unlit3d 0.1.0-dev.2

//! Load glTF documents and patch them into another world's
//! [`MeshSource`]: upload textures, materials and meshes into an existing
//! resource graph, unload them again, and spawn entities that draw the
//! document's default scene.
//!
//! [`UnlitGltf`] owns only the slim, replayable state of a loaded document —
//! the parsed [`gltf::Document`], its decoded buffers and images, and each
//! node's world-space transform — plus nothing else. It holds no [`World`],
//! no [`MeshSource`] and no GPU resources of its own. Everything GPU lives in
//! the graph of the `MeshSource` the caller patches, so a document can be
//! inserted into any number of worlds, and each insertion keeps the handles
//! that unload it again.
//!
//! # Usage
//!
//! ```rust,no_run
//! use unlit3d::gltf::UnlitGltf;
//! # fn run() -> Result<(), gltf::Error> {
//! let gltf = UnlitGltf::load("model.glb")?;
//! # Ok(()) }
//! ```
//!
//! The document is inserted in one call, which returns every handle that keeps
//! what it uploaded alive:
//!
//! ```rust,no_run
//! # use unlit3d::gltf::UnlitGltf;
//! # use unlit3d::prelude::{MeshSource, World};
//! # fn run(source: &mut MeshSource, world: &mut World) -> Result<(), gltf::Error> {
//! let gltf = UnlitGltf::load("model.glb")?;
//! let resources = gltf.insert_resources(source, world);
//! let entities = gltf.spawn_default_scene(world, &resources);
//! // ... render ...
//! // Giving the handles up makes their resources collectable.
//! drop(resources);
//! # Ok(()) }
//! ```
//!
//! A caller that wants to insert one kind of resource at a time — to reuse a
//! texture between documents, say — can still call
//! [`UnlitGltf::insert_images`], [`UnlitGltf::insert_materials`] and
//! [`UnlitGltf::insert_meshes`] directly and build a [`GltfResources`] of its
//! own.
//!
//! Spawning entities never spawns a camera: the renderer draws with the first
//! [`Camera`](crate::components::Camera) in the world, so one belongs to the
//! caller.
//!
//! # Texture formats
//!
//! An image is uploaded in the GPU format closest to the pixels the loader
//! decoded, so a document's textures keep their channels and their precision
//! instead of all being widened to RGBA8: 8-bit layouts upload as
//! `R8Unorm`/`Rg8Unorm`/`Rgba8UnormSrgb`, 16-bit ones as the half-float
//! formats of the same width, and 32-bit float ones as `Rgba32Float`. Two
//! shapes have no format of their own and widen by one channel: the
//! three-channel layouts, because neither an sRGB nor a float format comes in
//! three channels, and a 32-bit float one for the same reason.
//!
//! A one- or two-channel image is the interesting case: WebGPU has no
//! luminance format and no component swizzle, so a grayscale texture samples
//! as `(l, 0, 0, 1)` — the red channel carrying the luma — unless the shader
//! expands it. The built-in unlit shader does, under
//! `BASE_COLOR_LUMINANCE` and `BASE_COLOR_LUMINANCE_ALPHA`, which
//! [`UnlitGltf::pipeline_key`] sets for exactly those uploads: the texel
//! fans out to RGB (and its second channel to alpha) and the luminance is
//! decoded from sRGB, which is the encoding the glTF spec asks of a base-color
//! texture and the one an RGB upload would have been decoded by the sampler.
//! A caller writing its own shader either sets the same flags or takes the
//! luma from the red channel as it is.
//!
//! `Rgba32Float` is also the only upload a device may refuse to filter: it is
//! `unfilterable-float` without `Features::FLOAT32_FILTERABLE`, and then the
//! material that samples it binds a non-filtering sampler — a different
//! bind-group layout from the filtering one. Every handle carries the answer
//! in [`GltfImage::filtering`], and the two derive it the same way, so a
//! material always fits the pipeline its primitive drew with.
//!
//! # Supported subset
//!
//! The unlit renderer draws static meshes with a base-color texture and an
//! instance tint, and composites a material whose `alphaMode` is `BLEND` —
//! straight alpha, drawn z-sorted so overlapping surfaces layer correctly.
//! A `MASK` material cuts its fragments off below its `alphaCutoff` instead,
//! so it draws binary coverage: opaque where the texel is opaque enough and
//! absent everywhere else, blending with nothing. A primitive carrying both
//! `JOINTS_0` and `WEIGHTS_0` uploads its actual joint stream and draws
//! skinned; [`UnlitGltf::skin_pose`] turns the node's skin into the
//! [`SkinPose`] its joints deform by, and spawning such a node creates the
//! pose entity its mesh binds to. A primitive whose mesh declares a morph
//! target that displaces positions uploads those displacements and spawns with
//! a [`MorphBinding`] to a [`MorphWeights`] entity holding the node's starting
//! weights — see [`UnlitGltf::morph_weights`] — so writing that entity is how
//! a caller morphs the mesh. A `doubleSided` material rasterizes its back
//! faces instead of culling them, so its geometry draws from either side.
//! A document's animations are sampled on demand:
//! [`UnlitGltf::animation_count`], [`UnlitGltf::animation_name`] and
//! [`UnlitGltf::animation_duration`] describe what the document carries, and
//! [`UnlitGltf::apply_animation`] samples one at a time into the entities a
//! spawn produced — moving each animated node's children with it, rebuilding
//! the skin poses and writing the morph weights the clip targets. Nothing
//! plays by itself; the caller picks the time.
//! Everything else a glTF document can carry is ignored: no normals or
//! tangents. A mesh whose primitive uses techniques outside this subset
//! still uploads and spawns — it just renders without them.
//!
//! Every handle returned here keeps what it names alive: the next
//! [`MeshSource::maintain`](crate::mesh_source::MeshSource::maintain) collects
//! a resource once the last handle to it — and the last resource depending on
//! it — is gone, so giving up a handle is the unload.
//!
//! `UnlitGltf::load("model.glb")` in the examples above opens a local file —
//! see [`UnlitGltf::load`] and [`UnlitGltf::from_bytes`] for how a document
//! reaches the module in the first place.

use std::path::Path;

use crate::components::{
    GpuMaterial, GpuMesh, MorphBinding, MorphWeights, SkinBinding, SkinPose, Transform,
    ZSortedDrawing,
};
use crate::mesh::MorphDeltas;
use crate::mesh_source::MeshSource;
use crate::unlit::{
    InstanceColor, InstanceCutoff, MeshSourceUnlitExt, UnlitMeshDesc, UnlitPipeline,
    UnlitPipelineKey,
};
use gltf::animation::util::ReadOutputs;
use gltf::animation::{Interpolation, Property};
use unlit_ecs::{
    ArchetypeBuilder,
    prelude::{Entity, World},
};
use unlit_wgpu::pipeline::{BaseColorChannels, UnlitOptions};
use unlit_wgpu::resources::{ResHandle, TextureExt, TextureView};

/// The cutoff the glTF spec gives a `MASK` material that leaves `alphaCutoff`
/// out.
const DEFAULT_ALPHA_CUTOFF: f32 = 0.5;

/// A loaded glTF document, ready to be patched into another world.
///
/// Construct with [`Self::load`] or [`Self::from_bytes`]. The document is
/// parsed and its buffers and images decoded eagerly; no GPU resource exists
/// until an `insert_*` call uploads one into some world's
/// [`MeshSource`].
pub struct UnlitGltf {
    document: gltf::Document,
    buffers: Vec<gltf::buffer::Data>,
    images: Vec<gltf::image::Data>,
    /// World-space transform of every node, indexed by node index.
    node_transforms: Vec<Transform>,
    /// World-space matrix of every node, indexed by node index.
    ///
    /// Kept beside the decomposed transform because animation recomposes the
    /// matrices from the node's own local transform and its parents, and a
    /// shear in that chain has to survive the recomposition.
    node_matrices: Vec<glam::Mat4>,
    /// The parent of every node, indexed by node index; `None` at a root.
    ///
    /// glTF nodes list their children rather than their parent, so the table
    /// is built by walking every `children` list once. Animation recomposes
    /// the world matrices through it.
    parents: Vec<Option<usize>>,
    /// The local-space transform of every node, indexed by node index.
    ///
    /// This is what an animation channel overwrites: a channel targets one
    /// node's local translation, rotation or scale, and the world matrices are
    /// then recomposed from these locals and [`Self::parents`].
    locals: Vec<Transform>,
}

/// A texture uploaded from a glTF image.
///
/// Produced by [`UnlitGltf::insert_resources`] or
/// [`UnlitGltf::insert_image`] / [`UnlitGltf::insert_images`]. Dropping the
/// handle makes the texture collectable by the next maintain.
#[derive(Clone, Debug)]
pub struct GltfImage {
    /// The index of the image in the glTF document.
    pub image: usize,
    /// The uploaded texture holding the image's decoded pixels.
    pub texture: ResHandle<wgpu::Texture>,
    /// A default view of that texture, for callers that sample it directly.
    pub view: ResHandle<TextureView>,
    /// Whether the texture may be sampled with a filtering sampler.
    ///
    /// An image is uploaded in the GPU format closest to its own pixels, and
    /// some of those — 32-bit float, unless the device has
    /// `Features::FLOAT32_FILTERABLE` — can only be bound to a non-filtering
    /// sampler. A material built from this image has to declare the matching
    /// bind-group layout, which is what this flag decides.
    pub filtering: bool,
}

/// A material bind group built from a glTF material's base-color texture.
///
/// Produced by [`UnlitGltf::insert_resources`] or
/// [`UnlitGltf::insert_material`] / [`UnlitGltf::insert_materials`]. Dropping
/// the handle makes the bind group collectable; the view and sampler it names
/// are the caller's own handles.
#[derive(Clone, Debug)]
pub struct GltfMaterial {
    /// The index of the material in the glTF document.
    pub material: usize,
    /// The bind group that samples the base-color texture.
    pub bind_group: GpuMaterial,
    /// The texture view the bind group samples.
    pub view: ResHandle<TextureView>,
    /// The sampler the bind group uses.
    pub sampler: ResHandle<wgpu::Sampler>,
}

/// A mesh uploaded from one glTF primitive.
///
/// Produced by [`UnlitGltf::insert_resources`] or [`UnlitGltf::insert_mesh`] /
/// [`UnlitGltf::insert_meshes`]. Dropping the handle makes the mesh
/// collectable by the next maintain.
///
/// The key is derived from the primitive's own attributes and is the key the
/// mesh was uploaded with; spawning an entity from this handle reuses the same
/// key, so the drawn variant always matches the uploaded vertex layout.
#[derive(Clone, Debug)]
pub struct GltfMesh {
    /// The index of the mesh in the glTF document.
    pub mesh_index: usize,
    /// The index of the primitive inside that mesh.
    pub primitive_index: usize,
    /// The key the mesh was uploaded with.
    pub key: UnlitPipelineKey,
    /// The uploaded GPU mesh.
    pub mesh: GpuMesh,
}

/// The entities a spawned glTF node consists of.
///
/// Produced by [`UnlitGltf::spawn_node`] / [`UnlitGltf::spawn_default_scene`]
/// and given back to [`UnlitGltf::apply_animation`], which drives the node's
/// transform, its [`SkinPose`] and its [`MorphWeights`] through the entities
/// this names.
#[derive(Clone, Debug)]
pub struct GltfNode {
    /// The index of the node in the glTF document.
    pub node: usize,
    /// One entity per primitive of the node's mesh; empty for a node with no
    /// mesh.
    pub entities: Vec<Entity>,
    /// The entity holding the node's [`SkinPose`], when the node is skinned.
    pub pose: Option<Entity>,
    /// The entity holding the node's [`MorphWeights`], when the node's mesh
    /// has morph targets.
    pub weights: Option<Entity>,
}

