 8) | mul(0)
}

/// Unpack a packed `0x..RRGGBB` colour to linear-ish `0..1` float channels
/// (RGB only; the high byte is ignored here — sprite voxels are flat-lit).
#[inline]
fn rgb_to_f32(c: u32) -> [f32; 3] {
    [
        ((c >> 16) & 0xff) as f32 / 255.0,
        ((c >> 8) & 0xff) as f32 / 255.0,
        (c & 0xff) as f32 / 255.0,
    ]
}

/// Repack `0..1` float channels (clamped) into `0x80RRGGBB` — the
/// full-brightness packing the flat-lit sprite path writes.
#[inline]
#[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
fn f32_to_rgb(c: [f32; 3]) -> u32 {
    let q = |v: f32| (v.clamp(0.0, 1.0) * 255.0 + 0.5) as u32;
    0x8000_0000 | (q(c[0]) << 16) | (q(c[1]) << 8) | q(c[2])
}

/// roxlap world up — voxlap z-down, so up is `-z`. Used by
/// [`SpriteLightMode::WorldUp`] as a fixed shading normal.
const SPRITE_WORLD_UP: [f32; 3] = [0.0, 0.0, -1.0];

/// Per-instance billboard lighting mode (BB.2b), decoded from the sprite
/// `flags` (bits 6/7). Controls the surface normal / direct-light handling at
/// the sprite shade site so a camera-facing billboard needn't suffer the
/// camera-dependent N·L of its (camera-tracking) face normal.
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum SpriteLightMode {
    /// The DDA hit-face normal (default; today's DL.7 look).
    FaceNormal,
    /// A fixed world-up normal (stable directional shading).
    WorldUp,
    /// Ambient only — no sun / point-light direct term (flat cutout).
    AmbientOnly,
    /// Full-bright / emissive — the voxel colour at full intensity, ignoring
    /// lighting (glows: fire, spell auras). Encoded as **both** flag bits set.
    FullBright,
}

impl SpriteLightMode {
    /// Decode the mode from a sprite instance's `flags` word (bits
    /// 6/7 — `SPRITE_FLAG_LIGHT_WORLD_UP` /
    /// `SPRITE_FLAG_LIGHT_AMBIENT_ONLY`). Neither bit ⇒
    /// [`FaceNormal`](Self::FaceNormal) (the default); both ⇒
    /// [`FullBright`](Self::FullBright).
    #[must_use]
    pub fn from_flags(flags: u32) -> Self {
        let world_up = flags & SPRITE_FLAG_LIGHT_WORLD_UP != 0;
        let ambient_only = flags & SPRITE_FLAG_LIGHT_AMBIENT_ONLY != 0;
        match (ambient_only, world_up) {
            (true, true) => Self::FullBright, // both bits set
            (true, false) => Self::AmbientOnly,
            (false, true) => Self::WorldUp,
            (false, false) => Self::FaceNormal,
        }
    }
}

/// Shade a sprite voxel under a [`SpriteLightMode`] (BB.2b): `FaceNormal` is
/// the plain [`shade_dynamic`]; `WorldUp` swaps in a fixed world-up normal;
/// `AmbientOnly` drops the sun + point lights (and stylization) for a flat
/// ambient term. Shared by both sprite shade sites (opaque + translucent).
fn shade_dynamic_mode(
    mode: SpriteLightMode,
    albedo: [f32; 3],
    n_world: [f32; 3],
    center: [f32; 3],
    lights: &CpuLights<'_>,
    tester: Option<&mut dyn ShadowTester>,
) -> u32 {
    match mode {
        SpriteLightMode::FaceNormal => shade_dynamic(albedo, 1.0, n_world, center, lights, tester),
        SpriteLightMode::WorldUp => {
            shade_dynamic(albedo, 1.0, SPRITE_WORLD_UP, center, lights, tester)
        }
        SpriteLightMode::AmbientOnly => {
            let mut amb = *lights;
            amb.sun = false;
            amb.points = &[];
            amb.bands = 0; // smooth ambient (no cel ramp toward shadow_tint)
            shade_dynamic(albedo, 1.0, n_world, center, &amb, None)
        }
        // Emissive: the voxel colour at full intensity, ignoring the rig.
        SpriteLightMode::FullBright => f32_to_rgb(albedo),
    }
}

/// Cast one ray (in sprite-local voxel space) accumulating translucent
/// voxels front-to-back until an opaque voxel, transmittance exhaustion, or
/// the `max_t` cutoff (the terrain depth, so the march stops at geometry it
/// can't see past). `fwd_dot = dir·camera-forward` converts the ray
/// parameter to perpendicular depth. Returns `None` if the ray contributes
/// nothing (missed the box, or every voxel was clipped / behind terrain).
#[allow(clippy::cast_possible_truncation, clippy::too_many_arguments)]
fn cast_local_layers(
    dense: &SpriteDense,
    origin: [f32; 3],
    dir: [f32; 3],
    fwd_dot: f32,
    max_t: f32,
    shade_ctx: SpriteShade,
    // Instance basis (s,h,f) + world position — only used to light each layer
    // (rotate the model-local face normal + voxel centre to world). Ignored
    // when the rig is disabled (then every layer is the baked `shade`).
    s: [f32; 3],
    h: [f32; 3],
    f: [f32; 3],
    pos: [f32; 3],
    light_mode: SpriteLightMode,
) -> Option<LayerAccum> {
    #[allow(clippy::cast_precision_loss)]
    let hi = [
        dense.dims[0] as f32,
        dense.dims[1] as f32,
        dense.dims[2] as f32,
    ];
    let (t0, t1) = intersect_aabb(origin, dir, [0.0; 3], hi)?;
    let start = t0 + 1e-4;
    let p = [
        origin[0] + dir[0] * start,
        origin[1] + dir[1] * start,
        origin[2] + dir[2] * start,
    ];
    let mut cell = [
        (p[0].floor() as i32).clamp(0, dense.dims[0] - 1),
        (p[1].floor() as i32).clamp(0, dense.dims[1] - 1),
        (p[2].floor() as i32).clamp(0, dense.dims[2] - 1),
    ];
    let (step, mut t_max, t_delta) = dda_setup(origin, dir, cell, 1.0);
    let mut t_curr = t0;
    let max_steps = (dense.dims[0] + dense.dims[1] + dense.dims[2]) as usize + 8;

    let mut acc = LayerAccum {
        rgb: [0.0; 3],
        trans: 1.0,
        opaque: None,
    };
    let mut touched = false;
    // Per-span compositing: a translucent voxel contributes one alpha layer
    // only when the ray *enters* a contiguous solid run (the previous cell
    // was air). Without this, a ray clipping the shared boundary between two
    // adjacent surface voxels passes through both and double-composites a thin
    // strip — the model reads as "diced" by a voxel grid. Treating each solid
    // run as one surface makes a wall contribute exactly one alpha regardless
    // of how many of its voxels the ray grazes. A run is also re-entered on a
    // material change (TV.3: two adjacent translucent materials each count),
    // and an opaque voxel stops the ray on every cell (the opaque core of a
    // mixed model).
    let mut prev_solid = false;
    let mut prev_mat = 0u8;
    // Local ray length per ray-parameter unit — converts a cell's `t` span to
    // its path length in voxel units for the `Volumetric` Beer–Lambert weight.
    let dir_len = (dir[0] * dir[0] + dir[1] * dir[1] + dir[2] * dir[2]).sqrt();
    // Model-local face normal of the cell currently being shaded (the face the
    // ray crossed to enter it); zero for the AABB-entry voxel, matching
    // `cast_local`. Updated after each step.
    let mut normal = [0.0f32; 3];

    // XS.0/XS.2 — per-layer dynamic lighting (mirror of the opaque path):
    // rotate the model-local face normal + voxel centre to world via the
    // instance basis and call `shade_dynamic` (flat per voxel). Disabled rig ⇒
    // baked `shade` (byte-identical). XS.2 — when a scene occluder is present
    // the layer also receives hard shadows (world-space query, identity ctx).
    let lights = shade_ctx.lights;
    let tint = shade_ctx.tint;
    let mut tester = shade_ctx.shadow.map(|occ| WorldShadow {
        ctx: WorldShadowCtx::identity(occ),
    });
    let mut shade_layer = |idx: usize, cell: [i32; 3], n_local: [f32; 3]| -> u32 {
        if !lights.enabled {
            return tint_packed(shade(dense.col[idx], 0), tint);
        }
        let to_world = |v: [f32; 3]| {
            [
                v[0] * s[0] + v[1] * h[0] + v[2] * f[0],
                v[0] * s[1] + v[1] * h[1] + v[2] * f[1],
                v[0] * s[2] + v[1] * h[2] + v[2] * f[2],
            ]
        };
        let n_world = to_world(n_local);
        let rel = [
            cell[0] as f32 + 0.5 - dense.pivot[0],
            cell[1] as f32 + 0.5 - dense.pivot[1],
            cell[2] as f32 + 0.5 - dense.pivot[2],
        ];
        let wc = to_world(rel);
        let center = [pos[0] + wc[0], pos[1] + wc[1], pos[2] + wc[2]];
        let albedo = [
            ((dense.col[idx] >> 16) & 0xff) as f32 / 255.0,
            ((dense.col[idx] >> 8) & 0xff) as f32 / 255.0,
            (dense.col[idx] & 0xff) as f32 / 255.0,
        ];
        let t = tester.as_mut().map(|t| t as &mut dyn ShadowTester);
        tint_packed(
            shade_dynamic_mode(light_mode, albedo, n_world, center, &lights, t),
            tint,
        )
    };