/// Every resource one document uploaded, in document order.
///
/// Produced by [`UnlitGltf::insert_resources`], which uploads the whole
/// document in one call, and taken by [`UnlitGltf::spawn_node`] and
/// [`UnlitGltf::spawn_default_scene`] to draw it. Each vector is indexed by
/// the document index it came from: `images[i]` is image `i`, `materials[m]`
/// is material `m` — `None` when it names no base-color texture — and
/// `meshes` holds every primitive of every mesh, document-mesh-major and then
/// primitive-minor.
///
/// Holding the struct is what keeps the uploaded resources alive: dropping it
/// lets the next [`MeshSource::maintain`] collect them.
///
/// A caller that inserts the pieces itself — to share textures between
/// documents, say — builds one of these to spawn with.
#[derive(Clone, Debug, Default)]
pub struct GltfResources {
    /// Every uploaded image, indexed by document image.
    pub images: Vec<GltfImage>,
    /// Every uploaded material, indexed by document material.
    pub materials: Vec<Option<GltfMaterial>>,
    /// Every uploaded primitive, document-mesh-major then primitive-minor.
    pub meshes: Vec<GltfMesh>,
}

impl UnlitGltf {
    /// Load a glTF document from `path`, decoding its buffers and images.
    ///
    /// External URIs resolve against the file's directory. Fails with
    /// [`gltf::Error`] when the file cannot be read or parsed, or a referenced
    /// buffer or image cannot be decoded.
    pub fn load(path: impl AsRef<Path>) -> Result<Self, gltf::Error> {
        let (document, buffers, images) = gltf::import(path)?;
        Ok(Self::from_parts(document, buffers, images))
    }

    /// Parse a glTF document from `bytes`.
    ///
    /// The bytes may be either glTF JSON or a binary GLB container. Buffers
    /// arrive as embedded data URIs or, for a GLB, the `BIN` chunk; images
    /// must be embedded too (as data URIs or buffer views), because there is
    /// no base URI to resolve external references against.
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, gltf::Error> {
        let (document, buffers, images) = gltf::import_slice(bytes)?;
        Ok(Self::from_parts(document, buffers, images))
    }

    fn from_parts(
        document: gltf::Document,
        buffers: Vec<gltf::buffer::Data>,
        images: Vec<gltf::image::Data>,
    ) -> Self {
        let parents = node_parents(&document);
        let locals = node_locals(&document);
        let node_matrices = node_world_matrices(&document, &parents);
        let node_transforms = node_matrices.iter().map(|&m| decompose(m)).collect();
        Self {
            document,
            buffers,
            images,
            node_transforms,
            node_matrices,
            parents,
            locals,
        }
    }

    /// The [`UnlitPipelineKey`] the primitive renders with.
    ///
    /// The key carries only what a caller picks: the texture filtering the
    /// material's base-color texture needs, the luminance expansion that
    /// texture's format calls for, whether a `BLEND` material blends and
    /// whether a `MASK` one cuts. Everything geometric — which vertex
    /// channels the primitive carries, whether it samples its base-color
    /// texture, whether it skins or morphs — is a property of the draw, so
    /// [`Self::insert_mesh`] uploads the streams the primitive's own
    /// attributes declare and the variant a draw resolves derives the rest
    /// from the mesh it draws. A `doubleSided` material turns back-face
    /// culling off, so its geometry is drawn from either side; everything
    /// else follows [`UnlitOptions::standard`].
    pub fn pipeline_key(
        &self,
        device: &wgpu::Device,
        mesh: usize,
        primitive: usize,
    ) -> UnlitPipelineKey {
        let primitive = self.primitive(mesh, primitive);
        let material = primitive.material();
        let mut options = UnlitOptions::standard(device);
        // The texture the material samples decides whether the base-color
        // binding is a filtering one: the pipeline and the material bind group
        // have to agree, or the group does not fit the pipeline.
        let texture = material.pbr_metallic_roughness().base_color_texture();
        if samples_base_color_texture(&primitive)
            && let Some(info) = &texture
        {
            let image = info.texture().source().index();
            options.texture_filtering = self.image_filtering(image, device);
            // A one- or two-channel texture uploads in a format that cannot
            // decode sRGB for itself, so the shader expands the texel and
            // decodes the luminance instead.
            options.base_color = self.image_luminance(image);
        }
        // glTF base colors and textures are straight (non-premultiplied)
        // alpha, which is exactly what `ALPHA_BLENDING` composites.
        if material.alpha_mode() == gltf::material::AlphaMode::Blend {
            options.color_target.blend = Some(wgpu::BlendState::ALPHA_BLENDING);
        }
        // A `MASK` material is not a translucent one: its fragments are either
        // drawn whole or discarded, so it neither blends nor needs the
        // back-to-front sort a blended one does. The cutoff itself is
        // per-instance state, so it rides the instance stream rather than the
        // material group — see [`Self::spawn_node`].
        if alpha_cutoff(&material).is_some() {
            options.alpha_cutoff = true;
        }
        // A `doubleSided` material is visible from behind, so its back faces
        // are rasterized rather than culled. Nothing else about the variant
        // changes: the unlit fragment shader reads no normal, so there is no
        // back-face normal to reverse, and the geometry simply draws from both
        // sides.
        if material.double_sided() {
            options.primitive.cull_mode = None;
        }
        UnlitPipelineKey::new(options)
    }

    // -- images ---------------------------------------------------------------

    /// Upload `image` into `source`'s resource graph and return its handle.
    ///
    /// The texture holds the image's decoded pixels in the GPU format closest
    /// to them — see the [module docs](self#texture-formats); its sampler is
    /// not uploaded here, because that belongs to the material that samples
    /// it.
    pub fn insert_image(&self, source: &mut MeshSource, world: &World, image: usize) -> GltfImage {
        let data = &self.images[image];
        let device = source.device(world);
        let format = image_format(data, &device);
        let (width, height) = (data.width, data.height);
        let texture = device.create_texture(&wgpu::TextureDescriptor {
            label: None,
            size: wgpu::Extent3d {
                width,
                height,
                depth_or_array_layers: 1,
            },
            mip_level_count: 1,
            sample_count: 1,
            dimension: wgpu::TextureDimension::D2,
            format: format.format,
            usage: wgpu::TextureUsages::TEXTURE_BINDING | wgpu::TextureUsages::COPY_DST,
            view_formats: &[],
        });

        // `encode_rows` pads every row to the 256-byte stride a buffer-to-texture
        // copy needs; the padding bytes stay zero.
        let bytes_per_row = align_up(
            width * format.texel_bytes(),
            wgpu::COPY_BYTES_PER_ROW_ALIGNMENT,
        );
        let rows = encode_rows(data, format.format);
        source.queue(world).write_texture(
            wgpu::TexelCopyTextureInfo {
                texture: &texture,
                mip_level: 0,
                origin: wgpu::Origin3d::ZERO,
                aspect: wgpu::TextureAspect::All,
            },
            &rows,
            wgpu::TexelCopyBufferLayout {
                offset: 0,
                bytes_per_row: Some(bytes_per_row),
                rows_per_image: Some(height),
            },
            wgpu::Extent3d {
                width,
                height,
                depth_or_array_layers: 1,
            },
        );

        let (texture, view) = source.register_texture_and_default_view(world, texture);
        GltfImage {
            image,
            texture,
            view,
            filtering: format.filtering,
        }
    }

    /// Upload every image of the document.
    ///
    /// The returned vector is aligned with the document's image indices: the
    /// handle for image `i` sits at index `i`.
    pub fn insert_images(&self, source: &mut MeshSource, world: &World) -> Vec<GltfImage> {
        (0..self.document.images().len())
            .map(|image| self.insert_image(source, world, image))
            .collect()
    }

    // -- materials ------------------------------------------------------------

    /// Build the material bind group for `material` and return its handle.
    ///
    /// Returns `None` when the material declares no base-color texture: such a
    /// material has nothing to bind and spawns with no `GpuMaterial`.
    ///
    /// # Panics
    ///
    /// If the material's base-color texture names an image that is not in
    /// `images`.
    pub fn insert_material(
        &self,
        source: &mut MeshSource,
        world: &World,
        material: usize,
        images: &[GltfImage],
    ) -> Option<GltfMaterial> {
        let data = self.material(material);
        let info = data.pbr_metallic_roughness().base_color_texture()?;
        let image = images
            .iter()
            .find(|image| image.image == info.texture().source().index())
            .expect("the base-color texture's image must be inserted before the material");

        // A material group exists only for a variant that samples a base-color
        // texture; build the bind group against exactly that variant. Its
        // layout is the filtering one only when the image can be filtered, and
        // the sampler has to be non-filtering in step with it.
        let key = textured_key(&source.device(world), image.filtering);
        let descriptor = sampler_descriptor(&info.texture().sampler());
        let descriptor = if image.filtering {
            descriptor
        } else {
            wgpu::SamplerDescriptor {
                mag_filter: wgpu::FilterMode::Nearest,
                min_filter: wgpu::FilterMode::Nearest,
                mipmap_filter: wgpu::MipmapFilterMode::Nearest,
                ..descriptor
            }
        };
        let sampler = source.register_sampler(world, Some(descriptor));
        let view = {
            let texture = {
                let graph = MeshSource::graph(world, source.context());
                graph
                    .get(&image.texture)
                    .expect("the image's texture is in the graph")
                    .clone()
            };
            let view = TextureExt::create_view(&texture, &wgpu::TextureViewDescriptor::default());
            let mut graph = MeshSource::graph(world, source.context());
            let view = graph.insert(view, None);
            graph.add_dependency(&view, &image.texture);
            view
        };

        let bind_group = source.allocate_unlit_material(world, &key, view.clone(), sampler.clone());
        Some(GltfMaterial {
            material,
            bind_group,
            view,
            sampler,
        })
    }

    /// Build the bind group for every material of the document.
    ///
    /// The returned vector is aligned with the document's material indices:
    /// the entry for material `i` sits at index `i`, and is `None` for a
    /// material with no base-color texture.
    ///
    /// # Panics
    ///
    /// If a material's base-color texture names an image that is not in
    /// `images`.
    pub fn insert_materials(
        &self,
        source: &mut MeshSource,
        world: &World,
        images: &[GltfImage],
    ) -> Vec<Option<GltfMaterial>> {
        (0..self.document.materials().len())
            .map(|material| self.insert_material(source, world, material, images))
            .collect()
    }

    // -- meshes ---------------------------------------------------------------

    /// Upload the primitive into `source`'s resource graph and return its
    /// handle.
    ///
    /// Which streams are uploaded follows the primitive's own attributes:
    /// `TEXCOORD_0` with a base-color-textured material enables the UV stream,
    /// `COLOR_0` the vertex-color stream, `JOINTS_0` and `WEIGHTS_0` the joint
    /// stream, and a position-displacing morph target the displacement stream.
    /// A primitive read with streams its attributes do not declare would
    /// panic, so the derivation is the only safe choice here. Normals and
    /// tangents are ignored (see the module docs).
    ///
    /// # Panics
    ///
    /// If the primitive carries no `POSITION` attribute.
    pub fn insert_mesh(
        &self,
        source: &mut MeshSource,
        world: &World,
        mesh: usize,
        primitive: usize,
    ) -> GltfMesh {
        let key = self.pipeline_key(&source.device(world), mesh, primitive);
        let primitive = self.primitive(mesh, primitive);
        let reader = primitive.reader(|buffer| Some(&self.buffers[buffer.index()]));

        let positions: Vec<[f32; 3]> = reader
            .read_positions()
            .expect("a glTF primitive carries its POSITION attribute")
            .collect();
        // Read only the streams the primitive declares: an attribute it does
        // not carry is not uploaded, so decoding it would be wasted work.
        let uvs: Option<Vec<[f32; 2]>> = samples_base_color_texture(&primitive).then(|| {
            reader
                .read_tex_coords(0)
                .expect("the primitive samples a texture, so it has TEXCOORD_0")
                .into_f32()
                .collect()
        });
        let colors: Option<Vec<[u8; 4]>> = primitive
            .get(&gltf::Semantic::Colors(0))
            .is_some()
            .then(|| {
                reader
                    .read_colors(0)
                    .expect("the primitive has COLOR_0")
                    .into_rgba_u8()
                    .collect()
            });
        let indices: Option<Vec<u32>> = reader.read_indices().map(|i| i.into_u32().collect());
        let skinned = primitive.get(&gltf::Semantic::Joints(0)).is_some()
            && primitive.get(&gltf::Semantic::Weights(0)).is_some();
        let joints: Option<Vec<[u16; 4]>> = skinned.then(|| {
            reader
                .read_joints(0)
                .expect("the primitive has JOINTS_0")
                .into_u16()
                .collect()
        });
        let weights: Option<Vec<[f32; 4]>> = skinned.then(|| {
            reader
                .read_weights(0)
                .expect("the primitive has WEIGHTS_0")
                .into_f32()
                .collect()
        });
        // The displacements the mesh's morph stream reads, packed the way a
        // mesh wants them: for each vertex, every position-displacing target in
        // order, three components each. Writing straight into the packed array
        // keeps one allocation for the whole thing, however many targets the
        // primitive declares; a target of normals alone is skipped here.
        let morph_deltas = (morph_target_count(&primitive) > 0).then(|| {
            let target_count = morph_target_count(&primitive);
            let mut deltas = vec![0.0; positions.len() * target_count * 3];
            let mut target = 0;
            for (displacements, _, _) in reader.read_morph_targets() {
                let Some(displacements) = displacements else {
                    continue;
                };
                for (vertex, displacement) in displacements.enumerate() {
                    let start = (vertex * target_count + target) * 3;
                    deltas[start..start + 3].copy_from_slice(&displacement);
                }
                target += 1;
            }
            MorphDeltas {
                deltas,
                target_count: target_count as u32,
            }
        });

        let gpu_mesh = source.allocate_unlit_mesh(
            world,
            &key,
            UnlitMeshDesc {
                positions: &positions,
                uvs: uvs.as_deref(),
                colors: colors.as_deref(),
                indices: indices.as_deref(),
                joints: joints.as_deref(),
                weights: weights.as_deref(),
                morph_deltas,
            },
        );