    for _ in 0..max_steps {
        if cell[0] < 0
            || cell[0] >= dense.dims[0]
            || cell[1] < 0
            || cell[1] >= dense.dims[1]
            || cell[2] < 0
            || cell[2] >= dense.dims[2]
            || t_curr > t1
        {
            break;
        }
        // Stop at the terrain depth: everything past it is occluded, and the
        // already-drawn framebuffer pixel becomes the background.
        let depth = t_curr * fwd_dot;
        if depth >= max_t {
            break;
        }
        // Exit `t` of the current cell — the next boundary crossing. Its span
        // from `t_curr` is the ray's path through this cell (Volumetric).
        let exit_axis = min_axis(t_max);
        let t_exit = t_max[exit_axis];
        let idx = dense.idx_of(cell);
        let solid_here = dense.occ[idx];
        if solid_here && depth >= NEAR_Z {
            let mat_id = if dense.mat.is_empty() {
                shade_ctx.material
            } else {
                dense.mat[idx]
            };
            let m = shade_ctx.materials.get(mat_id);
            if m.is_opaque() {
                acc.opaque = Some((shade_layer(idx, cell, normal), t_curr));
                touched = true;
                break;
            }
            let a = f32::from(m.alpha) / 255.0 * (f32::from(shade_ctx.alpha_mul) / 255.0);
            if m.mode == BlendMode::Volumetric {
                // Per-cell Beer–Lambert: opacity weighted by traversed length
                // (in voxel units), so a thin sliver contributes ≈0 and a
                // filled volume thickens smoothly with depth. Always occludes.
                let seg_len = (t_exit - t_curr).max(0.0) * dir_len;
                let eff_a = 1.0 - (1.0 - a).powf(seg_len);
                let lit = rgb_to_f32(shade_layer(idx, cell, normal));
                acc.rgb[0] += acc.trans * eff_a * lit[0];
                acc.rgb[1] += acc.trans * eff_a * lit[1];
                acc.rgb[2] += acc.trans * eff_a * lit[2];
                acc.trans *= 1.0 - eff_a;
                touched = true;
                prev_mat = mat_id;
                if acc.trans < 1.0 / 256.0 {
                    break;
                }
            } else if !prev_solid || mat_id != prev_mat {
                // AlphaBlend / Additive: one alpha layer per solid-run entry or
                // material change (thickness-independent — shells, glass).
                let lit = rgb_to_f32(shade_layer(idx, cell, normal));
                acc.rgb[0] += acc.trans * a * lit[0];
                acc.rgb[1] += acc.trans * a * lit[1];
                acc.rgb[2] += acc.trans * a * lit[2];
                if m.mode == BlendMode::AlphaBlend {
                    acc.trans *= 1.0 - a; // Additive glow does not occlude.
                }
                touched = true;
                prev_mat = mat_id;
                if acc.trans < 1.0 / 256.0 {
                    break;
                }
            }
        }
        prev_solid = solid_here;
        t_curr = t_exit;
        cell[exit_axis] += step[exit_axis];
        t_max[exit_axis] += t_delta[exit_axis];
        normal = [0.0; 3];
        normal[exit_axis] = -(step[exit_axis] as f32);
    }

    touched.then_some(acc)
}

/// Draw one KV6 [`Sprite`] into `(fb, zb)` by per-pixel ray casting,
/// depth-compositing against whatever the terrain pass already wrote.
/// Returns the number of pixels written.
///
/// `cam` / `settings` are the **same** per-frame projection the DDA
/// terrain pass used (build via [`crate::camera_math::derive`]), so
/// sprite and terrain share one pinhole and z convention. `pitch_pixels`
/// is the framebuffer row stride. Honours `SPRITE_FLAG_INVISIBLE`
/// (skip) and `SPRITE_FLAG_NO_Z` (write without the depth test).
#[allow(
    clippy::too_many_arguments,
    clippy::cast_possible_truncation,
    clippy::cast_sign_loss
)]
#[must_use]
pub fn draw_sprite_dda(
    fb: &mut [u32],
    zb: &mut [f32],
    pitch_pixels: usize,
    width: u32,
    height: u32,
    cam: &CameraState,
    settings: &OpticastSettings,
    sprite: &Sprite,
) -> u32 {
    if sprite.flags & SPRITE_FLAG_INVISIBLE != 0 {
        return 0;
    }
    draw_sprite_dda_shaded(
        fb,
        zb,
        pitch_pixels,
        width,
        height,
        cam,
        settings,
        sprite,
        None,
    )
}

/// Draw one KV6 [`Sprite`], optionally with a translucent material (TV
/// stage) — the [`draw_sprite_dense_shaded`] counterpart of
/// [`draw_sprite_dda`]. `shade_ctx == None` (or an opaque effective
/// material) renders the sprite opaque, byte-for-byte unchanged.
#[allow(clippy::too_many_arguments)]
#[must_use]
pub fn draw_sprite_dda_shaded(
    fb: &mut [u32],
    zb: &mut [f32],
    pitch_pixels: usize,
    width: u32,
    height: u32,
    cam: &CameraState,
    settings: &OpticastSettings,
    sprite: &Sprite,
    shade_ctx: Option<SpriteShade>,
) -> u32 {
    if sprite.flags & SPRITE_FLAG_INVISIBLE != 0 {
        return 0;
    }
    // Decodes the KV6 to a dense grid each call (the per-frame cost an
    // animated clip avoids via [`ClipFlipbook`]'s cached grids). A non-empty
    // `material_map` classifies voxels into per-voxel materials (TV.3 mixed
    // models); an empty one is the plain uniform-material decode.
    let dense = if sprite.material_map.is_empty() {
        SpriteDense::from_kv6(&sprite.kv6)
    } else {
        SpriteDense::from_kv6_with_materials(&sprite.kv6, &sprite.material_map)
    };
    draw_sprite_dense_shaded(
        fb,
        zb,
        pitch_pixels,
        width,
        height,
        cam,
        settings,
        &dense,
        sprite.p,
        sprite.s,
        sprite.h,
        sprite.f,
        sprite.flags,
        shade_ctx,
    )
}

/// Draw a pre-decoded [`SpriteDense`] at a world pose — the generalised
/// core of [`draw_sprite_dda`], shared by the KV6 path and animated
/// [`ClipFlipbook`] frames. `pos` is the world pivot; `s`/`h`/`f` are the
/// model→world basis columns (local +x/+y/+z); `flags` honours
/// [`SPRITE_FLAG_INVISIBLE`] / [`SPRITE_FLAG_NO_Z`]. Returns pixels written.
///
/// Fully opaque (the existing first-hit path). For translucent sprites use
/// [`draw_sprite_dense_shaded`].
#[allow(clippy::too_many_arguments)]
#[must_use]
pub fn draw_sprite_dense(
    fb: &mut [u32],
    zb: &mut [f32],
    pitch_pixels: usize,
    width: u32,
    height: u32,
    cam: &CameraState,
    settings: &OpticastSettings,
    dense: &SpriteDense,
    pos: [f32; 3],
    s: [f32; 3],
    h: [f32; 3],
    f: [f32; 3],
    flags: u32,
) -> u32 {
    draw_sprite_dense_shaded(
        fb,
        zb,
        pitch_pixels,
        width,
        height,
        cam,
        settings,
        dense,
        pos,
        s,
        h,
        f,
        flags,
        None,
    )
}

/// Draw a pre-decoded [`SpriteDense`] at a world pose, optionally with a
/// translucent material (TV stage). `shade_ctx`:
/// - `None`, or a `Some` whose effective material is opaque ⇒ the existing
///   first-hit, depth-tested opaque path, **byte-for-byte unchanged**.
/// - `Some` with a translucent uniform material (or a non-empty per-voxel
///   `mat` array) ⇒ front-to-back accumulate-and-continue: each ray marches
///   through the sprite compositing `AlphaBlend`/`Additive` layers over what
///   lies behind (terrain/sky already in the framebuffer, or an opaque voxel
///   of the model). Opaque voxels write the model surface depth; purely
///   translucent pixels composite over the framebuffer without touching the
///   z-buffer (they do not occlude). See `PORTING-TRANSPARENCY.md`.
#[allow(
    clippy::too_many_arguments,
    clippy::cast_possible_truncation,
    clippy::cast_sign_loss
)]
#[must_use]
pub fn draw_sprite_dense_shaded(
    fb: &mut [u32],
    zb: &mut [f32],
    pitch_pixels: usize,
    width: u32,
    height: u32,
    cam: &CameraState,
    settings: &OpticastSettings,
    dense: &SpriteDense,
    pos: [f32; 3],
    s: [f32; 3],
    h: [f32; 3],
    f: [f32; 3],
    flags: u32,
    shade_ctx: Option<SpriteShade>,
) -> u32 {
    if flags & SPRITE_FLAG_INVISIBLE != 0 || dense.occ.is_empty() {
        return 0;
    }
    // Clip render scale: `basis · ((v − pivot) · vws)` ≡
    // `(basis · vws) · (v − pivot)`, so scaling the basis once here
    // reproduces the GPU shader's per-voxel `* voxel_world_size`
    // without touching any transform below. Exact no-op at 1.0.
    let sc = dense.voxel_world_size;
    let (s, h, f) = (bb_scale3(s, sc), bb_scale3(h, sc), bb_scale3(f, sc));
    let Some(minv) = invert_basis(s, h, f) else {
        return 0;
    };
    let pivot = dense.pivot;
    let no_z = flags & SPRITE_FLAG_NO_Z != 0;
    // BB.2b — per-instance billboard lighting mode (flags bits 6/7).
    let light_mode = SpriteLightMode::from_flags(flags);

    // Screen bounding box from the 8 corners of the local voxel box.
    let Some(rect) = project_screen_rect(dense, pos, s, h, f, cam, settings, width, height) else {
        return 0;
    };

    // Per-sprite gate: a sprite whose effective material is opaque (the
    // common case, and every sprite while no translucent material is
    // defined) takes the original loop unchanged — so the opaque world stays
    // bit-identical. Only a genuinely translucent sprite runs the accumulate
    // loop.
    let layers =
        shade_ctx.filter(|s| !dense.mat.is_empty() || !s.materials.get(s.material).is_opaque());

    debug_assert_eq!(fb.len(), zb.len());
    let target = RasterTarget::new(fb, zb);
    // PF.8 — one row of the sprite's screen rect; rows are disjoint, every
    // pixel reads/writes only its own index, so rows parallelise safely
    // (the terrain DDA's `RasterTarget` band contract).
    let draw_row = |py: u32| -> u32 {
        let mut written = 0u32;
        let row = py as usize * pitch_pixels;
        for px in rect.0..rect.2 {
            let (origin, dir) = pixel_ray(cam, settings, px, py);
            // World ray → sprite-local voxel space.
            let rel = [origin[0] - pos[0], origin[1] - pos[1], origin[2] - pos[2]];
            let ol = mat_apply(&minv, rel);
            let origin_local = [ol[0] + pivot[0], ol[1] + pivot[1], ol[2] + pivot[2]];
            let dir_local = mat_apply(&minv, dir);
            let fwd_dot =
                dir[0] * cam.forward[0] + dir[1] * cam.forward[1] + dir[2] * cam.forward[2];
            let idx = row + px as usize;

            if let Some(shade_ctx) = layers {
                // ---- translucent: accumulate front-to-back ----
                if fwd_dot <= 1e-6 {
                    continue;
                }
                // Terrain/opaque depth cutoff (perpendicular distance, the
                // same units `cast_local_layers` compares `t_curr·fwd_dot`
                // against — NOT a ray parameter, since the CPU `dir` is
                // unnormalised). SAFETY: idx in rect ⊂ (width,height).
                let max_t = if no_z {
                    f32::INFINITY
                } else {
                    unsafe { target.read_depth(idx) }
                };
                let Some(acc) = cast_local_layers(
                    dense,
                    origin_local,
                    dir_local,
                    fwd_dot,
                    max_t,
                    shade_ctx,
                    s,
                    h,
                    f,
                    pos,
                    light_mode,
                ) else {
                    continue;
                };
                // SAFETY: idx in bounds; single-threaded writer.
                let wrote = unsafe {
                    match acc.opaque {
                        Some((bg_color, t)) => {
                            // Opaque model surface behind the translucent
                            // layers: composite over it, write surface depth.
                            let bg = rgb_to_f32(bg_color);
                            let out = f32_to_rgb([
                                acc.rgb[0] + acc.trans * bg[0],
                                acc.rgb[1] + acc.trans * bg[1],
                                acc.rgb[2] + acc.trans * bg[2],
                            ]);
                            let depth = t * fwd_dot;
                            if no_z {
                                target.write_color(idx, out);
                                target.write_depth(idx, depth);
                                true
                            } else {
                                target.z_test_write(idx, out, depth)
                            }
                        }
                        None => {
                            // Ray exited (or fully attenuated) with no opaque
                            // model voxel: composite over the framebuffer
                            // (terrain/sky). Translucent layers do not occlude,
                            // so the z-buffer is left untouched.
                            let bg = rgb_to_f32(target.read_color(idx));
                            let out = f32_to_rgb([
                                acc.rgb[0] + acc.trans * bg[0],
                                acc.rgb[1] + acc.trans * bg[1],
                                acc.rgb[2] + acc.trans * bg[2],
                            ]);
                            target.write_color(idx, out);
                            true
                        }
                    }
                };
                written += u32::from(wrote);
            } else {
                // ---- opaque: first-hit path ----
                let Some((color, t, n_local, cell)) = cast_local(dense, origin_local, dir_local)
                else {
                    continue;
                };
                let depth = t * fwd_dot;
                if depth < NEAR_Z {
                    continue;
                }
                // DL.7 — dynamic lighting when a rig is active (sun + point
                // lights + cel + ramp, flat per voxel); else the baked `shade`
                // (byte-identical). The model-local face normal + voxel centre
                // are rotated into world space via the instance basis (s,h,f).
                let dl = shade_ctx.map_or(CpuLights::default(), |s| s.lights);
                let lit = if dl.enabled {
                    let to_world = |v: [f32; 3]| {
                        [
                            v[0] * s[0] + v[1] * h[0] + v[2] * f[0],
                            v[0] * s[1] + v[1] * h[1] + v[2] * f[1],
                            v[0] * s[2] + v[1] * h[2] + v[2] * f[2],
                        ]
                    };
                    let n_world = to_world(n_local);
                    let rel = [
                        cell[0] as f32 + 0.5 - pivot[0],
                        cell[1] as f32 + 0.5 - pivot[1],
                        cell[2] as f32 + 0.5 - pivot[2],
                    ];
                    let wc = to_world(rel);
                    let center = [pos[0] + wc[0], pos[1] + wc[1], pos[2] + wc[2]];
                    let albedo = [
                        ((color >> 16) & 0xff) as f32 / 255.0,
                        ((color >> 8) & 0xff) as f32 / 255.0,
                        (color & 0xff) as f32 / 255.0,
                    ];
                    // XS.2 — sprite receives shadows: a world-space query
                    // against the scene occluder (sprite shading is already in
                    // world space, so an identity transform). `None` ⇒ unshadowed.
                    let mut ws = shade_ctx.and_then(|s| s.shadow).map(|occ| WorldShadow {
                        ctx: WorldShadowCtx::identity(occ),
                    });
                    let tester = ws.as_mut().map(|t| t as &mut dyn ShadowTester);
                    shade_dynamic_mode(light_mode, albedo, n_world, center, &dl, tester)
                } else {
                    shade(color, 0)
                };
                // Per-instance RGB tint (white ⇒ no-op).
                let lit = tint_packed(lit, shade_ctx.map_or(0x00FF_FFFF, |s| s.tint));
                // SAFETY: idx in-bounds for the rect within (width, height);
                // single-threaded writer.
                let wrote = unsafe {
                    if no_z {
                        target.write_color(idx, lit);
                        target.write_depth(idx, depth);
                        true
                    } else {
                        target.z_test_write(idx, lit, depth)
                    }
                };
                written += u32::from(wrote);
            }
        }
        written
    };
    // PF.8 — the sprite pass was entirely single-threaded after the
    // rayon-parallel terrain; large footprints (a close-up character,
    // a big translucent volume) now use the same worker pool. Small
    // rects stay serial — per-sprite rayon overhead would dominate.
    let rows = rect.3.saturating_sub(rect.1) as usize;
    let cols = rect.2.saturating_sub(rect.0) as usize;
    const SPRITE_PAR_MIN_PIXELS: usize = 64 * 64;
    if rows >= 2 && rows * cols >= SPRITE_PAR_MIN_PIXELS {
        use rayon::prelude::*;
        (rect.1..rect.3).into_par_iter().map(draw_row).sum()
    } else {
        (rect.1..rect.3).map(draw_row).sum()
    }
}

/// Project the sprite's local voxel AABB to a clamped screen rectangle
/// `(x0, y0, x1, y1)` (half-open). `None` if it can't appear; falls back
/// to the full viewport when the box straddles the near plane (rare).
#[allow(
    clippy::cast_possible_truncation,
    clippy::cast_sign_loss,
    clippy::cast_precision_loss
)]
fn project_screen_rect(
    dense: &SpriteDense,
    pos: [f32; 3],
    s: [f32; 3],
    h: [f32; 3],
    f: [f32; 3],
    cam: &CameraState,
    settings: &OpticastSettings,
    width: u32,
    height: u32,
) -> Option<(u32, u32, u32, u32)> {
    let (xs, ys, zs) = (
        dense.dims[0] as f32,
        dense.dims[1] as f32,
        dense.dims[2] as f32,
    );
    let (xp, yp, zp) = (dense.pivot[0], dense.pivot[1], dense.pivot[2]);
    let (mut x0, mut y0, mut x1, mut y1) = (f32::MAX, f32::MAX, f32::MIN, f32::MIN);
    let mut all_front = true;
    for &cx in &[0.0, xs] {
        for &cy in &[0.0, ys] {
            for &cz in &[0.0, zs] {
                // Local → world via the sprite basis about the pivot.
                let lx = cx - xp;
                let ly = cy - yp;
                let lz = cz - zp;
                let world = [
                    pos[0] + lx * s[0] + ly * h[0] + lz * f[0],
                    pos[1] + lx * s[1] + ly * h[1] + lz * f[1],
                    pos[2] + lx * s[2] + ly * h[2] + lz * f[2],
                ];
                let rel = [
                    world[0] - cam.pos[0],
                    world[1] - cam.pos[1],
                    world[2] - cam.pos[2],
                ];
                let cz_cam =
                    rel[0] * cam.forward[0] + rel[1] * cam.forward[1] + rel[2] * cam.forward[2];
                if cz_cam < NEAR_Z {
                    all_front = false;
                    continue;
                }
                let cx_cam = rel[0] * cam.right[0] + rel[1] * cam.right[1] + rel[2] * cam.right[2];
                let cy_cam = rel[0] * cam.down[0] + rel[1] * cam.down[1] + rel[2] * cam.down[2];
                let sx = settings.hx + cx_cam / cz_cam * settings.hz;
                let sy = settings.hy + cy_cam / cz_cam * settings.hz;
                x0 = x0.min(sx);
                y0 = y0.min(sy);
                x1 = x1.max(sx);
                y1 = y1.max(sy);
            }
        }
    }
    let (w, h) = (width as f32, height as f32);
    let (rx0, ry0, rx1, ry1) = if all_front {
        (
            (x0 - 1.0).max(0.0),
            (y0 - 1.0).max(0.0),
            (x1 + 1.0).min(w),
            (y1 + 1.0).min(h),
        )
    } else {
        // Straddles the near plane → scan the whole viewport.
        (0.0, 0.0, w, h)
    };
    if rx0 >= rx1 || ry0 >= ry1 {
        return None;
    }
    Some((rx0 as u32, ry0 as u32, rx1.ceil() as u32, ry1.ceil() as u32))
}

/// CPU-side decoded animated voxel clip: every frame's [`SpriteDense`]
/// is cached at construction, so per-frame playback is a grid **select**
/// — not the per-frame voxel-volume decode [`draw_sprite_dda`] pays each
/// call. The CPU counterpart to the GPU flipbook (VCL.2). Build once from
/// a [`DecodedClip`], then [`draw_frame`](ClipFlipbook::draw_frame) the
/// active frame each render.
pub struct ClipFlipbook {
    /// PF.8 — `Arc` per frame so the shadow occluder shares the cached
    /// decode instead of deep-cloning the current frame per rebuild.
    frames: Vec<Arc<SpriteDense>>,
}

impl ClipFlipbook {
    /// An empty flipbook (no frames) — a tombstone for a removed clip;
    /// [`draw_frame`](Self::draw_frame) always draws nothing.
    #[must_use]
    pub fn empty() -> Self {
        Self { frames: Vec::new() }
    }

    /// Decode + cache every frame of `clip` (one [`SpriteDense`] each).
    #[must_use]
    pub fn from_decoded(clip: &DecodedClip) -> Self {
        Self::from_decoded_with_materials(clip, &[])
    }