        GltfMesh {
            mesh_index: mesh,
            primitive_index: primitive.index(),
            key,
            mesh: gpu_mesh,
        }
    }

    /// Upload every primitive of every mesh of the document.
    ///
    /// The vector is ordered document-mesh-major, then primitive-minor: first
    /// all of mesh 0's primitives, then mesh 1's, and so on.
    pub fn insert_meshes(&self, source: &mut MeshSource, world: &World) -> Vec<GltfMesh> {
        let count: usize = self
            .document
            .meshes()
            .map(|mesh| mesh.primitives().len())
            .sum();
        let mut meshes = Vec::with_capacity(count);
        for mesh in 0..self.document.meshes().len() {
            for primitive in 0..self.mesh(mesh).primitives().count() {
                meshes.push(self.insert_mesh(source, world, mesh, primitive));
            }
        }
        meshes
    }

    /// Insert the whole document and return every handle that keeps it alive.
    ///
    /// This is the one call that uploads a document: it inserts the images,
    /// builds the materials from them and uploads every primitive, and hands
    /// back a [`GltfResources`] holding all of it. Pass that to
    /// [`Self::spawn_node`] or [`Self::spawn_default_scene`] to draw the
    /// document; keep it alive for as long as the entities live.
    ///
    /// A caller that inserts the pieces itself — to share a texture between
    /// documents, say — can call [`Self::insert_images`],
    /// [`Self::insert_materials`] and [`Self::insert_meshes`] instead and
    /// build a [`GltfResources`] of its own.
    pub fn insert_resources(&self, source: &mut MeshSource, world: &World) -> GltfResources {
        let images = self.insert_images(source, world);
        let materials = self.insert_materials(source, world, &images);
        let meshes = self.insert_meshes(source, world);
        GltfResources {
            images,
            materials,
            meshes,
        }
    }

    // -- skins ----------------------------------------------------------------

    /// The [`SkinPose`] of a node's skin, at the node's rest transform.
    ///
    /// A document carries a skin as a list of joint nodes and one inverse bind
    /// matrix per joint; the pose a renderer wants is a matrix per joint, which
    /// is what this builds:
    ///
    /// ```text
    /// joint matrix = inverse(mesh node world) * joint node world * inverse bind matrix
    /// ```
    ///
    /// The pose is the *rest* pose — the document's node transforms as loaded,
    /// which is the transform each joint was bound in, so the deformation is
    /// the identity everywhere. A caller that animates joints moves the joint
    /// nodes and rebuilds the pose, or writes the matrices itself; what this
    /// gives is a valid pose to start from and the joint count the mesh's
    /// joint indices address.
    ///
    /// `node` is the node the mesh is attached to, whose skin the pose comes
    /// from. A node with no skin, or a skin with no `inverseBindMatrices`, gets
    /// matrices that deform nothing: the identity per joint, which is what a
    /// skin bound at rest means.
    ///
    /// The returned pose has one matrix per joint of the skin, in the order the
    /// mesh's joint indices address them.
    pub fn skin_pose(&self, node: usize) -> SkinPose {
        self.skin_pose_with_matrices(node, &self.node_matrices)
    }

    /// The [`SkinPose`] a node's skin deforms by, evaluated against a given
    /// set of node world matrices.
    ///
    /// [`Self::skin_pose`] evaluates the rest pose; [`Self::apply_animation`]
    /// evaluates one where the joints have been animated.
    fn skin_pose_with_matrices(&self, node: usize, matrices: &[glam::Mat4]) -> SkinPose {
        let node_data = self.node(node);
        let Some(skin) = node_data.skin() else {
            return SkinPose::default();
        };
        let reader = skin.reader(|buffer| Some(&self.buffers[buffer.index()]));
        let inverse_bind: Option<Vec<glam::Mat4>> = reader.read_inverse_bind_matrices().map(|m| {
            m.map(|matrix| glam::Mat4::from_cols_array_2d(&matrix))
                .collect()
        });
        // A skinned mesh is deformed in its own space, so the mesh node's world
        // matrix has to come back out of the joint's: a joint's inverse bind
        // matrix already encodes the joint's rest transform, and the mesh node
        // moves the whole skinned result.
        let world_to_mesh = matrices[node].inverse();
        let poses: Vec<glam::Mat4> = skin
            .joints()
            .enumerate()
            .map(|(joint, joint_node)| {
                let joint_world = matrices[joint_node.index()];
                let bind = inverse_bind.as_ref().map_or(glam::Mat4::IDENTITY, |ibm| {
                    *ibm.get(joint).expect("one inverse bind matrix per joint")
                });
                world_to_mesh * joint_world * bind
            })
            .collect();
        SkinPose::new(poses)
    }

    /// The number of joints a node's skin has.
    ///
    /// `None` when the node names no skin, so a caller can walk a document
    /// without asking first.
    pub fn skin_joint_count(&self, node: usize) -> Option<usize> {
        Some(self.node(node).skin()?.joints().count())
    }

    /// The morph weights a node's mesh starts at, one per target that
    /// displaces positions.
    ///
    /// glTF lets a node override the mesh's own weights, so a node states them
    /// first and the mesh's `weights` is the fallback; a mesh that states
    /// neither starts at zero, which is to say undeformed. The vector is as
    /// long as the deforming target list, so it is always the right length for
    /// the [`MorphWeights`] a spawned mesh binds to — a document that states
    /// fewer weights than it has targets is padded, and one that states more is
    /// truncated, because the renderer rejects a mismatch.
    ///
    /// Returns an empty vector for a node whose mesh has no positional morph
    /// targets.
    pub fn morph_weights(&self, node: usize) -> Vec<f32> {
        let Some(mesh) = self.node(node).mesh() else {
            return Vec::new();
        };
        let node_weights = self.node(node).weights();
        let mut weights = node_weights
            .or_else(|| mesh.weights())
            .map(<[f32]>::to_vec)
            .unwrap_or_default();
        let count = mesh
            .primitives()
            .map(|primitive| morph_target_count(&primitive))
            .max()
            .unwrap_or(0);
        weights.resize(count, 0.0);
        weights
    }

    // -- animation ------------------------------------------------------------

    /// The number of animations the document carries.
    pub fn animation_count(&self) -> usize {
        self.document.animations().count()
    }

    /// The name of an animation, or `None` when it carries none.
    ///
    /// # Panics
    ///
    /// If `animation` is out of bounds.
    pub fn animation_name(&self, animation: usize) -> Option<&str> {
        self.animation(animation).name()
    }

    /// The duration of an animation in seconds: the latest input keyframe
    /// across all of its samplers.
    ///
    /// # Panics
    ///
    /// If `animation` is out of bounds.
    pub fn animation_duration(&self, animation: usize) -> f32 {
        let animation = self.animation(animation);
        let mut duration: f32 = 0.0;
        for channel in animation.channels() {
            let reader = channel.reader(|buffer| Some(&self.buffers[buffer.index()]));
            if let Some(inputs) = reader.read_inputs()
                && let Some(last) = inputs.last()
            {
                duration = duration.max(last);
            }
        }
        duration
    }

    /// Drive an animation to `time` seconds, writing what it moves back into
    /// the world.
    ///
    /// `nodes` is the list [`Self::spawn_node`] /
    /// [`Self::spawn_default_scene`] returned: for every node it names, the
    /// animated world transform is written into the entity's [`Transform`],
    /// a skinned node's [`SkinPose`] is rebuilt against the animated joints,
    /// and a morphed node's [`MorphWeights`] are set to the sampled weights.
    ///
    /// `world` is borrowed shared, not mutable: components are written through
    /// the world's interior mutability, so the same call site can keep reading
    /// the world while it animates.
    ///
    /// A channel's inputs outside its keyframe range clamp to the nearest end,
    /// and translation, scale and rotation interpolate by the sampler's
    /// `interpolation`: `STEP` holds the previous keyframe, `LINEAR` blends
    /// (with a spherical blend for a rotation) and `CUBICSPLINE` evaluates the
    /// Hermite spline through the frame's tangents. A node no channel targets
    /// keeps its rest transform, and a node that is neither spawned nor an
    /// ancestor of one costs nothing.
    ///
    /// # Panics
    ///
    /// If `animation` is out of bounds.
    pub fn apply_animation(&self, world: &World, animation: usize, time: f32, nodes: &[GltfNode]) {
        let animation = self.animation(animation);
        let count = self.parents.len();
        let mut locals = self.locals.clone();
        let mut animated = vec![false; count];
        let mut weights: Vec<Option<Vec<f32>>> = vec![None; count];

        for channel in animation.channels() {
            let node = channel.target().node().index();
            let property = channel.target().property();
            let interpolation = channel.sampler().interpolation();
            let reader = channel.reader(|buffer| Some(&self.buffers[buffer.index()]));
            let Some(inputs) = reader.read_inputs() else {
                continue;
            };
            let times: Vec<f32> = inputs.collect();
            // Interpolation needs an interval to work in, and the spec
            // requires at least two keyframes anyway.
            if times.len() < 2 {
                continue;
            }
            let Some(outputs) = reader.read_outputs() else {
                continue;
            };
            let required = times.len() * interpolation_stride(interpolation);
            match (property, outputs) {
                (Property::Translation, ReadOutputs::Translations(values)) => {
                    let values: Vec<[f32; 3]> = values.collect();
                    if values.len() < required {
                        continue;
                    }
                    locals[node].translation =
                        sample_vec3(interpolation, &times, &values, time).into();
                    animated[node] = true;
                }
                (Property::Scale, ReadOutputs::Scales(values)) => {
                    let values: Vec<[f32; 3]> = values.collect();
                    if values.len() < required {
                        continue;
                    }
                    locals[node].scale = sample_vec3(interpolation, &times, &values, time).into();
                    animated[node] = true;
                }
                (Property::Rotation, ReadOutputs::Rotations(rotations)) => {
                    let values: Vec<[f32; 4]> = rotations.into_f32().collect();
                    if values.len() < required {
                        continue;
                    }
                    locals[node].rotation = sample_rotation(interpolation, &times, &values, time);
                    animated[node] = true;
                }
                (Property::MorphTargetWeights, ReadOutputs::MorphTargetWeights(outputs)) => {
                    let values: Vec<f32> = outputs.into_f32().collect();
                    let targets = self.morph_weights(node).len();
                    let needed = times.len() * targets * interpolation_stride(interpolation);
                    if targets == 0 || values.len() < needed {
                        continue;
                    }
                    weights[node] = Some(sample_weights(
                        interpolation,
                        &times,
                        &values,
                        targets,
                        time,
                    ));
                }
                _ => {}
            }
        }

        if !animated.iter().any(|&moved| moved) && weights.iter().all(Option::is_none) {
            return;
        }

        // A node whose ancestor moved has moved too, even when no channel
        // names it: mark the descendants before recomposing.
        let mut children: Vec<Vec<usize>> = vec![Vec::new(); count];
        for (node, parent) in self.parents.iter().enumerate() {
            if let Some(parent) = parent {
                children[*parent].push(node);
            }
        }
        let mut dirty = animated.clone();
        let mut stack: Vec<usize> = (0..count).filter(|&node| dirty[node]).collect();
        while let Some(node) = stack.pop() {
            for &child in &children[node] {
                if !dirty[child] {
                    dirty[child] = true;
                    stack.push(child);
                }
            }
        }

        let mut matrices = self.node_matrices.clone();
        for &node in &node_order(&self.document, &self.parents) {
            if !dirty[node] {
                continue;
            }
            let local = glam::Mat4::from(locals[node].compute_matrix());
            matrices[node] = match self.parents[node] {
                Some(parent) => matrices[parent] * local,
                None => local,
            };
        }

        for handle in nodes {
            let node = handle.node;
            if dirty[node] {
                let transform = decompose(matrices[node]);
                for &entity in &handle.entities {
                    let _ = world.with_mut::<Transform, _>(entity, |current| {
                        *current = transform.clone();
                    });
                }
            }
            if let Some(entity) = handle.pose {
                let pose = self.skin_pose_with_matrices(node, &matrices);
                let _ = world.with_mut::<SkinPose, _>(entity, |current| *current = pose.clone());
            }
            if let (Some(entity), Some(sampled)) = (handle.weights, weights[node].as_ref()) {
                let _ = world.with_mut::<MorphWeights, _>(entity, |current| {
                    current.weights = sampled.clone();
                });
            }
        }
    }

    fn animation(&self, animation: usize) -> gltf::animation::Animation<'_> {
        self.document
            .animations()
            .nth(animation)
            .expect("glTF animation index in bounds")
    }