    /// Like [`from_decoded`](Self::from_decoded) but classifies every frame's
    /// voxels into per-voxel material ids by colour (TV.3 mixed models) via
    /// `material_map` — the clip analogue of
    /// [`SpriteDense::from_kv6_with_materials`]. An empty map yields the same
    /// all-opaque result as `from_decoded`.
    #[must_use]
    pub fn from_decoded_with_materials(clip: &DecodedClip, material_map: &[(Rgb, u8)]) -> Self {
        let frames = clip
            .frames
            .iter()
            .map(|frame| {
                Arc::new(
                    SpriteDense::from_voxel_frame_with_materials(
                        frame,
                        clip.dims,
                        clip.pivot,
                        material_map,
                    )
                    .with_voxel_world_size(clip.voxel_world_size),
                )
            })
            .collect();
        Self { frames }
    }

    /// Number of cached frames. `0` for a tombstone from
    /// [`empty`](Self::empty).
    #[must_use]
    pub fn frame_count(&self) -> usize {
        self.frames.len()
    }

    /// Borrow frame `frame`'s cached dense grid, if in range.
    #[must_use]
    pub fn frame(&self, frame: usize) -> Option<&SpriteDense> {
        self.frames.get(frame).map(Arc::as_ref)
    }

    /// Share frame `frame`'s cached dense grid (PF.8) — a cheap refcount
    /// clone for the shadow occluder (was a deep clone per rebuild).
    #[must_use]
    pub fn frame_arc(&self, frame: usize) -> Option<Arc<SpriteDense>> {
        self.frames.get(frame).cloned()
    }

    /// Replace one frame's cached dense grid in place — the CPU side of an
    /// editor's single-frame edit (no re-decode of the other frames).
    /// Returns `false` if `frame` is out of range.
    pub fn set_frame(&mut self, frame: usize, dense: SpriteDense) -> bool {
        match self.frames.get_mut(frame) {
            Some(slot) => {
                *slot = Arc::new(dense);
                true
            }
            None => false,
        }
    }

    /// Draw frame `frame` at a world pose via [`draw_sprite_dense`] —
    /// `pos` is the world pivot, `s`/`h`/`f` the model→world basis columns.
    /// Returns pixels written (0 if `frame` is out of range).
    #[allow(clippy::too_many_arguments)]
    #[must_use]
    pub fn draw_frame(
        &self,
        fb: &mut [u32],
        zb: &mut [f32],
        pitch_pixels: usize,
        width: u32,
        height: u32,
        cam: &CameraState,
        settings: &OpticastSettings,
        frame: usize,
        pos: [f32; 3],
        s: [f32; 3],
        h: [f32; 3],
        f: [f32; 3],
        flags: u32,
    ) -> u32 {
        self.draw_frame_shaded(
            fb,
            zb,
            pitch_pixels,
            width,
            height,
            cam,
            settings,
            frame,
            pos,
            s,
            h,
            f,
            flags,
            None,
        )
    }

    /// Draw frame `frame`, optionally with a translucent material (TV stage)
    /// — the [`draw_sprite_dense_shaded`] counterpart of
    /// [`draw_frame`](Self::draw_frame). `shade_ctx == None` (or an opaque
    /// effective material) renders the frame opaque, unchanged.
    #[allow(clippy::too_many_arguments)]
    #[must_use]
    pub fn draw_frame_shaded(
        &self,
        fb: &mut [u32],
        zb: &mut [f32],
        pitch_pixels: usize,
        width: u32,
        height: u32,
        cam: &CameraState,
        settings: &OpticastSettings,
        frame: usize,
        pos: [f32; 3],
        s: [f32; 3],
        h: [f32; 3],
        f: [f32; 3],
        flags: u32,
        shade_ctx: Option<SpriteShade>,
    ) -> u32 {
        let Some(dense) = self.frames.get(frame) else {
            return 0;
        };
        draw_sprite_dense_shaded(
            fb,
            zb,
            pitch_pixels,
            width,
            height,
            cam,
            settings,
            dense,
            pos,
            s,
            h,
            f,
            flags,
            shade_ctx,
        )
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::camera_math;
    use crate::Camera;
    use roxlap_formats::kv6::Kv6;
    use roxlap_formats::material::{Material, MaterialTable};
    use roxlap_formats::VoxColor;

    /// BB.2b — `WorldUp` lights a side-facing billboard as if it faced world
    /// up (the sun directly overhead); `AmbientOnly` drops the sun term.
    #[test]
    fn sprite_light_mode_world_up_and_ambient_only() {
        let lights = CpuLights {
            enabled: true,
            sun: true,
            sun_dir: [0.0, 0.0, -1.0], // toward the sun (world up, z-down)
            sun_color: [1.0, 1.0, 1.0],
            sun_intensity: 1.0,
            sun_casts_shadow: false,
            points: &[],
            ambient: [0.2, 0.2, 0.2],
            bands: 0,
            shadow_tint: [0.0; 3],
            shadow_strength: 0.0,
            shadow_bias: 0.0,
            shadow_max_dist: 0.0,
        };
        let a = [1.0, 1.0, 1.0];
        let c = [0.0, 0.0, 0.0];
        let g = |packed: u32| (packed >> 8) & 0xff; // green channel
        let up_n = [0.0, 0.0, -1.0];
        let side_n = [1.0, 0.0, 0.0];
        let face_up = g(shade_dynamic_mode(
            SpriteLightMode::FaceNormal,
            a,
            up_n,
            c,
            &lights,
            None,
        ));
        let face_side = g(shade_dynamic_mode(
            SpriteLightMode::FaceNormal,
            a,
            side_n,
            c,
            &lights,
            None,
        ));
        let amb = g(shade_dynamic_mode(
            SpriteLightMode::AmbientOnly,
            a,
            up_n,
            c,
            &lights,
            None,
        ));
        let world_up = g(shade_dynamic_mode(
            SpriteLightMode::WorldUp,
            a,
            side_n,
            c,
            &lights,
            None,
        ));
        assert!(
            face_up > face_side,
            "a sun-facing face is brighter than a side face"
        );
        assert!(amb < face_up, "ambient-only drops the sun term");
        assert_eq!(
            world_up, face_up,
            "world-up shades a side-facing billboard as if it faced up"
        );
        let full = g(shade_dynamic_mode(
            SpriteLightMode::FullBright,
            a,
            side_n,
            c,
            &lights,
            None,
        ));
        // Full-bright = the albedo at full intensity, ignoring the rig (so a
        // glow isn't dimmed by ambient like AmbientOnly is).
        assert_eq!(full, 255, "full-bright emits the colour at full intensity");
        assert!(full > amb, "full-bright glow is brighter than ambient-only");
    }

    /// DL.7 — `cast_local` reports the hit's model-local face normal (used to
    /// light sprites/clips). A ray crossing air then a solid block via the
    /// z face gets a back-facing (-z) normal; the entry voxel (immediately
    /// solid) gets a zero normal.
    #[test]
    fn cast_local_reports_face_normal() {
        // Solid only at z >= 4 (air below), full in x/y.
        let kv6 = Kv6::from_fn(8, 8, 8, |_, _, z| {
            (z >= 4).then_some(VoxColor(0x80_C0_40_20))
        });
        let dense = SpriteDense::from_kv6(&kv6);
        // Ray from below, travelling +z: air (z<4) then the block's top face.
        let (_c, _t, n, cell) =
            cast_local(&dense, [4.0, 4.0, -5.0], [0.0, 0.0, 1.0]).expect("ray hits the block");
        assert_eq!(cell[2], 4, "first solid voxel is the z=4 surface");
        assert!(
            n[2] < -0.5 && n[0].abs() < 1e-6 && n[1].abs() < 1e-6,
            "z-crossing face normal points back toward the ray (-z): {n:?}",
        );
    }
    use roxlap_formats::sprite::Sprite;
    use roxlap_formats::voxel_clip::{LoopMode, VoxelClip, VoxelFrame};

    fn settings(w: u32, h: u32) -> OpticastSettings {
        OpticastSettings::for_oracle_framebuffer(w, h)
    }

    /// Camera at the origin looking down +y at a sprite ahead.
    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],
        }
    }

    /// PS.1 — the CPU sprite DDA honours a **scaled** basis (voxlap
    /// heritage: `s/h/f` magnitude = scale). The same cube drawn with
    /// 2× / 0.5× columns covers roughly 4× / 0.25× the pixels of the
    /// unit pose — the parity the particle system's scale-over-life
    /// relies on. Loose bounds: perspective (the scaled cube's front
    /// face sits nearer/farther) skews the exact ratio.
    #[test]
    fn scaled_basis_scales_drawn_extent() {
        let kv6 = Kv6::from_fn(8, 8, 8, |_, _, _| Some(VoxColor(0x80_C0_40_20)));
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 32.0, 32.0, 32.0);
        let cfg = settings(w, h);

        let px_at = |k: f32| -> u32 {
            let mut sprite = Sprite::axis_aligned(kv6.clone(), [0.0, 40.0, 0.0]);
            for a in 0..3 {
                sprite.s[a] *= k;
                sprite.h[a] *= k;
                sprite.f[a] *= k;
            }
            let mut fb = vec![0u32; n];
            let mut zb = vec![f32::INFINITY; n];
            draw_sprite_dda(&mut fb, &mut zb, w as usize, w, h, &cs, &cfg, &sprite)
        };

        let (unit, double, half) = (px_at(1.0), px_at(2.0), px_at(0.5));
        assert!(unit > 0, "unit-scale cube must draw ({unit} px)");
        let r2 = f64::from(double) / f64::from(unit);
        let rh = f64::from(half) / f64::from(unit);
        assert!(
            (3.0..8.0).contains(&r2),
            "2× scale should roughly quadruple coverage: {unit} → {double} px (×{r2:.2})"
        );
        assert!(
            (0.08..0.5).contains(&rh),
            "0.5× scale should roughly quarter coverage: {unit} → {half} px (×{rh:.2})"
        );
    }