    // -- spawning -------------------------------------------------------------

    /// Spawn the entities that draw `node`'s mesh into `world`.
    ///
    /// Returns a [`GltfNode`] naming one entity per primitive of the node's
    /// mesh — each carrying the
    /// node's world-space [`Transform`], the uploaded [`GpuMesh`], the
    /// pipeline for the mesh's key and an [`InstanceColor`] tinted with the
    /// material's base-color factor. A primitive that samples its material's
    /// base-color texture also carries the matching [`GpuMaterial`]. A blended
    /// primitive ([`gltf::material::AlphaMode::Blend`]) also carries
    /// [`ZSortedDrawing`], so it is drawn after the opaque geometry and
    /// composited back-to-front. A skinned primitive (one whose mesh carries a
    /// joint stream) carries a [`SkinBinding`] to the node's [`SkinPose`], which is
    /// spawned on an entity of its own so several meshes can share it — see
    /// [`Self::skin_pose`]. Children are not spawned; use
    /// [`Self::spawn_default_scene`] for the whole scene.
    ///
    /// # Panics
    ///
    /// If the node's mesh (or a mesh it shares primitives with) was not
    /// inserted into `resources`, or a primitive's material is `None` there
    /// while it samples a base-color texture.
    pub fn spawn_node(
        &self,
        world: &mut World,
        node: usize,
        resources: &GltfResources,
    ) -> GltfNode {
        let index = node;
        let transform = self.node_transforms[node].clone();
        let node = self.node(node);
        let mut entities = Vec::new();
        // One pose entity per skinned node, shared by every primitive it
        // draws: the joints are the node's skin, and two primitives of one
        // mesh deform together. The morph weights get an entity of their own
        // for the same reason — they are the node's, not the primitive's.
        let mut pose: Option<Entity> = None;
        let mut morphs: Option<Entity> = None;
        if let Some(mesh) = node.mesh() {
            for primitive in mesh.primitives() {
                let mesh_handle = resources
                    .meshes
                    .iter()
                    .find(|handle| {
                        handle.mesh_index == mesh.index()
                            && handle.primitive_index == primitive.index()
                    })
                    .expect("a document mesh must be inserted before the node that draws it");
                // The base-color factor tints the instance whether or not the
                // material samples a texture; a primitive naming no material
                // falls back to the glTF default, opaque white.
                let tint: glam::Vec4 = primitive
                    .material()
                    .pbr_metallic_roughness()
                    .base_color_factor()
                    .into();
                // The `alphaMode: MASK` cutoff the shader compares against is
                // read before the primitive is handed to the material lookup,
                // which takes it by value.
                let cutoff = alpha_cutoff(&primitive.material());
                let material = samples_base_color_texture(&primitive)
                    .then(|| self.primitive_material(primitive, &resources.materials));
                let z_sorted = mesh_handle.key.options.color_target.blend.is_some();
                let skinned = mesh_handle.mesh.skinned;
                let binding = skinned.then(|| {
                    let pose =
                        *pose.get_or_insert_with(|| world.spawn((self.skin_pose(node.index()),)));
                    SkinBinding::new(pose)
                });
                let morphed = mesh_handle.mesh.morph_targets > 0;
                let morph_binding = morphed.then(|| {
                    let weights = *morphs.get_or_insert_with(|| {
                        world.spawn((MorphWeights::new(self.morph_weights(node.index())),))
                    });
                    MorphBinding::new(weights)
                });
                // The components a primitive carries vary by material,
                // skin and blend mode, so the bundle is assembled rather than
                // written as a tuple: an absent material is a component left
                // out, not a `None` the renderer would have to look through.
                let mut bundle = ArchetypeBuilder::new();
                bundle.push(transform.clone());
                bundle.push(mesh_handle.mesh.clone());
                bundle.push(UnlitPipeline::new(mesh_handle.key.clone()));
                bundle.push(InstanceColor::new(tint));
                // The cutoff is per-instance state, so the entity carries it
                // and the shader reads it out of the instance stream.
                if let Some(cutoff) = cutoff {
                    bundle.push(InstanceCutoff::new(cutoff));
                }
                if let Some(material) = material {
                    bundle.push(material.bind_group.clone());
                }
                if let Some(binding) = binding {
                    bundle.push(binding);
                }
                if let Some(binding) = morph_binding {
                    bundle.push(binding);
                }
                if z_sorted {
                    bundle.push(ZSortedDrawing);
                }
                entities.push(world.spawn(bundle));
            }
        }
        GltfNode {
            node: index,
            entities,
            pose,
            weights: morphs,
        }
    }

    /// Spawn every node reachable from the document's default scene.
    ///
    /// Returns one [`GltfNode`] per visited node, in the order the walk
    /// reaches them; a node with no mesh has none of its own but is still
    /// returned, so the list maps a document node to its entities. Nodes are
    /// visited root-first and each subtree fully; every node with a mesh
    /// contributes its primitives exactly where its world-space transform sits
    /// in the hierarchy.
    ///
    /// # Panics
    ///
    /// If the document declares no default scene, or any spawn fails (see
    /// [`Self::spawn_node`]).
    pub fn spawn_default_scene(
        &self,
        world: &mut World,
        resources: &GltfResources,
    ) -> Vec<GltfNode> {
        let scene = self
            .document
            .default_scene()
            .expect("the glTF document declares a default scene");
        let mut nodes = Vec::new();
        let mut stack: Vec<usize> = scene.nodes().map(|node| node.index()).collect();
        while let Some(node) = stack.pop() {
            nodes.push(self.spawn_node(world, node, resources));
            stack.extend(self.node(node).children().map(|child| child.index()));
        }
        nodes
    }

    /// The inserted material a primitive draws with.
    ///
    /// A primitive without a material index, or whose material is `None` in
    /// `materials` (no base-color texture), draws untextured.
    fn primitive_material<'m>(
        &self,
        primitive: gltf::Primitive<'_>,
        materials: &'m [Option<GltfMaterial>],
    ) -> &'m GltfMaterial {
        match primitive.material().index() {
            Some(index) => materials
                .get(index)
                .and_then(Option::as_ref)
                .expect("every material a spawned mesh reads must have been inserted"),
            None => panic!("a primitive that samples a texture must name a material"),
        }
    }

    fn node(&self, node: usize) -> gltf::scene::Node<'_> {
        self.document
            .nodes()
            .nth(node)
            .expect("glTF node index in bounds")
    }

    fn mesh(&self, mesh: usize) -> gltf::Mesh<'_> {
        self.document
            .meshes()
            .nth(mesh)
            .expect("glTF mesh index in bounds")
    }

    fn primitive(&self, mesh: usize, primitive: usize) -> gltf::Primitive<'_> {
        self.mesh(mesh)
            .primitives()
            .nth(primitive)
            .expect("glTF primitive index in bounds")
    }

    /// Whether image `image` uploads as a texture a filtering sampler can
    /// sample — the same answer [`Self::insert_image`] bakes into the handle.
    fn image_filtering(&self, image: usize, device: &wgpu::Device) -> bool {
        image_format(&self.images[image], device).filtering
    }

    /// How image `image`'s texels have to be expanded once sampled: nothing for
    /// a texture that already carries RGB, and the matching base-color layout
    /// for one carrying a single luma channel, or luma and alpha.
    ///
    /// Both bit depths are affected. Neither `R8Unorm` nor `R16Float` can
    /// decode sRGB for itself — WebGPU attaches the transfer function only to
    /// four-channel formats — so the shader does it, on the assumption the
    /// grayscale source is sRGB-encoded just as an RGB one would be.
    fn image_luminance(&self, image: usize) -> BaseColorChannels {
        match self.images[image].format {
            gltf::image::Format::R8 | gltf::image::Format::R16 => BaseColorChannels::Luminance,
            gltf::image::Format::R8G8 | gltf::image::Format::R16G16 => {
                BaseColorChannels::LuminanceAlpha
            }
            _ => BaseColorChannels::Rgba,
        }
    }

    fn material(&self, material: usize) -> gltf::Material<'_> {
        self.document
            .materials()
            .nth(material)
            .expect("glTF material index in bounds")
    }
}

/// Round `value` up to the next multiple of `alignment`.
fn align_up(value: u32, alignment: u32) -> u32 {
    value.div_ceil(alignment) * alignment
}

/// The uploaded form of a glTF image: its texture format, and whether a
/// sampler is allowed to filter it.
///
/// glTF images arrive in ten pixel layouts — one to four channels of 8-bit,
/// 16-bit or 32-bit float — and there is a GPU format of nearly the same shape
/// for each, so an image keeps its channels and its precision instead of being
/// widened to RGBA8.
///
/// `filtering` is what the material's sampler and bind-group layout are built
/// around: a 32-bit float texture is an `unfilterable-float` unless the device
/// has `Features::FLOAT32_FILTERABLE`, and an unfilterable texture may only be
/// bound to a non-filtering sampler, which is a different bind-group layout
/// from a filtering one.
#[derive(Clone, Copy, Debug)]
struct ImageFormat {
    format: wgpu::TextureFormat,
    filtering: bool,
}

impl ImageFormat {
    /// The bytes one texel of the format occupies.
    fn texel_bytes(self) -> u32 {
        texel_bytes(self.format)
    }
}

/// The bytes one texel of an upload format occupies.
fn texel_bytes(format: wgpu::TextureFormat) -> u32 {
    format
        .block_copy_size(Some(wgpu::TextureAspect::All))
        .expect("a sampled color format has a texel block size")
}

/// Pick the upload format for a decoded glTF image.
///
/// The 8-bit layouts keep their channel count, and their pixels are uploaded
/// in the sRGB format of that width so sampling decodes them — which is what
/// the glTF spec asks of a base-color texture. `R8G8B8` widens to
/// `Rgba8UnormSrgb` because there is no three-channel sRGB format; `R8G8`
/// stays two channels, which is the luma/alpha pair the glTF loader decodes a
/// grayscale-alpha PNG into.
///
/// The 16-bit and 32-bit layouts upload as half and single precision floats.
/// Half float needs no device feature at all — unlike `R16Unorm` and friends,
/// which need `Features::TEXTURE_FORMAT_16BIT_NORM` — so an image never has to
/// fall back to a narrower format. `Rgba32Float` is the only upload whose
/// filtering depends on the device.
fn image_format(data: &gltf::image::Data, device: &wgpu::Device) -> ImageFormat {
    use gltf::image::Format as F;
    use wgpu::TextureFormat as T;
    let format = match data.format {
        F::R8 => T::R8Unorm,
        F::R8G8 => T::Rg8Unorm,
        // There is no three-channel sRGB format, so the three-channel layouts
        // widen by one opaque channel.
        F::R8G8B8 => T::Rgba8UnormSrgb,
        F::R8G8B8A8 => T::Rgba8UnormSrgb,
        F::R16 => T::R16Float,
        F::R16G16 => T::Rg16Float,
        // Nor a three-channel float one.
        F::R16G16B16 => T::Rgba16Float,
        F::R16G16B16A16 => T::Rgba16Float,
        F::R32G32B32FLOAT => T::Rgba32Float,
        F::R32G32B32A32FLOAT => T::Rgba32Float,
    };
    let filtering = matches!(
        format.sample_type(Some(wgpu::TextureAspect::All), Some(device.features())),
        Some(wgpu::TextureSampleType::Float { filterable: true })
    );
    ImageFormat { format, filtering }
}