    /// Build a [`VoxelFrame`] from a dense `fill(x,y,z) -> Option<color>`.
    fn clip_frame(dims: [u32; 3], fill: impl Fn(u32, u32, u32) -> Option<u32>) -> VoxelFrame {
        let owpc = dims[2].div_ceil(32).max(1) as usize;
        let cols = (dims[0] * dims[1]) as usize;
        let mut occupancy = vec![0u32; cols * owpc];
        let mut color_offsets = vec![0u32; cols + 1];
        let mut colors = Vec::new();
        for y in 0..dims[1] {
            for x in 0..dims[0] {
                let col = (x + y * dims[0]) as usize;
                color_offsets[col] = colors.len() as u32;
                for z in 0..dims[2] {
                    if let Some(c) = fill(x, y, z) {
                        occupancy[col * owpc + (z >> 5) as usize] |= 1u32 << (z & 31);
                        colors.push(c);
                    }
                }
            }
        }
        color_offsets[cols] = colors.len() as u32;
        VoxelFrame {
            occupancy,
            colors,
            color_offsets,
        }
    }

    /// A cached [`ClipFlipbook`] draws distinct frames distinctly — the
    /// CPU flipbook select. Frame 0 fills the bottom half (red), frame 1
    /// the top half (green); rendered at the same pose they cover
    /// different screen pixels in different colours.
    #[test]
    fn clip_flipbook_frames_render_differently() {
        let dims = [8u32, 8, 8];
        let f0 = clip_frame(dims, |_x, _y, z| (z < 4).then_some(0x00FF_0000)); // red, low z
        let f1 = clip_frame(dims, |_x, _y, z| (z >= 4).then_some(0x0000_FF00)); // green, high z
        let clip = VoxelClip::from_frames(
            dims,
            [4.0, 4.0, 4.0],
            1.0,
            LoopMode::Loop,
            &[f0, f1],
            &[],
            33,
            0,
        );
        let decoded = clip.decode().expect("decode");
        let book = ClipFlipbook::from_decoded(&decoded);
        assert_eq!(book.frame_count(), 2);
        assert!(book.frame(0).is_some() && book.frame(2).is_none());

        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 32.0, 32.0, 32.0);
        let cfg = settings(w, h);
        let pose = [0.0, 40.0, 0.0];
        let (s, hh, f) = ([1.0, 0.0, 0.0], [0.0, 1.0, 0.0], [0.0, 0.0, 1.0]);

        let render = |frame: usize| -> Vec<u32> {
            let mut fb = vec![0u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let wrote = book.draw_frame(
                &mut fb, &mut zb, w as usize, w, h, &cs, &cfg, frame, pose, s, hh, f, 0,
            );
            assert!(wrote > 0, "frame {frame} should draw some pixels");
            fb
        };
        let fb0 = render(0);
        let fb1 = render(1);
        assert_ne!(fb0, fb1, "distinct frames must render distinct pixels");
        // Each frame shows its own channel: red present in frame 0, green
        // present in frame 1.
        assert!(fb0.iter().any(|&p| (p & 0x00FF_0000) != 0));
        assert!(fb1.iter().any(|&p| (p & 0x0000_FF00) != 0));
        // Out-of-range frame draws nothing.
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        assert_eq!(
            book.draw_frame(&mut fb, &mut zb, w as usize, w, h, &cs, &cfg, 9, pose, s, hh, f, 0),
            0
        );
    }

    #[test]
    fn clip_flipbook_set_frame_replaces_one_frame() {
        // The single-frame edit primitive: replace frame 0's dense with
        // frame 1's content, in place. Out-of-range → false.
        let dims = [8u32, 8, 8];
        let f0 = clip_frame(dims, |_, _, z| (z < 4).then_some(0x00FF_0000)); // red
        let f1 = clip_frame(dims, |_, _, z| (z >= 4).then_some(0x0000_FF00)); // green
        let clip =
            VoxelClip::from_frames(dims, [4.0; 3], 1.0, LoopMode::Loop, &[f0, f1], &[], 33, 0);
        let decoded = clip.decode().unwrap();
        let mut book = ClipFlipbook::from_decoded(&decoded);

        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let cfg = settings(w, h);
        let render0 = |b: &ClipFlipbook| -> Vec<u32> {
            let mut fb = vec![0u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let _ = b.draw_frame(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &cfg,
                0,
                [0.0, 40.0, 0.0],
                [1.0, 0.0, 0.0],
                [0.0, 1.0, 0.0],
                [0.0, 0.0, 1.0],
                0,
            );
            fb
        };

        let before = render0(&book);
        assert!(
            before.iter().any(|&p| (p & 0x00FF_0000) != 0),
            "frame 0 is red"
        );

        // Replace frame 0 with frame 1's dense.
        let replacement = SpriteDense::from_voxel_frame(&decoded.frames[1], dims, decoded.pivot);
        assert!(book.set_frame(0, replacement));
        let extra = SpriteDense::from_voxel_frame(&decoded.frames[1], dims, decoded.pivot);
        assert!(!book.set_frame(9, extra), "out-of-range set_frame is false");

        let after = render0(&book);
        assert!(
            after.iter().any(|&p| (p & 0x0000_FF00) != 0),
            "frame 0 now green"
        );
        assert_ne!(before, after);
    }

    /// A solid cube sprite in front of the camera is drawn, with the
    /// cube colour (shaded) and a sensible centre depth.
    #[test]
    fn cube_sprite_renders() {
        let kv6 = Kv6::solid_cube(8, VoxColor(0x80_C0_40_20));
        let sprite = Sprite::axis_aligned(kv6, [0.0, 40.0, 0.0]);
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 32.0, 32.0, 32.0);
        let wrote = draw_sprite_dda(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &sprite,
        );

        assert!(wrote > 20, "cube should cover many pixels (got {wrote})");
        let centre = (h / 2 * w + w / 2) as usize;
        assert_eq!(
            fb[centre] & 0x00ff_ffff,
            0x00_C0_40_20,
            "got {:08x}",
            fb[centre]
        );
        // Pivot at world y=40, cube spans y in [36,44] → near face ~36.
        assert!(
            (zb[centre] - 36.0).abs() < 3.0,
            "centre depth {} not ≈ 36",
            zb[centre]
        );
    }

    /// A KV6 whose voxel colours store a `0x00` high byte (voxlap's
    /// unused `dir` slot, e.g. `sprite_meltsphere.kv6`) must still
    /// render its authored RGB, not black — the brightness byte is
    /// normalised to full on decode.
    #[test]
    fn zero_high_byte_sprite_not_black() {
        let kv6 = Kv6::solid_cube(8, VoxColor(0x00_C0_40_20));
        let sprite = Sprite::axis_aligned(kv6, [0.0, 40.0, 0.0]);
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 32.0, 32.0, 32.0);
        let wrote = draw_sprite_dda(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &sprite,
        );
        assert!(wrote > 20, "cube should cover many pixels (got {wrote})");
        let centre = (h / 2 * w + w / 2) as usize;
        assert_eq!(
            fb[centre] & 0x00ff_ffff,
            0x00_C0_40_20,
            "zero-high-byte sprite rendered as {:08x} (black bug)",
            fb[centre]
        );
    }

    /// A sprite occludes / is occluded by the z-buffer: a nearer
    /// pre-filled depth blocks the sprite; a farther one lets it win.
    #[test]
    fn sprite_respects_zbuffer() {
        let kv6 = Kv6::solid_cube(8, VoxColor(0x80_FF_FF_FF));
        let sprite = Sprite::axis_aligned(kv6, [0.0, 40.0, 0.0]);
        let (w, h) = (32u32, 32u32);
        let n = (w * h) as usize;
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 16.0, 16.0, 16.0);
        let centre = (h / 2 * w + w / 2) as usize;

        // Terrain in front (depth 10 < ~36) → sprite blocked at centre.
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        fb[centre] = 0x80_11_22_33;
        zb[centre] = 10.0;
        let _ = draw_sprite_dda(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &sprite,
        );
        assert_eq!(
            fb[centre], 0x80_11_22_33,
            "near terrain must occlude sprite"
        );

        // Terrain behind (depth 100) → sprite wins.
        let mut fb2 = vec![0u32; n];
        let mut zb2 = vec![f32::INFINITY; n];
        fb2[centre] = 0x80_11_22_33;
        zb2[centre] = 100.0;
        let _ = draw_sprite_dda(
            &mut fb2,
            &mut zb2,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &sprite,
        );
        assert_ne!(fb2[centre], 0x80_11_22_33, "sprite must beat far terrain");
        assert!(zb2[centre] < 100.0, "sprite depth must replace terrain's");
    }

    /// The covered screen rect (min/max px,py) of whatever the sprite
    /// painted — used to compare an axis-aligned vs a rotated pose.
    fn covered_rect(fb: &[u32], w: u32, h: u32) -> (u32, u32, u32, u32) {
        let (mut x0, mut y0, mut x1, mut y1) = (w, h, 0u32, 0u32);
        for py in 0..h {
            for px in 0..w {
                if fb[(py * w + px) as usize] & 0x00ff_ffff != 0 {
                    x0 = x0.min(px);
                    y0 = y0.min(py);
                    x1 = x1.max(px);
                    y1 = y1.max(py);
                }
            }
        }
        (x0, y0, x1, y1)
    }

    /// A non-cube box drawn axis-aligned vs. drawn with a per-instance
    /// transform that swaps its long axis onto the screen's other axis
    /// flips the silhouette's aspect ratio. Pins that the `s/h/f` basis
    /// (the path `DynSpriteTransform` feeds) actually reorients the model.
    #[test]
    fn posed_basis_reorients_silhouette() {
        // Wide-in-local-x, short-in-local-z box → appears wide on screen
        // (screen-x = world-x via `right`, screen-y = world-z via `down`).
        let kv6 = Kv6::solid_box(16, 4, 4, VoxColor(0x80_C0_40_20));
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 32.0, 32.0, 32.0);

        // Axis-aligned: wide silhouette.
        let aa = Sprite::axis_aligned(kv6.clone(), [0.0, 40.0, 0.0]);
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        let _ = draw_sprite_dda(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &aa,
        );
        let (ax0, ay0, ax1, ay1) = covered_rect(&fb, w, h);
        let aa_wide = (ax1 - ax0) as i32 - (ay1 - ay0) as i32;
        assert!(
            aa_wide > 4,
            "axis-aligned box should be wider than tall (got w-h={aa_wide})"
        );

        // Posed: map local +x onto world +z and local +z onto world +x
        // (det = -1 ≠ 0). Same box now reads tall on screen.
        let mut posed = aa.clone();
        posed.s = [0.0, 0.0, 1.0]; // local +x ↦ world +z (screen down)
        posed.h = [0.0, 1.0, 0.0]; // local +y ↦ world +y (depth)
        posed.f = [1.0, 0.0, 0.0]; // local +z ↦ world +x (screen right)
        let mut fb2 = vec![0u32; n];
        let mut zb2 = vec![f32::INFINITY; n];
        let _ = draw_sprite_dda(
            &mut fb2,
            &mut zb2,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &posed,
        );
        let (bx0, by0, bx1, by1) = covered_rect(&fb2, w, h);
        let posed_tall = (by1 - by0) as i32 - (bx1 - bx0) as i32;
        assert!(
            posed_tall > 4,
            "posed box should be taller than wide (got h-w={posed_tall})"
        );
    }

    /// A degenerate (singular) basis — `det == 0` — makes the sprite
    /// silently skip rather than panic (the `DynSpriteTransform` guard).
    #[test]
    fn degenerate_basis_draws_nothing() {
        let kv6 = Kv6::solid_cube(8, VoxColor(0x80_FF_FF_FF));
        let mut sprite = Sprite::axis_aligned(kv6, [0.0, 40.0, 0.0]);
        sprite.f = sprite.s; // two equal columns → det 0
        let (w, h) = (32u32, 32u32);
        let n = (w * h) as usize;
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 16.0, 16.0, 16.0);
        let wrote = draw_sprite_dda(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &sprite,
        );
        assert_eq!(wrote, 0, "singular basis must skip, not panic");
    }

    /// An invisible sprite draws nothing.
    #[test]
    fn invisible_sprite_skipped() {
        let kv6 = Kv6::solid_cube(8, VoxColor(0x80_FF_FF_FF));
        let mut sprite = Sprite::axis_aligned(kv6, [0.0, 40.0, 0.0]);
        sprite.flags |= roxlap_formats::sprite::SPRITE_FLAG_INVISIBLE;
        let (w, h) = (32u32, 32u32);
        let n = (w * h) as usize;
        let mut fb = vec![0u32; n];
        let mut zb = vec![f32::INFINITY; n];
        let cam = cam_looking_y();
        let cs = camera_math::derive(&cam, w, h, 16.0, 16.0, 16.0);
        let wrote = draw_sprite_dda(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &sprite,
        );
        assert_eq!(wrote, 0);
    }

    // ---------- TV.1a: translucent accumulate-and-continue path ----------

    /// Draw a uniform-material 8³ cube (RGB `0xC0_40_20`) at world y=40 over
    /// a `bg`-filled framebuffer with z-buffer `zb_v`, using palette id 1 =
    /// `mat`. Returns `(centre_pixel, full_framebuffer)`.
    fn draw_cube_shaded(mat: Material, alpha_mul: u8, bg: u32, zb_v: f32) -> (u32, Vec<u32>) {
        let mut table = MaterialTable::new();
        table.set(1, mat);
        let dense = SpriteDense::from_kv6(&Kv6::solid_cube(8, VoxColor(0x80_C0_40_20)));
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let mut fb = vec![bg; n];
        let mut zb = vec![zb_v; n];
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let sh = SpriteShade {
            materials: &table,
            lights: CpuLights::default(),
            material: 1,
            alpha_mul,
            tint: 0x00FF_FFFF,
            shadow: None,
        };
        let _ = draw_sprite_dense_shaded(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &dense,
            [0.0, 40.0, 0.0],
            [1.0, 0.0, 0.0],
            [0.0, 1.0, 0.0],
            [0.0, 0.0, 1.0],
            0,
            Some(sh),
        );
        (fb[(h / 2 * w + w / 2) as usize], fb)
    }

    /// An additive sprite over a dark background brightens it (glow) — and
    /// never darkens any channel below the background.
    #[test]
    fn additive_sprite_brightens_background() {
        let bg = 0x80_20_20_20;
        let (centre, _) = draw_cube_shaded(Material::additive(255), 255, bg, f32::INFINITY);
        let (cr, cg, cb) = ((centre >> 16) & 0xff, (centre >> 8) & 0xff, centre & 0xff);
        assert!(
            cr > 0x20 && cg > 0x20 && cb >= 0x20,
            "centre {centre:08x} should be brighter than bg"
        );
        // Red channel (sprite 0xC0) lifts the most.
        assert!(
            cr >= cg && cr >= cb,
            "additive of a red-dominant cube stays red-dominant"
        );
    }

    /// An alpha-blend sprite composites *between* the background and its own
    /// colour — neither equal to the bare background nor the opaque colour.
    #[test]
    fn alpha_blend_sprite_between_bg_and_color() {
        let bg = 0x80_20_20_20;
        let (centre, _) = draw_cube_shaded(Material::alpha_blend(128), 255, bg, f32::INFINITY);
        let cr = (centre >> 16) & 0xff;
        assert!(
            cr > 0x20,
            "blended red must rise above bg 0x20 (got {cr:02x})"
        );
        assert!(
            cr < 0xC0,
            "blended red must stay below opaque 0xC0 (got {cr:02x})"
        );
        // Distinct from both endpoints.
        assert_ne!(centre & 0x00ff_ffff, bg & 0x00ff_ffff);
        assert_ne!(centre & 0x00ff_ffff, 0x00_C0_40_20);
    }

    /// The per-instance `alpha_mul` scales opacity: a lower multiplier keeps
    /// more of the background (less of the sprite colour).
    #[test]
    fn alpha_mul_scales_opacity() {
        let bg = 0x80_20_20_20;
        let (full, _) = draw_cube_shaded(Material::alpha_blend(255), 255, bg, f32::INFINITY);
        let (faded, _) = draw_cube_shaded(Material::alpha_blend(255), 64, bg, f32::INFINITY);
        let r_full = (full >> 16) & 0xff;
        let r_faded = (faded >> 16) & 0xff;
        // Both lift red above bg, but the faded one stays closer to bg.
        assert!(
            r_full > r_faded,
            "alpha_mul=255 ({r_full:02x}) more opaque than 64 ({r_faded:02x})"
        );
        assert!(r_faded > 0x20, "even faded lifts above bg");
    }

    /// A `SpriteShade` whose effective material is **opaque** (id 0) renders
    /// byte-for-byte identically to the plain opaque path — the per-sprite
    /// gate that keeps the opaque world unchanged.
    #[test]
    fn opaque_shade_ctx_matches_plain_path() {
        let table = MaterialTable::new();
        let dense = SpriteDense::from_kv6(&Kv6::solid_cube(8, VoxColor(0x80_C0_40_20)));
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let pose = (
            [0.0, 40.0, 0.0],
            [1.0, 0.0, 0.0],
            [0.0, 1.0, 0.0],
            [0.0, 0.0, 1.0],
        );

        let mut fb_plain = vec![0u32; n];
        let mut zb_plain = vec![f32::INFINITY; n];
        let _ = draw_sprite_dense(
            &mut fb_plain,
            &mut zb_plain,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &dense,
            pose.0,
            pose.1,
            pose.2,
            pose.3,
            0,
        );

        let mut fb_sh = vec![0u32; n];
        let mut zb_sh = vec![f32::INFINITY; n];
        let sh = SpriteShade {
            materials: &table,
            lights: CpuLights::default(),
            material: 0, // opaque
            alpha_mul: 255,
            tint: 0x00FF_FFFF,
            shadow: None,
        };
        let _ = draw_sprite_dense_shaded(
            &mut fb_sh,
            &mut zb_sh,
            w as usize,
            w,
            h,
            &cs,
            &settings(w, h),
            &dense,
            pose.0,
            pose.1,
            pose.2,
            pose.3,
            0,
            Some(sh),
        );

        assert_eq!(
            fb_plain, fb_sh,
            "opaque shade-ctx must match the plain path bit-for-bit"
        );
        assert_eq!(zb_plain, zb_sh, "opaque shade-ctx z-buffer must match too");
    }

    /// A translucent (additive) sprite behind nearer terrain is occluded:
    /// the front depth (~36) is past the z-buffer cutoff (5), so the march
    /// stops before contributing and the background pixel is untouched.
    #[test]
    fn translucent_sprite_occluded_by_near_terrain() {
        let bg = 0x80_20_20_20;
        let (centre, _) = draw_cube_shaded(Material::additive(255), 255, bg, 5.0);
        assert_eq!(
            centre, bg,
            "near terrain (z=5) must occlude the sprite at y≈36"
        );
    }

    /// Per-span compositing: a translucent voxel contributes one alpha layer
    /// per contiguous solid run, so a 2-voxel-thick slab composites the same
    /// as a 1-voxel-thick one (adjacent voxels are not double-counted). This
    /// is the fix for the voxel-grid striping where a ray clipping a shared
    /// voxel boundary passed through two cells of one wall.
    #[test]
    fn per_span_thickness_independent() {
        fn centre(ysiz: u32) -> u32 {
            let mut table = MaterialTable::new();
            table.set(1, Material::alpha_blend(128));
            let dense = SpriteDense::from_kv6(&Kv6::solid_box(8, ysiz, 8, VoxColor(0x80_C0_40_20)));
            let (w, h) = (64u32, 64u32);
            let n = (w * h) as usize;
            let mut fb = vec![0x80_10_10_10u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
            let sh = SpriteShade {
                materials: &table,
                lights: CpuLights::default(),
                material: 1,
                alpha_mul: 255,
                tint: 0x00FF_FFFF,
                shadow: None,
            };
            let _ = draw_sprite_dense_shaded(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &settings(w, h),
                &dense,
                [0.0, 40.0, 0.0],
                [1.0, 0.0, 0.0],
                [0.0, 1.0, 0.0],
                [0.0, 0.0, 1.0],
                0,
                Some(sh),
            );
            fb[(h / 2 * w + w / 2) as usize] & 0x00ff_ffff
        }
        // A 2-deep box is solid through (surface-only of a 2-thick box is both
        // y-layers); per-span treats the straight-through ray's two adjacent
        // voxels as one surface → identical to the 1-deep slab.
        assert_eq!(
            centre(1),
            centre(2),
            "per-span: a 2-thick slab must match a 1-thick one (no double-count)"
        );
    }