/// Encode `data`'s pixels as the rows of a `format` texture.
///
/// A channel the source does not carry — the alpha of a three-channel image,
/// say — is filled with an opaque one, and every value is clamped to `0..=1`
/// on the way in, which is all the wider float formats need. Each row is
/// padded to the 256-byte stride a buffer-to-texture copy asks for.
fn encode_rows(data: &gltf::image::Data, format: wgpu::TextureFormat) -> Vec<u8> {
    let (width, height) = (data.width as usize, data.height as usize);
    let channels = source_channels(data.format);
    let texel = texel_bytes(format) as usize;
    let bytes_per_row = align_up(
        width as u32 * texel as u32,
        wgpu::COPY_BYTES_PER_ROW_ALIGNMENT,
    ) as usize;
    let mut rows = vec![0u8; bytes_per_row * height];
    let mut scratch = [0u8; 16];

    for (y, out) in rows.chunks_exact_mut(bytes_per_row).enumerate() {
        for x in 0..width {
            let pixel = &data.pixels[(y * width + x) * channels..];
            let bytes = encode_texel(data.format, format, pixel, &mut scratch);
            out[x * texel..][..texel].copy_from_slice(bytes);
        }
    }
    rows
}

/// The number of bytes one channel of a glTF pixel format occupies.
fn source_channel_bytes(format: gltf::image::Format) -> usize {
    match format {
        gltf::image::Format::R8
        | gltf::image::Format::R8G8
        | gltf::image::Format::R8G8B8
        | gltf::image::Format::R8G8B8A8 => 1,
        gltf::image::Format::R16
        | gltf::image::Format::R16G16
        | gltf::image::Format::R16G16B16
        | gltf::image::Format::R16G16B16A16 => 2,
        gltf::image::Format::R32G32B32FLOAT | gltf::image::Format::R32G32B32A32FLOAT => 4,
    }
}

/// The number of channels a glTF pixel format stores per texel.
fn source_channels(format: gltf::image::Format) -> usize {
    match format {
        gltf::image::Format::R8 | gltf::image::Format::R16 => 1,
        gltf::image::Format::R8G8 | gltf::image::Format::R16G16 => 2,
        gltf::image::Format::R8G8B8 | gltf::image::Format::R16G16B16 => 3,
        gltf::image::Format::R8G8B8A8 | gltf::image::Format::R16G16B16A16 => 4,
        gltf::image::Format::R32G32B32FLOAT => 3,
        gltf::image::Format::R32G32B32A32FLOAT => 4,
    }
}

/// The number of channels an upload format stores per texel.
fn format_channels(format: wgpu::TextureFormat) -> usize {
    match format {
        wgpu::TextureFormat::R8Unorm | wgpu::TextureFormat::R16Float => 1,
        wgpu::TextureFormat::Rg8Unorm | wgpu::TextureFormat::Rg16Float => 2,
        wgpu::TextureFormat::Rgba8UnormSrgb
        | wgpu::TextureFormat::Rgba16Float
        | wgpu::TextureFormat::Rgba32Float => 4,
        _ => unreachable!("a glTF image uploads in one of the formats above"),
    }
}

/// Encode one source texel into `format`, returning its bytes.
///
/// `scratch` backs the result, so a caller can reuse it across texels.
fn encode_texel<'a>(
    source: gltf::image::Format,
    format: wgpu::TextureFormat,
    pixel: &[u8],
    scratch: &'a mut [u8; 16],
) -> &'a [u8] {
    let channel = source_channel_bytes(source);
    let wanted = format_channels(format);
    let available = source_channels(source);
    for slot in 0..wanted {
        // A channel the source does not carry is an opaque one.
        let value = if slot < available {
            read_channel(source, &pixel[slot * channel..])
        } else {
            1.0
        };
        write_channel(format, slot, value, scratch);
    }
    &scratch[..texel_bytes(format) as usize]
}

/// One channel of a glTF pixel, as a `0..=1` float.
fn read_channel(format: gltf::image::Format, bytes: &[u8]) -> f32 {
    match format {
        gltf::image::Format::R8
        | gltf::image::Format::R8G8
        | gltf::image::Format::R8G8B8
        | gltf::image::Format::R8G8B8A8 => bytes[0] as f32 / 255.0,
        gltf::image::Format::R16
        | gltf::image::Format::R16G16
        | gltf::image::Format::R16G16B16
        | gltf::image::Format::R16G16B16A16 => {
            u16::from_le_bytes([bytes[0], bytes[1]]) as f32 / 65535.0
        }
        gltf::image::Format::R32G32B32FLOAT | gltf::image::Format::R32G32B32A32FLOAT => {
            f32::from_le_bytes([bytes[0], bytes[1], bytes[2], bytes[3]])
        }
    }
}

/// Write one `0..=1` channel of `format`'s `slot` into `scratch`.
fn write_channel(format: wgpu::TextureFormat, slot: usize, value: f32, scratch: &mut [u8; 16]) {
    // `f32::clamp` leaves NaN alone, and a NaN texel would poison every sample
    // it reaches, so fold it to the bottom of the range instead.
    let clamped = if value.is_nan() {
        0.0
    } else {
        value.clamp(0.0, 1.0)
    };
    match format {
        wgpu::TextureFormat::R8Unorm
        | wgpu::TextureFormat::Rg8Unorm
        | wgpu::TextureFormat::Rgba8UnormSrgb => {
            scratch[slot] = (clamped * 255.0).round() as u8;
        }
        wgpu::TextureFormat::R16Float
        | wgpu::TextureFormat::Rg16Float
        | wgpu::TextureFormat::Rgba16Float => {
            let bits = half::f16::from_f32(clamped).to_bits();
            scratch[slot * 2..][..2].copy_from_slice(&bits.to_le_bytes());
        }
        wgpu::TextureFormat::Rgba32Float => {
            scratch[slot * 4..][..4].copy_from_slice(&clamped.to_le_bytes());
        }
        _ => unreachable!("a glTF image uploads in one of the formats above"),
    }
}

/// The key a material bind group is built against.
///
/// Only `texture_filtering` shapes the material group's layout, so any key
/// carrying it works; this is the one a base-color-textured primitive's
/// [`UnlitGltf::pipeline_key`] derives to. The per-vertex channels a draw adds
/// do not enter the material group, and wgpu deduplicates identical layout
/// descriptors, so the group fits the pipeline whatever else its key reads.
fn textured_key(device: &wgpu::Device, filtering: bool) -> UnlitPipelineKey {
    UnlitPipelineKey::new(UnlitOptions {
        texture_filtering: filtering,
        ..UnlitOptions::standard(device)
    })
}

/// Whether a primitive samples its material's base-color texture.
///
/// A texture is sampled with the per-vertex UV, so a primitive that carries no
/// `TEXCOORD_0` cannot sample one and is drawn untextured even when its
/// material declares a texture. The answer is a property of the draw — the
/// material it names and the attributes it carries — rather than of anything a
/// caller picks, which is why it is derived here rather than stored in the key.
fn samples_base_color_texture(primitive: &gltf::Primitive<'_>) -> bool {
    primitive.get(&gltf::Semantic::TexCoords(0)).is_some()
        && primitive
            .material()
            .pbr_metallic_roughness()
            .base_color_texture()
            .is_some()
}

/// How many of a primitive's morph targets displace positions.
///
/// A target may displace only normals or tangents; those are ignored here, and
/// a target that carries no positions at all would contribute nothing to a
/// displacement array, so it is not counted and the mesh is not uploaded as
/// morphing. The count is therefore of the targets that *do* displace, in the
/// order they appear — which is the order the shader's weights are indexed in,
/// not the document's own target indices.
fn morph_target_count(primitive: &gltf::Primitive<'_>) -> usize {
    primitive
        .morph_targets()
        .filter(|target| target.positions().is_some())
        .count()
}

/// The alpha a `MASK` material's fragments are cut off at.
///
/// Returns `None` for every other `alphaMode`, for which there is nothing to
/// cut. The value is the material's own `alphaCutoff`, or the spec's default
/// when the material leaves it out.
fn alpha_cutoff(material: &gltf::Material<'_>) -> Option<f32> {
    (material.alpha_mode() == gltf::material::AlphaMode::Mask)
        .then(|| material.alpha_cutoff().unwrap_or(DEFAULT_ALPHA_CUTOFF))
}

/// Translate a glTF sampler to wgpu terms.
///
/// Unspecified filters default to `Linear`, unspecified wrap modes default to
/// `Repeat` — the glTF spec defaults.
fn sampler_descriptor(sampler: &gltf::texture::Sampler<'_>) -> wgpu::SamplerDescriptor<'static> {
    use gltf::texture::{MagFilter, MinFilter};
    let (min_filter, mipmap_filter) = match sampler.min_filter() {
        Some(MinFilter::Nearest) | Some(MinFilter::NearestMipmapNearest) => {
            (wgpu::FilterMode::Nearest, wgpu::MipmapFilterMode::Nearest)
        }
        Some(MinFilter::NearestMipmapLinear) => {
            (wgpu::FilterMode::Nearest, wgpu::MipmapFilterMode::Linear)
        }
        Some(MinFilter::LinearMipmapNearest) => {
            (wgpu::FilterMode::Linear, wgpu::MipmapFilterMode::Nearest)
        }
        Some(MinFilter::Linear) | Some(MinFilter::LinearMipmapLinear) | None => {
            (wgpu::FilterMode::Linear, wgpu::MipmapFilterMode::Linear)
        }
    };
    let mag_filter = match sampler.mag_filter() {
        Some(MagFilter::Nearest) => wgpu::FilterMode::Nearest,
        Some(MagFilter::Linear) | None => wgpu::FilterMode::Linear,
    };
    wgpu::SamplerDescriptor {
        label: None,
        address_mode_u: address_mode(sampler.wrap_s()),
        address_mode_v: address_mode(sampler.wrap_t()),
        address_mode_w: wgpu::AddressMode::ClampToEdge,
        mag_filter,
        min_filter,
        mipmap_filter,
        ..wgpu::SamplerDescriptor::default()
    }
}

fn address_mode(mode: gltf::texture::WrappingMode) -> wgpu::AddressMode {
    use gltf::texture::WrappingMode;
    match mode {
        WrappingMode::ClampToEdge => wgpu::AddressMode::ClampToEdge,
        WrappingMode::MirroredRepeat => wgpu::AddressMode::MirrorRepeat,
        WrappingMode::Repeat => wgpu::AddressMode::Repeat,
    }
}

/// The parent of every node, indexed by node index; `None` at a root.
///
/// glTF nodes have no parent handle, so the table is built from each node's
/// children list, which is the document's own statement of the hierarchy.
fn node_parents(document: &gltf::Document) -> Vec<Option<usize>> {
    let mut parents = vec![None; document.nodes().len()];
    for node in document.nodes() {
        for child in node.children() {
            parents[child.index()] = Some(node.index());
        }
    }
    parents
}

/// The local-space transform of every node, indexed by node index.
///
/// A matrix-form node transform is composed back into translation, rotation
/// and scale so that an animation can address a single one of them; the
/// decomposition is lossy for shear, which a `Transform` cannot carry.
fn node_locals(document: &gltf::Document) -> Vec<Transform> {
    document
        .nodes()
        .map(|node| decompose(glam::Mat4::from_cols_array_2d(&node.transform().matrix())))
        .collect()
}

/// The world-space matrix of every node, indexed by node index.
///
/// World matrices are accumulated root-first so a child always sees its
/// parent's matrix before its own is computed. Hierarchies are traversed
/// iteratively so arbitrarily deep scenes cannot overflow the stack.
fn node_world_matrices(document: &gltf::Document, parents: &[Option<usize>]) -> Vec<glam::Mat4> {
    let mut world = vec![glam::Mat4::IDENTITY; document.nodes().len()];
    for &root in &node_order(document, parents) {
        let node_data = document.nodes().nth(root).expect("node index in bounds");
        let local = glam::Mat4::from_cols_array_2d(&node_data.transform().matrix());
        world[root] = match parents[root] {
            Some(parent) => world[parent] * local,
            None => local,
        };
    }
    world
}

/// The node indices in an order where every node follows its parent.
///
/// The order is the same whether a node's own local matrix is the document's
/// or one an animation replaced it with, so recomposing from it can reuse the
/// cached `parents` table.
fn node_order(document: &gltf::Document, parents: &[Option<usize>]) -> Vec<usize> {
    let roots: Vec<usize> = (0..document.nodes().len())
        .filter(|&index| parents[index].is_none())
        .collect();
    let mut order = Vec::with_capacity(document.nodes().len());
    let mut stack = roots;
    while let Some(node) = stack.pop() {
        order.push(node);
        stack.extend(
            document
                .nodes()
                .nth(node)
                .expect("node index in bounds")
                .children()
                .map(|child| child.index()),
        );
    }
    order
}

/// Split a world-space matrix into the parts an entity's
/// [`Transform`] stores: translation, rotation and scale.
///
/// The decomposition is lossy for sheared matrices, and an axis scaled to
/// zero keeps an arbitrary rotation (there is no rotation to preserve along a
/// collapsed axis). Both are fine for an unlit renderer.
fn decompose(matrix: glam::Mat4) -> Transform {
    let translation = matrix.w_axis.truncate();
    let x = matrix.x_axis.truncate();
    let y = matrix.y_axis.truncate();
    let z = matrix.z_axis.truncate();
    let (sx, sy, sz) = (x.length(), y.length(), z.length());
    let rotation = glam::Quat::from_mat3(&glam::Mat3::from_cols(
        axis_or_identity(x, sx),
        axis_or_identity(y, sy),
        axis_or_identity(z, sz),
    ));
    Transform {
        translation,
        rotation,
        scale: glam::Vec3::new(sx, sy, sz),
    }
}

/// A unit-length axis, keeping its direction; identity when collapsed.
fn axis_or_identity(axis: glam::Vec3, length: f32) -> glam::Vec3 {
    if length > f32::EPSILON {
        axis / length
    } else {
        glam::Vec3::X // direction is meaningless along a zero-length axis
    }
}

/// How many values a keyframe stores under an interpolation.
///
/// A cubic spline stores three — in-tangent, value, out-tangent — and every
/// other interpolation stores one.
fn interpolation_stride(interpolation: Interpolation) -> usize {
    match interpolation {
        Interpolation::CubicSpline => 3,
        Interpolation::Step | Interpolation::Linear => 1,
    }
}

/// The keyframe interval `time` falls in, clamped to the keyframes' range.
///
/// Returns the left frame's index, always in `0..times.len() - 1` so a caller
/// can read both ends of the interval, and the fraction of the way through it.
fn keyframe(times: &[f32], time: f32) -> (usize, f32) {
    let last = times.len() - 1;
    if time <= times[0] {
        return (0, 0.0);
    }
    if time >= times[last] {
        return (last - 1, 1.0);
    }
    let frame = times
        .partition_point(|&key| key <= time)
        .saturating_sub(1)
        .min(last - 1);
    let span = times[frame + 1] - times[frame];
    let t = if span > 0.0 {
        (time - times[frame]) / span
    } else {
        0.0
    };
    (frame, t)
}

/// The keyframe a step interpolation holds at `time`: the latest one at or
/// before it, so the value jumps when `time` reaches the next.
fn step_frame(times: &[f32], time: f32) -> usize {
    times.partition_point(|&key| key <= time).saturating_sub(1)
}

/// A glTF `translation` or `scale` sampled at `time`.
fn sample_vec3(
    interpolation: Interpolation,
    times: &[f32],
    values: &[[f32; 3]],
    time: f32,
) -> [f32; 3] {
    let stride = interpolation_stride(interpolation);
    match interpolation {
        Interpolation::Step => values[step_frame(times, time) * stride],
        Interpolation::Linear => {
            let (frame, t) = keyframe(times, time);
            glam::Vec3::from(values[frame])
                .lerp(glam::Vec3::from(values[frame + 1]), t)
                .to_array()
        }
        Interpolation::CubicSpline => {
            let (frame, t) = keyframe(times, time);
            cubic_vec3(
                glam::Vec3::from(values[frame * 3 + 1]),
                glam::Vec3::from(values[frame * 3 + 2]),
                glam::Vec3::from(values[(frame + 1) * 3 + 1]),
                glam::Vec3::from(values[(frame + 1) * 3]),
                t,
                times[frame + 1] - times[frame],
            )
            .to_array()
        }
    }
}

/// A glTF `rotation` sampled at `time`, always a unit quaternion.
///
/// A linear rotation blends spherically, the spec's own requirement; a cubic
/// one interpolates each component and renormalizes.
fn sample_rotation(
    interpolation: Interpolation,
    times: &[f32],
    values: &[[f32; 4]],
    time: f32,
) -> glam::Quat {
    let stride = interpolation_stride(interpolation);
    match interpolation {
        Interpolation::Step => quat(values[step_frame(times, time) * stride]),
        Interpolation::Linear => {
            let (frame, t) = keyframe(times, time);
            quat(values[frame]).slerp(quat(values[frame + 1]), t)
        }
        Interpolation::CubicSpline => {
            let (frame, t) = keyframe(times, time);
            let value = cubic_vec4(
                glam::Vec4::from(values[frame * 3 + 1]),
                glam::Vec4::from(values[frame * 3 + 2]),
                glam::Vec4::from(values[(frame + 1) * 3 + 1]),
                glam::Vec4::from(values[(frame + 1) * 3]),
                t,
                times[frame + 1] - times[frame],
            );
            quat(value.to_array())
        }
    }
}

/// A glTF `weights` channel sampled at `time`, one weight per morph target.
///
/// A cubic keyframe groups its tangents and values kind-first — every
/// in-tangent, then every value, then every out-tangent — which is a different
/// layout from the one the node channels use.
fn sample_weights(
    interpolation: Interpolation,
    times: &[f32],
    values: &[f32],
    targets: usize,
    time: f32,
) -> Vec<f32> {
    match interpolation {
        Interpolation::Step => {
            let frame = step_frame(times, time);
            values[frame * targets..(frame + 1) * targets].to_vec()
        }
        Interpolation::Linear => {
            let (frame, t) = keyframe(times, time);
            let a = &values[frame * targets..(frame + 1) * targets];
            let b = &values[(frame + 1) * targets..(frame + 2) * targets];
            a.iter().zip(b).map(|(&a, &b)| a + (b - a) * t).collect()
        }
        Interpolation::CubicSpline => {
            let (frame, t) = keyframe(times, time);
            let span = times[frame + 1] - times[frame];
            let base = frame * 3 * targets;
            let next = (frame + 1) * 3 * targets;
            (0..targets)
                .map(|target| {
                    cubic_f32(
                        values[base + targets + target],
                        values[base + 2 * targets + target],
                        values[next + targets + target],
                        values[next + target],
                        t,
                        span,
                    )
                })
                .collect()
        }
    }
}

/// One segment of a cubic Hermite spline, the spec's `CUBICSPLINE` formula.
fn cubic_f32(v0: f32, b0: f32, v1: f32, a1: f32, t: f32, span: f32) -> f32 {
    let t2 = t * t;
    let t3 = t2 * t;
    (2.0 * t3 - 3.0 * t2 + 1.0) * v0
        + span * (t3 - 2.0 * t2 + t) * b0
        + (-2.0 * t3 + 3.0 * t2) * v1
        + span * (t3 - t2) * a1
}

/// [`cubic_f32`] over three components.
fn cubic_vec3(
    v0: glam::Vec3,
    b0: glam::Vec3,
    v1: glam::Vec3,
    a1: glam::Vec3,
    t: f32,
    span: f32,
) -> glam::Vec3 {
    let t2 = t * t;
    let t3 = t2 * t;
    (2.0 * t3 - 3.0 * t2 + 1.0) * v0
        + span * (t3 - 2.0 * t2 + t) * b0
        + (-2.0 * t3 + 3.0 * t2) * v1
        + span * (t3 - t2) * a1
}

/// [`cubic_f32`] over four components.
fn cubic_vec4(
    v0: glam::Vec4,
    b0: glam::Vec4,
    v1: glam::Vec4,
    a1: glam::Vec4,
    t: f32,
    span: f32,
) -> glam::Vec4 {
    let t2 = t * t;
    let t3 = t2 * t;
    (2.0 * t3 - 3.0 * t2 + 1.0) * v0
        + span * (t3 - 2.0 * t2 + t) * b0
        + (-2.0 * t3 + 3.0 * t2) * v1
        + span * (t3 - t2) * a1
}

/// A unit quaternion from the `[x, y, z, w]` a glTF rotation stores, falling
/// back to the identity when the value is degenerate.
fn quat(value: [f32; 4]) -> glam::Quat {
    let quat = glam::Quat::from_xyzw(value[0], value[1], value[2], value[3]);
    let length = quat.length();
    if length > f32::EPSILON {
        quat / length
    } else {
        glam::Quat::IDENTITY
    }
}

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

    /// A one-by-one image of `pixels` in the given format.
    fn image(format: gltf::image::Format, pixels: Vec<u8>) -> gltf::image::Data {
        gltf::image::Data {
            pixels,
            format,
            width: 1,
            height: 1,
        }
    }

    /// The texel `data` encodes into `format`, without its row padding.
    fn encode(format: wgpu::TextureFormat, data: &gltf::image::Data) -> Vec<u8> {
        let texel = texel_bytes(format) as usize;
        let mut out = Vec::new();
        let mut scratch = [0u8; 16];
        out.extend_from_slice(encode_texel(
            data.format,
            format,
            &data.pixels,
            &mut scratch,
        ));
        assert_eq!(out.len(), texel);
        out
    }

    #[test]
    fn a_luma_alpha_image_keeps_luma_in_every_channel_slot_it_has() {
        use gltf::image::Format as F;
        // The loader decodes a grayscale-alpha PNG into two channels, luma then
        // alpha, and `Rg8Unorm` keeps both — the bug this replaces wrote them
        // out as red and green.
        let data = image(F::R8G8, vec![0x40, 0x80]);
        assert_eq!(
            encode(wgpu::TextureFormat::Rg8Unorm, &data),
            vec![0x40, 0x80]
        );
    }

    #[test]
    fn a_three_channel_image_gains_an_opaque_alpha() {
        use gltf::image::Format as F;
        let data = image(F::R8G8B8, vec![1, 2, 3]);
        assert_eq!(
            encode(wgpu::TextureFormat::Rgba8UnormSrgb, &data),
            vec![1, 2, 3, 255]
        );
    }

    #[test]
    fn a_sixteen_bit_image_narrows_to_half_float() {
        use gltf::image::Format as F;
        let data = image(F::R16G16B16A16, vec![0x00, 0x80, 0x00, 0x40, 0, 0, 0, 0]);
        let encoded = encode(wgpu::TextureFormat::Rgba16Float, &data);
        // `32768 / 65535` is a half above a half, `16384 / 65535` a quarter,
        // and the trailing zeroes stay zero.
        let channels = encoded
            .as_chunks::<2>()
            .0
            .iter()
            .map(|bytes| half::f16::from_bits(u16::from_le_bytes(*bytes)).to_f32())
            .collect::<Vec<_>>();
        assert!((channels[0] - 0.5).abs() < 1e-3, "{channels:?}");
        assert!((channels[1] - 0.25).abs() < 1e-3, "{channels:?}");
        assert_eq!(&channels[2..], &[0.0, 0.0]);
    }

    #[test]
    fn a_float_image_out_of_range_clamps() {
        use gltf::image::Format as F;
        let data = image(F::R32G32B32A32FLOAT, {
            let mut pixels = Vec::new();
            pixels.extend_from_slice(&(-1.0f32).to_le_bytes());
            pixels.extend_from_slice(&0.5f32.to_le_bytes());
            pixels.extend_from_slice(&2.0f32.to_le_bytes());
            pixels.extend_from_slice(&f32::NAN.to_le_bytes());
            pixels
        });
        let encoded = encode(wgpu::TextureFormat::Rgba32Float, &data);
        let channels = encoded
            .as_chunks::<4>()
            .0
            .iter()
            .map(|bytes| f32::from_le_bytes(*bytes))
            .collect::<Vec<_>>();
        assert_eq!(channels, vec![0.0, 0.5, 1.0, 0.0]);
    }