    /// Volumetric (Beer–Lambert) is the thickness-*dependent* counterpart of
    /// per-span: a deeper **filled** volume absorbs more, so its centre pixel
    /// sits closer to the volume colour (less background shows through) than a
    /// shallow one — the opposite of `per_span_thickness_independent`.
    #[test]
    fn volumetric_thickness_deepens_opacity() {
        // Centre-pixel red channel of a filled red box `depth` voxels deep,
        // Volumetric material, over a dark background.
        fn red_at(depth: u32) -> u32 {
            let mut table = MaterialTable::new();
            table.set(1, Material::volumetric(128));
            // FILLED box (every cell solid, interior kept) so the ray actually
            // traverses `depth` absorbing voxels. `from_fn` would cull the
            // interior to a hollow shell (front+back faces only) — no genuine
            // depth accumulation — so use `from_fn_keep_interior`.
            let kv6 = Kv6::from_fn_keep_interior(
                8,
                depth,
                8,
                |_, _, _| Some(VoxColor(0x80_C0_20_20)),
                |_| true,
            );
            let dense = SpriteDense::from_kv6(&kv6);
            let (w, h) = (64u32, 64u32);
            let n = (w * h) as usize;
            let mut fb = vec![0x80_10_10_10u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
            let sh = SpriteShade {
                materials: &table,
                lights: CpuLights::default(),
                material: 1,
                alpha_mul: 255,
                tint: 0x00FF_FFFF,
                shadow: None,
            };
            let _ = draw_sprite_dense_shaded(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &settings(w, h),
                &dense,
                [0.0, 40.0, 0.0],
                [1.0, 0.0, 0.0],
                [0.0, 1.0, 0.0],
                [0.0, 0.0, 1.0],
                0,
                Some(sh),
            );
            (fb[(h / 2 * w + w / 2) as usize] >> 16) & 0xff
        }
        let shallow = red_at(1);
        let deep = red_at(12);
        // Both lift red above the 0x10 background; the deeper volume absorbs
        // more of its own colour in, so its red is higher (more opaque).
        assert!(
            shallow > 0x10,
            "even a 1-deep volume tints (got {shallow:02x})"
        );
        assert!(
            deep > shallow,
            "deeper Volumetric volume is more opaque: deep {deep:02x} > shallow {shallow:02x}"
        );
    }

    /// CPU/GPU parity for scaled clips: rendering a dense volume with
    /// `voxel_world_size = k` at a unit basis must be pixel-identical
    /// to rendering it with `voxel_world_size = 1` at a `k`-scaled
    /// basis — the two factorings of `basis · ((v − pivot) · k)`. The
    /// occluder must agree so shadows match the pixels.
    #[test]
    fn voxel_world_size_matches_scaled_basis() {
        use crate::dda::WorldOccluder;
        let kv6 = Kv6::solid_cube(8, VoxColor(0x80_40_C0_40));
        let scaled = SpriteDense::from_kv6(&kv6).with_voxel_world_size(2.0);
        let unit = SpriteDense::from_kv6(&kv6);

        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let draw = |dense: &SpriteDense, basis: f32| {
            let mut fb = vec![0x80_10_10_10u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
            let written = draw_sprite_dense(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &settings(w, h),
                dense,
                [0.0, 40.0, 0.0],
                [basis, 0.0, 0.0],
                [0.0, basis, 0.0],
                [0.0, 0.0, basis],
                0,
            );
            assert!(written > 0, "sprite must be visible");
            fb
        };
        assert_eq!(
            draw(&scaled, 1.0),
            draw(&unit, 2.0),
            "vws=2 @ unit basis == vws=1 @ 2x basis, pixel for pixel"
        );

        let occludes = |dense: &SpriteDense, basis: f32, x: f64| {
            let mut occ = SpriteOccluder::new();
            occ.push(
                Arc::new(dense.clone()),
                [0.0, 40.0, 0.0],
                [basis, 0.0, 0.0],
                [0.0, basis, 0.0],
                [0.0, 0.0, basis],
            );
            occ.occluded_world([x, 0.0, 0.0], [0.0, 1.0, 0.0], 100.0)
        };
        // x = 6: inside the DOUBLED half-extent (8) but outside the
        // unit one (4) — blocked only under the 2x scale, from either
        // factoring.
        assert!(occludes(&scaled, 1.0, 6.0));
        assert!(occludes(&unit, 2.0, 6.0));
        assert!(!occludes(&unit, 1.0, 6.0), "unscaled cube ends at 4");
    }

    /// XS.2 — the sprite occluder reports a world ray blocked by a sprite
    /// volume (the mechanism by which sprites **cast** shadows, and **receive**
    /// them from each other). A ray through the cube is occluded; one well to
    /// the side is not.
    #[test]
    fn sprite_occluder_blocks_ray_through_volume() {
        use crate::dda::WorldOccluder;
        // 8³ cube, identity pose at world origin; `from_fn` centres the pivot
        // (4,4,4), so the cube spans world ≈ [-4, 4]³.
        let dense = Arc::new(SpriteDense::from_kv6(&Kv6::solid_cube(
            8,
            VoxColor(0x80_FF_FF_FF),
        )));
        let mut occ = SpriteOccluder::new();
        occ.push(
            dense,
            [0.0, 0.0, 0.0],
            [1.0, 0.0, 0.0],
            [0.0, 1.0, 0.0],
            [0.0, 0.0, 1.0],
        );
        assert!(!occ.is_empty());
        // Up the z-axis through the cube centre (world x=y=0 ⇒ local 4,4).
        assert!(
            occ.occluded_world([0.0, 0.0, -50.0], [0.0, 0.0, 1.0], 100.0),
            "a ray through the cube must be occluded"
        );
        // Far to the side: never enters the cube's AABB.
        assert!(
            !occ.occluded_world([50.0, 0.0, -50.0], [0.0, 0.0, 1.0], 100.0),
            "a ray missing the cube must not be occluded"
        );
        // Beyond max_t: the cube is at ~50 units, cap at 10 ⇒ unreached.
        assert!(
            !occ.occluded_world([0.0, 0.0, -50.0], [0.0, 0.0, 1.0], 10.0),
            "max_t shorter than the distance to the cube ⇒ unoccluded"
        );
    }

    /// XS.2 — a sprite **receives** a hard shadow: a blocker volume between the
    /// drawn sprite and the sun darkens it. The blocker lives only in the
    /// occluder (it isn't drawn), so the framebuffer difference is purely the
    /// shadow it casts on the visible sprite.
    #[test]
    fn sprite_receives_hard_shadow() {
        // Drawn target: a voxel sphere at world (0,40,0). A sphere (unlike a
        // face-on cube, whose visible face is the normal-less AABB-entry voxel)
        // gives real per-voxel normals, so its −y hemisphere is genuinely
        // sunlit (to-sun = −y, toward the camera). A blocker cube at (0,25,0)
        // between sphere and sun shadows that lit hemisphere.
        let target = SpriteDense::from_kv6(&Kv6::from_fn(16, 16, 16, |x, y, z| {
            let (dx, dy, dz) = (x as i32 - 8, y as i32 - 8, z as i32 - 8);
            (dx * dx + dy * dy + dz * dz <= 49).then_some(VoxColor(0x80_C0_C0_C0))
        }));
        let mut occ = SpriteOccluder::new();
        occ.push(
            Arc::new(SpriteDense::from_kv6(&Kv6::solid_cube(
                8,
                VoxColor(0x80_FF_FF_FF),
            ))),
            [0.0, 25.0, 0.0],
            [1.0, 0.0, 0.0],
            [0.0, 1.0, 0.0],
            [0.0, 0.0, 1.0],
        );
        let table = MaterialTable::new();
        let base = CpuLights {
            enabled: true,
            sun: true,
            sun_dir: [0.0, -1.0, 0.0], // to-sun: −y (toward the camera)
            sun_color: [1.0; 3],
            sun_intensity: 1.0,
            sun_casts_shadow: true,
            ambient: [0.3; 3],
            shadow_strength: 0.85,
            shadow_bias: 1.5,
            shadow_max_dist: 128.0,
            ..CpuLights::default()
        };
        let (w, h) = (64u32, 64u32);
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let sum_lum = |shadow: Option<&dyn crate::dda::WorldOccluder>| -> u64 {
            let n = (w * h) as usize;
            let mut fb = vec![0u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let sh = SpriteShade {
                materials: &table,
                lights: base,
                material: 0,
                alpha_mul: 255,
                tint: 0x00FF_FFFF,
                shadow,
            };
            let _ = draw_sprite_dense_shaded(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &settings(w, h),
                &target,
                [0.0, 40.0, 0.0],
                [1.0, 0.0, 0.0],
                [0.0, 1.0, 0.0],
                [0.0, 0.0, 1.0],
                0,
                Some(sh),
            );
            fb.iter()
                .map(|&p| u64::from((p & 0xff) + ((p >> 8) & 0xff) + ((p >> 16) & 0xff)))
                .sum()
        };
        let lit = sum_lum(None);
        let shadowed = sum_lum(Some(&occ));
        assert!(
            shadowed < lit,
            "the blocker must shadow the drawn sprite: shadowed={shadowed} lit={lit}"
        );
    }

    /// Per-instance RGB tint multiplies the sprite's colour: a red tint on a
    /// white cube zeroes green+blue and keeps red; a white tint is a no-op.
    #[test]
    fn sprite_rgb_tint_recolours() {
        let table = MaterialTable::new();
        let dense = SpriteDense::from_kv6(&Kv6::solid_cube(8, VoxColor(0x80_FF_FF_FF)));
        let (w, h) = (64u32, 64u32);
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let centre = |tint: u32| -> u32 {
            let n = (w * h) as usize;
            let mut fb = vec![0u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let sh = SpriteShade {
                materials: &table,
                lights: CpuLights::default(),
                material: 0,
                alpha_mul: 255,
                tint,
                shadow: None,
            };
            let _ = draw_sprite_dense_shaded(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &settings(w, h),
                &dense,
                [0.0, 40.0, 0.0],
                [1.0, 0.0, 0.0],
                [0.0, 1.0, 0.0],
                [0.0, 0.0, 1.0],
                0,
                Some(sh),
            );
            fb[(h / 2 * w + w / 2) as usize]
        };
        let r = |p: u32| (p >> 16) & 0xff;
        let g = |p: u32| (p >> 8) & 0xff;
        let b = |p: u32| p & 0xff;
        let white = centre(0x00FF_FFFF);
        let red = centre(0x00FF_0000);
        assert!(
            g(white) > 180 && b(white) > 180 && r(white) > 180,
            "white tint must be a no-op: {white:#08x}"
        );
        assert!(
            r(red) > 180 && g(red) < 20 && b(red) < 20,
            "red tint zeroes green/blue, keeps red: {red:#08x}"
        );
    }