    /// A GLB holding one skinned triangle: node 0 draws mesh 0 under skin 0,
    /// whose single joint is node 1 at `translation`.
    ///
    /// The buffer carries the positions, the joint indices and weights that
    /// name the joint, the skin's inverse bind matrix (the identity, so the
    /// joint's own transform is what moves the vertex) and the index buffer.
    /// The JSON is written out by hand rather than through a serializer,
    /// because the crate carries no JSON dependency beyond the loader's own.
    fn skinned_document(translation: [f32; 3]) -> Vec<u8> {
        let mut bin = Vec::new();
        let mut views: Vec<String> = Vec::new();
        let mut add = |bin: &mut Vec<u8>, bytes: &[u8], target: Option<u32>| {
            while !bin.len().is_multiple_of(4) {
                bin.push(0);
            }
            let target = target.map_or(String::new(), |target| format!(r#", "target": {target}"#));
            views.push(format!(
                r#"{{ "buffer": 0, "byteOffset": {}, "byteLength": {}{target} }}"#,
                bin.len(),
                bytes.len()
            ));
            bin.extend_from_slice(bytes);
            views.len() - 1
        };

        let positions: Vec<u8> = [0.0f32, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0]
            .iter()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let position_view = add(&mut bin, &positions, Some(34962));
        let joints: Vec<u8> = (0..3)
            .flat_map(|_| [0u16, 0, 0, 0])
            .flat_map(u16::to_le_bytes)
            .collect();
        let joint_view = add(&mut bin, &joints, Some(34962));
        let weights: Vec<u8> = (0..3)
            .flat_map(|_| [1.0f32, 0.0, 0.0, 0.0])
            .flat_map(f32::to_le_bytes)
            .collect();
        let weight_view = add(&mut bin, &weights, Some(34962));
        let identity: Vec<u8> = [
            1.0f32, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0,
        ]
        .iter()
        .flat_map(|f| f.to_le_bytes())
        .collect();
        let bind_view = add(&mut bin, &identity, None);
        let indices: Vec<u8> = [0u16, 1, 2].iter().flat_map(|i| i.to_le_bytes()).collect();
        let index_view = add(&mut bin, &indices, Some(34963));
        while !bin.len().is_multiple_of(4) {
            bin.push(0);
        }

        let json = format!(
            r#"{{"asset":{{"version":"2.0"}},
                "scene":0,
                "scenes":[{{"nodes":[0]}}],
                "nodes":[{{"mesh":0,"skin":0}},
                         {{"name":"joint","translation":[{x},{y},{z}]}}],
                "skins":[{{"joints":[1],"inverseBindMatrices":3}}],
                "meshes":[{{"primitives":[{{"attributes":{{"POSITION":0,"JOINTS_0":1,"WEIGHTS_0":2}},
                                            "indices":4}}]}}],
                "accessors":[{{"bufferView":{position_view},"componentType":5126,"count":3,
                               "type":"VEC3","min":[0.0,0.0,0.0],"max":[1.0,1.0,0.0]}},
                             {{"bufferView":{joint_view},"componentType":5123,"count":3,"type":"VEC4"}},
                             {{"bufferView":{weight_view},"componentType":5126,"count":3,"type":"VEC4"}},
                             {{"bufferView":{bind_view},"componentType":5126,"count":1,"type":"MAT4"}},
                             {{"bufferView":{index_view},"componentType":5123,"count":3,"type":"SCALAR"}}],
                "bufferViews":[{}],
                "buffers":[{{"byteLength":{}}}]}}"#,
            views.join(", "),
            bin.len(),
            x = translation[0],
            y = translation[1],
            z = translation[2],
        );
        let mut json = json.into_bytes();
        while !json.len().is_multiple_of(4) {
            json.push(b' ');
        }
        let mut glb = Vec::new();
        glb.extend_from_slice(&0x4654_6C67u32.to_le_bytes());
        glb.extend_from_slice(&2u32.to_le_bytes());
        glb.extend_from_slice(&((12 + 8 + json.len() + 8 + bin.len()) as u32).to_le_bytes());
        glb.extend_from_slice(&(json.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x4E4F_534Au32.to_le_bytes());
        glb.extend_from_slice(&json);
        glb.extend_from_slice(&(bin.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x004E_4942u32.to_le_bytes());
        glb.extend_from_slice(&bin);
        glb
    }

    /// A GLB holding one triangle whose mesh declares `targets` positions
    /// morph targets and states `mesh_weights` on the mesh, optionally
    /// overridden by `node_weights` on the node that draws it.
    ///
    /// Each target displaces a single vertex by `[0, 1, 0]`-ish amounts so a
    /// wrong target stride reads the wrong displacement. A target passed as
    /// `None` declares no positions at all, which is how a normals-only target
    /// is written here.
    fn morphed_document(
        mesh_weights: Option<&[f32]>,
        node_weights: Option<&[f32]>,
        targets: &[Option<[f32; 3]>],
    ) -> Vec<u8> {
        let mut bin = Vec::new();
        let mut views: Vec<String> = Vec::new();
        let mut add = |bin: &mut Vec<u8>, bytes: &[u8], target: Option<u32>| {
            while !bin.len().is_multiple_of(4) {
                bin.push(0);
            }
            let target = target.map_or(String::new(), |target| format!(r#", "target": {target}"#));
            views.push(format!(
                r#"{{ "buffer": 0, "byteOffset": {}, "byteLength": {}{target} }}"#,
                bin.len(),
                bytes.len()
            ));
            bin.extend_from_slice(bytes);
            views.len() - 1
        };

        let positions: Vec<u8> = [0.0f32, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0]
            .iter()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let mut accessors = vec![format!(
            r#"{{ "bufferView": {}, "componentType": 5126, "count": 3, "type": "VEC3",
                 "min": [0.0, 0.0, 0.0], "max": [1.0, 1.0, 0.0] }}"#,
            add(&mut bin, &positions, Some(34962))
        )];
        // One accessor per target that displaces, in target order.
        let mut displacement_accessors = Vec::new();
        for target in targets {
            let Some(delta) = target else {
                continue;
            };
            let mut deltas = [0.0f32; 9];
            deltas[3] = delta[0];
            deltas[4] = delta[1];
            deltas[5] = delta[2];
            let bytes: Vec<u8> = deltas.iter().flat_map(|f| f.to_le_bytes()).collect();
            let view = add(&mut bin, &bytes, None);
            displacement_accessors.push(accessors.len());
            accessors.push(format!(
                r#"{{ "bufferView": {view}, "componentType": 5126, "count": 3, "type": "VEC3" }}"#
            ));
        }
        let indices: Vec<u8> = [0u16, 1, 2].iter().flat_map(|i| i.to_le_bytes()).collect();
        let index_accessor = accessors.len();
        accessors.push(format!(
            r#"{{ "bufferView": {}, "componentType": 5123, "count": 3, "type": "SCALAR" }}"#,
            add(&mut bin, &indices, Some(34963))
        ));
        while !bin.len().is_multiple_of(4) {
            bin.push(0);
        }

        // A target that carries no positions is declared as an empty object,
        // which is what glTF allows and what the loader must skip.
        let target_json: Vec<String> = targets
            .iter()
            .enumerate()
            .map(|(index, target)| match target {
                Some(_) => format!(r#"{{"POSITION":{}}}"#, displacement_accessors[index]),
                None => "{}".to_string(),
            })
            .collect();
        let mesh_weights = mesh_weights
            .map(|weights| {
                let list: Vec<String> = weights.iter().map(|w| w.to_string()).collect();
                format!(r#","weights":[{}]"#, list.join(","))
            })
            .unwrap_or_default();
        let node_weights = node_weights
            .map(|weights| {
                let list: Vec<String> = weights.iter().map(|w| w.to_string()).collect();
                format!(r#","weights":[{}]"#, list.join(","))
            })
            .unwrap_or_default();
        let json = format!(
            r#"{{"asset":{{"version":"2.0"}},
                "scene":0,
                "scenes":[{{"nodes":[0]}}],
                "nodes":[{{"mesh":0{node_weights}}}],
                "meshes":[{{"primitives":[{{"attributes":{{"POSITION":0}},
                                            "indices":{index_accessor},
                                            "targets":[{}]}}]{mesh_weights}}}],
                "accessors":[{}],
                "bufferViews":[{}],
                "buffers":[{{"byteLength":{}}}]}}"#,
            target_json.join(", "),
            accessors.join(", "),
            views.join(", "),
            bin.len(),
        );
        let mut json = json.into_bytes();
        while !json.len().is_multiple_of(4) {
            json.push(b' ');
        }
        let mut glb = Vec::new();
        glb.extend_from_slice(&0x4654_6C67u32.to_le_bytes());
        glb.extend_from_slice(&2u32.to_le_bytes());
        glb.extend_from_slice(&((12 + 8 + json.len() + 8 + bin.len()) as u32).to_le_bytes());
        glb.extend_from_slice(&(json.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x4E4F_534Au32.to_le_bytes());
        glb.extend_from_slice(&json);
        glb.extend_from_slice(&(bin.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x004E_4942u32.to_le_bytes());
        glb.extend_from_slice(&bin);
        glb
    }

    /// A GLB holding one triangle whose material states `doubleSided` or not.
    fn sided_document(double_sided: bool) -> Vec<u8> {
        let mut bin = Vec::new();
        let mut views: Vec<String> = Vec::new();
        let mut add = |bin: &mut Vec<u8>, bytes: &[u8], target: Option<u32>| {
            while !bin.len().is_multiple_of(4) {
                bin.push(0);
            }
            let target = target.map_or(String::new(), |target| format!(r#", "target": {target}"#));
            views.push(format!(
                r#"{{ "buffer": 0, "byteOffset": {}, "byteLength": {}{target} }}"#,
                bin.len(),
                bytes.len()
            ));
            bin.extend_from_slice(bytes);
            views.len() - 1
        };
        let positions: Vec<u8> = [0.0f32, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0]
            .iter()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let position_view = add(&mut bin, &positions, Some(34962));
        let indices: Vec<u8> = [0u16, 1, 2].iter().flat_map(|i| i.to_le_bytes()).collect();
        let index_view = add(&mut bin, &indices, Some(34963));
        while !bin.len().is_multiple_of(4) {
            bin.push(0);
        }
        let sided = if double_sided {
            r#","doubleSided":true"#
        } else {
            ""
        };
        let json = format!(
            r#"{{"asset":{{"version":"2.0"}},
                "scene":0,
                "scenes":[{{"nodes":[0]}}],
                "nodes":[{{"mesh":0}}],
                "meshes":[{{"primitives":[{{"attributes":{{"POSITION":0}},
                                            "indices":1,
                                            "material":0}}]}}],
                "materials":[{{"pbrMetallicRoughness":{{"baseColorFactor":[1.0,1.0,1.0,1.0]}}{sided}}}],
                "accessors":[
                    {{"bufferView":{position_view},"componentType":5126,"count":3,"type":"VEC3",
                      "min":[0.0,0.0,0.0],"max":[1.0,1.0,0.0]}},
                    {{"bufferView":{index_view},"componentType":5123,"count":3,"type":"SCALAR"}}],
                "bufferViews":[{}],
                "buffers":[{{"byteLength":{}}}]}}"#,
            views.join(", "),
            bin.len(),
        );
        let mut json = json.into_bytes();
        while !json.len().is_multiple_of(4) {
            json.push(b' ');
        }
        let mut glb = Vec::new();
        glb.extend_from_slice(&0x4654_6C67u32.to_le_bytes());
        glb.extend_from_slice(&2u32.to_le_bytes());
        glb.extend_from_slice(&((12 + 8 + json.len() + 8 + bin.len()) as u32).to_le_bytes());
        glb.extend_from_slice(&(json.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x4E4F_534Au32.to_le_bytes());
        glb.extend_from_slice(&json);
        glb.extend_from_slice(&(bin.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x004E_4942u32.to_le_bytes());
        glb.extend_from_slice(&bin);
        glb
    }

    #[test]
    fn a_double_sided_material_culls_nothing() {
        let (device, _queue) = wgpu::Device::noop(&wgpu::DeviceDescriptor::default());

        let single = UnlitGltf::from_bytes(&sided_document(false))
            .expect("the single-sided document parses");
        assert_eq!(
            single
                .pipeline_key(&device, 0, 0)
                .options
                .primitive
                .cull_mode,
            Some(wgpu::Face::Back),
            "a material that is not double-sided culls back faces"
        );

        let double =
            UnlitGltf::from_bytes(&sided_document(true)).expect("the sided document parses");
        assert_eq!(
            double
                .pipeline_key(&device, 0, 0)
                .options
                .primitive
                .cull_mode,
            None,
            "a double-sided material culls nothing"
        );
    }

    #[test]
    fn culling_alone_distinguishes_the_two_sided_variants() {
        let (device, _queue) = wgpu::Device::noop(&wgpu::DeviceDescriptor::default());
        let single = UnlitGltf::from_bytes(&sided_document(false))
            .expect("the single-sided document parses")
            .pipeline_key(&device, 0, 0);
        let double = UnlitGltf::from_bytes(&sided_document(true))
            .expect("the double-sided document parses")
            .pipeline_key(&device, 0, 0);
        assert_ne!(
            single, double,
            "the two sides must be separate variants so each keeps its own pipeline"
        );
        assert_eq!(
            single.options.primitive.cull_mode,
            Some(wgpu::Face::Back),
            "a single-sided primitive culls its back faces"
        );
        assert_eq!(
            double.options.primitive.cull_mode, None,
            "a double-sided material culls nothing"
        );
        // Nothing but the cull mode sets the two apart. The options are
        // hashed, so the rewrite goes through `update`: the word follows the
        // changed field and the equality check stays sound.
        let mut normalized = single.options.clone();
        normalized
            .update(|options| options.primitive.cull_mode = double.options.primitive.cull_mode);
        assert_eq!(normalized, double.options);
    }