    /// XS.0 — translucent sprite layers are now **lit** (dynamic-lighting rig),
    /// not flat-baked: with the rig enabled at a dim ambient (sun off), the
    /// accumulated layer colour is darker than the disabled (baked, full-
    /// brightness) path. Pins that `cast_local_layers` runs `shade_dynamic`.
    #[test]
    fn translucent_sprite_layers_are_lit() {
        fn center_red(lights: CpuLights) -> u32 {
            let mut table = MaterialTable::new();
            table.set(1, Material::alpha_blend(160));
            let dense = SpriteDense::from_kv6(&Kv6::solid_box(8, 8, 8, VoxColor(0x80_E0_30_30)));
            let (w, h) = (64u32, 64u32);
            let n = (w * h) as usize;
            let mut fb = vec![0x80_10_10_10u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
            let sh = SpriteShade {
                materials: &table,
                lights,
                material: 1,
                alpha_mul: 255,
                tint: 0x00FF_FFFF,
                shadow: None,
            };
            let _ = draw_sprite_dense_shaded(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &settings(w, h),
                &dense,
                [0.0, 40.0, 0.0],
                [1.0, 0.0, 0.0],
                [0.0, 1.0, 0.0],
                [0.0, 0.0, 1.0],
                0,
                Some(sh),
            );
            (fb[(h / 2 * w + w / 2) as usize] >> 16) & 0xff
        }
        let baked = center_red(CpuLights::default()); // disabled ⇒ full-brightness baked
        let dim = center_red(CpuLights {
            enabled: true,
            ambient: [0.3; 3], // sun off, dim ambient ⇒ the layer should darken
            ..CpuLights::default()
        });
        assert!(
            dim < baked,
            "lit translucent layer must respond to the rig (dim ambient darkens): dim={dim:#x} baked={baked:#x}",
        );
    }

    /// The demo scenario: an **opaque** backdrop sprite drawn first, then a
    /// **translucent** sprite in front of it sharing the buffer. The glass
    /// must composite over the backdrop colour (tint it), not leave it
    /// unchanged. Pins the CPU opaque-then-translucent interaction.
    #[test]
    fn translucent_sprite_tints_opaque_sprite_behind() {
        let mut table = MaterialTable::new();
        table.set(1, Material::alpha_blend(128));
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let mut fb = vec![0x80_10_20_40u32; n]; // flat sky
        let mut zb = vec![f32::INFINITY; n];
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let cfg = settings(w, h);
        let id = [1.0, 0.0, 0.0];
        let up = [0.0, 1.0, 0.0];
        let fw = [0.0, 0.0, 1.0];
        let centre = (h / 2 * w + w / 2) as usize;

        // Opaque red backdrop (material 0), far.
        let backdrop = SpriteDense::from_kv6(&Kv6::solid_cube(12, VoxColor(0x80_FF_00_00)));
        let sh_op = SpriteShade {
            materials: &table,
            lights: CpuLights::default(),
            material: 0,
            alpha_mul: 255,
            tint: 0x00FF_FFFF,
            shadow: None,
        };
        let _ = draw_sprite_dense_shaded(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &cfg,
            &backdrop,
            [0.0, 80.0, 0.0],
            id,
            up,
            fw,
            0,
            Some(sh_op),
        );
        let after_backdrop = fb[centre];
        assert_eq!(
            after_backdrop & 0x00ff_ffff,
            0x00FF_0000,
            "backdrop red must be drawn first"
        );

        // Cyan glass (material 1), nearer + overlapping.
        let glass = SpriteDense::from_kv6(&Kv6::solid_cube(12, VoxColor(0x80_00_FF_FF)));
        let sh_gl = SpriteShade {
            materials: &table,
            lights: CpuLights::default(),
            material: 1,
            alpha_mul: 255,
            tint: 0x00FF_FFFF,
            shadow: None,
        };
        let wrote = draw_sprite_dense_shaded(
            &mut fb,
            &mut zb,
            w as usize,
            w,
            h,
            &cs,
            &cfg,
            &glass,
            [0.0, 40.0, 0.0],
            id,
            up,
            fw,
            0,
            Some(sh_gl),
        );
        let _ = wrote;
        let after_glass = fb[centre];
        assert_ne!(
            after_glass, after_backdrop,
            "glass must tint the backdrop (composite over it)"
        );
        // Cyan over red: red channel drops, blue/green rise.
        assert!(
            (after_glass >> 16) & 0xff < 0xFF,
            "glass should reduce the backdrop's red (got {after_glass:08x})"
        );
    }

    /// TV.3: `from_kv6_with_materials` classifies voxels into per-voxel
    /// material ids by colour — mapped colour → its id, unmapped → 0.
    #[test]
    fn from_kv6_with_materials_classifies_by_color() {
        let col = VoxColor(0x80_AA_BB_CC);
        let kv6 = Kv6::solid_cube(6, col);
        let dense = SpriteDense::from_kv6_with_materials(&kv6, &[(Rgb(0x00AA_BBCC), 2)]);
        assert_eq!(
            dense.mat.len(),
            dense.col.len(),
            "per-voxel mat array sized"
        );
        let mut solids = 0;
        for idx in 0..dense.occ.len() {
            if dense.occ[idx] {
                assert_eq!(dense.mat[idx], 2, "mapped colour → material 2");
                solids += 1;
            }
        }
        assert!(solids > 0, "cube has solid voxels");
        // A map that doesn't include the cube's colour → all opaque (0).
        let dense0 = SpriteDense::from_kv6_with_materials(&kv6, &[(Rgb(0x0012_3456), 5)]);
        assert!(
            dense0.mat.iter().all(|&m| m == 0),
            "unmapped colour → material 0"
        );
    }

    /// TV.3: a model whose every voxel maps to the *same* material id renders
    /// identically to drawing it with that material as the instance's uniform
    /// material — the per-voxel path reduces to the uniform path when
    /// homogeneous (and overrides the instance's `material`).
    #[test]
    fn per_voxel_material_matches_uniform_when_homogeneous() {
        let mut table = MaterialTable::new();
        table.set(1, Material::alpha_blend(120));
        let col = VoxColor(0x80_30_A0_F0);
        let kv6 = Kv6::solid_cube(10, col);
        let (w, h) = (64u32, 64u32);
        let n = (w * h) as usize;
        let cs = camera_math::derive(&cam_looking_y(), w, h, 32.0, 32.0, 32.0);
        let cfg = settings(w, h);
        let (pos, s, hh, f) = (
            [0.0, 40.0, 0.0],
            [1.0, 0.0, 0.0],
            [0.0, 1.0, 0.0],
            [0.0, 0.0, 1.0],
        );
        let render = |dense: &SpriteDense, material: u8| -> Vec<u32> {
            let mut fb = vec![0x80_10_10_10u32; n];
            let mut zb = vec![f32::INFINITY; n];
            let sh = SpriteShade {
                materials: &table,
                lights: CpuLights::default(),
                material,
                alpha_mul: 255,
                tint: 0x00FF_FFFF,
                shadow: None,
            };
            let _ = draw_sprite_dense_shaded(
                &mut fb,
                &mut zb,
                w as usize,
                w,
                h,
                &cs,
                &cfg,
                dense,
                pos,
                s,
                hh,
                f,
                0,
                Some(sh),
            );
            fb
        };
        // Per-voxel: every voxel → material 1; instance's uniform material is 0
        // (opaque) but the per-voxel id overrides it.
        let pv = render(
            &SpriteDense::from_kv6_with_materials(&kv6, &[(col.rgb_part(), 1)]),
            0,
        );
        // Uniform: no per-voxel data, instance material 1.
        let un = render(&SpriteDense::from_kv6(&kv6), 1);
        assert_eq!(pv, un, "homogeneous per-voxel material == uniform material");
        // And it's actually translucent (differs from the bare background).
        let centre = (h / 2 * w + w / 2) as usize;
        assert_ne!(pv[centre] & 0x00ff_ffff, 0x0010_1010, "translucent, not bg");
    }

    /// TV.3 (clip wiring): a [`ClipFlipbook`] built with a colour→material map
    /// carries per-voxel materials on every cached frame (the clip analogue of
    /// `from_kv6_with_materials`); an empty map leaves them all-opaque, so the
    /// flipbook is byte-identical to `from_decoded`.
    #[test]
    fn clip_flipbook_with_materials_classifies_every_frame() {
        let dims = [6u32, 6, 6];
        let glass = Rgb(0x00AA_BBCC);
        let glass_lit = 0x80AA_BBCC;
        // Two distinct frames, both filled with the glass colour.
        let f0 = clip_frame(dims, |_x, _y, z| (z < 3).then_some(glass_lit));
        let f1 = clip_frame(dims, |_x, _y, z| (z >= 3).then_some(glass_lit));
        let clip = VoxelClip::from_frames(
            dims,
            [3.0, 3.0, 3.0],
            1.0,
            LoopMode::Loop,
            &[f0, f1],
            &[],
            33,
            0,
        );
        let decoded = clip.decode().expect("decode");

        let book = ClipFlipbook::from_decoded_with_materials(&decoded, &[(glass, 2)]);
        assert_eq!(book.frame_count(), 2);
        for fr in 0..2 {
            let dense = book.frame(fr).expect("frame in range");
            assert_eq!(dense.mat.len(), dense.col.len(), "frame {fr} mat sized");
            let mut solids = 0;
            for idx in 0..dense.occ.len() {
                if dense.occ[idx] {
                    assert_eq!(dense.mat[idx], 2, "frame {fr}: glass → material 2");
                    solids += 1;
                }
            }
            assert!(solids > 0, "frame {fr} has solid voxels");
        }

        // An empty map ⇒ no per-voxel materials, identical to `from_decoded`.
        let plain = ClipFlipbook::from_decoded(&decoded);
        let plain_mat = ClipFlipbook::from_decoded_with_materials(&decoded, &[]);
        for fr in 0..2 {
            assert!(plain.frame(fr).unwrap().mat.is_empty());
            assert!(plain_mat.frame(fr).unwrap().mat.is_empty());
        }
    }
}