    #[test]
    fn a_morph_target_makes_the_mesh_morph() {
        let gltf = UnlitGltf::from_bytes(&morphed_document(None, None, &[Some([0.0, 1.0, 0.0])]))
            .expect("the morphed document parses");
        let primitive = gltf.primitive(0, 0);
        assert_eq!(morph_target_count(&primitive), 1);
    }

    #[test]
    fn a_target_without_positions_does_not_make_the_mesh_morph() {
        // A normals-only target displaces nothing this loader reads, so the
        // mesh stays in the rigid path rather than drawing an empty morph.
        let gltf = UnlitGltf::from_bytes(&morphed_document(None, None, &[None]))
            .expect("the morphed document parses");
        let primitive = gltf.primitive(0, 0);
        assert_eq!(morph_target_count(&primitive), 0);
        assert!(gltf.morph_weights(0).is_empty());
    }

    #[test]
    fn a_node_overrides_the_mesh_weights() {
        let gltf = UnlitGltf::from_bytes(&morphed_document(
            Some(&[0.25]),
            Some(&[0.75]),
            &[Some([0.0, 1.0, 0.0])],
        ))
        .expect("the morphed document parses");
        assert_eq!(gltf.morph_weights(0), vec![0.75]);

        let gltf = UnlitGltf::from_bytes(&morphed_document(
            Some(&[0.25]),
            None,
            &[Some([1.0, 0.0, 0.0])],
        ))
        .expect("the morphed document parses");
        assert_eq!(gltf.morph_weights(0), vec![0.25]);
    }

    #[test]
    fn morph_weights_are_padded_to_the_target_count() {
        // A document that states fewer weights than it has targets would make
        // the renderer reject the mesh, so the shortfall is padded with the
        // undeformed weight rather than passed on.
        let gltf = UnlitGltf::from_bytes(&morphed_document(
            Some(&[0.5]),
            None,
            &[Some([0.0, 1.0, 0.0]), Some([1.0, 0.0, 0.0])],
        ))
        .expect("the morphed document parses");
        assert_eq!(gltf.morph_weights(0), vec![0.5, 0.0]);

        // And one that states more is truncated, for the same reason.
        let gltf = UnlitGltf::from_bytes(&morphed_document(
            Some(&[0.5, 1.0]),
            None,
            &[Some([0.0, 1.0, 0.0])],
        ))
        .expect("the morphed document parses");
        assert_eq!(gltf.morph_weights(0), vec![0.5]);
    }

    #[test]
    fn a_mesh_without_targets_starts_undeformed() {
        let gltf = UnlitGltf::from_bytes(&morphed_document(None, None, &[]))
            .expect("the morphed document parses");
        assert!(gltf.morph_weights(0).is_empty());
        assert_eq!(morph_target_count(&gltf.primitive(0, 0)), 0);
    }

    #[test]
    fn a_rest_skin_pose_deforms_nothing() {
        // The joint sits at rest, so its world matrix is the inverse bind
        // matrix's inverse and every joint matrix comes out the identity: the
        // pose a document uploads at rest has to leave the mesh exactly where
        // it is.
        let gltf = UnlitGltf::from_bytes(&skinned_document([0.0, 0.0, 0.0]))
            .expect("the skinned document parses");
        let pose = gltf.skin_pose(0);
        assert_eq!(pose.matrices.len(), 1, "the skin has one joint");
        assert!(
            pose.matrices[0].abs_diff_eq(glam::Mat4::IDENTITY, 1e-6),
            "{}",
            pose.matrices[0]
        );
    }

    #[test]
    fn a_joint_away_from_its_bind_pose_moves_the_mesh() {
        // An inverse bind matrix records where a joint was *bound*, so a joint
        // node that sits somewhere else pulls the vertices it weights to
        // itself: the pose is the joint's world matrix against its bind
        // matrix, not the identity.
        let gltf = UnlitGltf::from_bytes(&skinned_document([3.0, -2.0, 1.0]))
            .expect("the skinned document parses");
        let pose = gltf.skin_pose(0);
        assert!(
            pose.matrices[0].abs_diff_eq(
                glam::Mat4::from_translation(glam::Vec3::new(3.0, -2.0, 1.0)),
                1e-6
            ),
            "{}",
            pose.matrices[0]
        );
    }

    #[test]
    fn a_skinned_primitive_declares_its_joint_stream() {
        // The primitive carries JOINTS_0 and WEIGHTS_0, so the mesh is
        // uploaded with them and the variant a draw resolves reads them.
        let gltf = UnlitGltf::from_bytes(&skinned_document([0.0, 0.0, 0.0]))
            .expect("the skinned document parses");
        let primitive = gltf.primitive(0, 0);
        assert!(primitive.get(&gltf::Semantic::Joints(0)).is_some());
        assert!(primitive.get(&gltf::Semantic::Weights(0)).is_some());
        assert!(!samples_base_color_texture(&primitive));
    }

    #[test]
    fn a_node_without_a_skin_has_no_pose() {
        let gltf = UnlitGltf::from_bytes(&skinned_document([0.0, 0.0, 0.0]))
            .expect("the skinned document parses");
        assert_eq!(gltf.skin_joint_count(0), Some(1));
        assert_eq!(gltf.skin_joint_count(1), None);
        assert!(gltf.skin_pose(1).matrices.is_empty());
    }

    #[test]
    fn rows_are_padded_to_the_copy_stride() {
        use gltf::image::Format as F;
        let data = gltf::image::Data {
            pixels: vec![7; 2 * 4],
            format: F::R8G8B8A8,
            width: 2,
            height: 1,
        };
        let rows = encode_rows(&data, wgpu::TextureFormat::Rgba8UnormSrgb);
        assert_eq!(rows.len(), wgpu::COPY_BYTES_PER_ROW_ALIGNMENT as usize);
        assert_eq!(&rows[..8], &[7; 8]);
        assert_eq!(&rows[8..], &vec![0; 256 - 8][..]);
    }
    // -- animation ------------------------------------------------------------

    /// Asserts a keyframe pair matches, allowing for the float fraction.
    fn assert_keyframe(actual: (usize, f32), expected: (usize, f32)) {
        assert_eq!(actual.0, expected.0, "{actual:?} vs {expected:?}");
        assert!(
            (actual.1 - expected.1).abs() < 1e-6,
            "{actual:?} vs {expected:?}"
        );
    }

    /// 4-aligns `bin`, records a bufferView for `bytes` and returns its index.
    fn add_view(
        bin: &mut Vec<u8>,
        views: &mut Vec<String>,
        bytes: &[u8],
        target: Option<u32>,
    ) -> usize {
        while !bin.len().is_multiple_of(4) {
            bin.push(0);
        }
        let target = target.map_or(String::new(), |target| format!(r#", "target": {target}"#));
        views.push(format!(
            r#"{{ "buffer": 0, "byteOffset": {}, "byteLength": {}{target} }}"#,
            bin.len(),
            bytes.len()
        ));
        bin.extend_from_slice(bytes);
        views.len() - 1
    }

    /// Wraps a JSON document and its binary chunk into a GLB.
    fn glb(json: String, bin: &[u8]) -> Vec<u8> {
        let mut json = json.into_bytes();
        while !json.len().is_multiple_of(4) {
            json.push(b' ');
        }
        let mut glb = Vec::new();
        glb.extend_from_slice(&0x4654_6C67u32.to_le_bytes());
        glb.extend_from_slice(&2u32.to_le_bytes());
        glb.extend_from_slice(&((12 + 8 + json.len() + 8 + bin.len()) as u32).to_le_bytes());
        glb.extend_from_slice(&(json.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x4E4F_534Au32.to_le_bytes());
        glb.extend_from_slice(&json);
        glb.extend_from_slice(&(bin.len() as u32).to_le_bytes());
        glb.extend_from_slice(&0x004E_4942u32.to_le_bytes());
        glb.extend_from_slice(bin);
        glb
    }

    /// The `min` and `max` a glTF accessor needs, as a JSON fragment.
    fn bounds(times: &[f32]) -> String {
        let min = times.first().copied().unwrap_or(0.0);
        let max = times.last().copied().unwrap_or(0.0);
        format!(r#", "min": [{min}], "max": [{max}]"#)
    }

    /// Adds the triangle both an animated mesh and a static one draw, and
    /// pushes its position and index accessors, returning the index accessor.
    fn add_triangle(
        bin: &mut Vec<u8>,
        views: &mut Vec<String>,
        accessors: &mut Vec<String>,
    ) -> usize {
        let positions: Vec<u8> = [0.0f32, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0]
            .iter()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let position = add_view(bin, views, &positions, Some(34962));
        accessors.push(format!(
            r#"{{ "bufferView": {position}, "componentType": 5126, "count": 3, "type": "VEC3", "min": [0.0, 0.0, 0.0], "max": [1.0, 1.0, 0.0] }}"#
        ));
        let index_accessor = accessors.len();
        let indices: Vec<u8> = [0u16, 1, 2].iter().flat_map(|i| i.to_le_bytes()).collect();
        let index = add_view(bin, views, &indices, Some(34963));
        accessors.push(format!(
            r#"{{ "bufferView": {index}, "componentType": 5123, "count": 3, "type": "SCALAR" }}"#
        ));
        index_accessor
    }

    /// A GLB whose node 0 carries a translation animation and whose child node
    /// 1 — drawing a triangle — carries a rotation animation.
    ///
    /// One document therefore exercises both node paths and the parent-to-child
    /// recomposition. `translations` and `rotations` hold one entry per
    /// keyframe, except under `CUBICSPLINE`, where each keyframe contributes
    /// its in-tangent, value and out-tangent in that order.
    fn parented_animated_document(
        interpolation: &str,
        times: &[f32],
        translations: &[[f32; 3]],
        rotations: &[[f32; 4]],
    ) -> Vec<u8> {
        let mut bin = Vec::new();
        let mut views: Vec<String> = Vec::new();
        let mut accessors: Vec<String> = Vec::new();

        let input_bytes: Vec<u8> = times.iter().flat_map(|t| t.to_le_bytes()).collect();
        let input = add_view(&mut bin, &mut views, &input_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {input}, "componentType": 5126, "count": {}, "type": "SCALAR"{} }}"#,
            times.len(),
            bounds(times),
        ));
        let translation_bytes: Vec<u8> = translations
            .iter()
            .flatten()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let translation = add_view(&mut bin, &mut views, &translation_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {translation}, "componentType": 5126, "count": {}, "type": "VEC3" }}"#,
            translations.len()
        ));
        let rotation_bytes: Vec<u8> = rotations
            .iter()
            .flatten()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let rotation = add_view(&mut bin, &mut views, &rotation_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {rotation}, "componentType": 5126, "count": {}, "type": "VEC4" }}"#,
            rotations.len()
        ));
        let index_accessor = add_triangle(&mut bin, &mut views, &mut accessors);

        let json = format!(
            r#"{{"asset":{{"version":"2.0"}},
                "scene":0,
                "scenes":[{{"nodes":[0]}}],
                "nodes":[{{"children":[1]}},{{"mesh":0}}],
                "meshes":[{{"primitives":[{{"attributes":{{"POSITION":3}},"indices":{index_accessor}}}]}}],
                "accessors":[{}],
                "bufferViews":[{}],
                "buffers":[{{"byteLength":{}}}],
                "animations":[{{"name":"walk",
                    "channels":[{{"sampler":0,"target":{{"node":0,"path":"translation"}}}},
                                {{"sampler":1,"target":{{"node":1,"path":"rotation"}}}}],
                    "samplers":[{{"input":0,"interpolation":"{interpolation}","output":1}},
                                {{"input":0,"interpolation":"{interpolation}","output":2}}]}}]}}"#,
            accessors.join(", "),
            views.join(", "),
            bin.len(),
        );
        glb(json, &bin)
    }

    /// A GLB holding one skinned triangle whose single joint node carries a
    /// translation animation.
    fn animated_skinned_document(times: &[f32], translations: &[[f32; 3]]) -> Vec<u8> {
        let mut bin = Vec::new();
        let mut views: Vec<String> = Vec::new();
        let mut accessors: Vec<String> = Vec::new();

        let input_bytes: Vec<u8> = times.iter().flat_map(|t| t.to_le_bytes()).collect();
        let input = add_view(&mut bin, &mut views, &input_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {input}, "componentType": 5126, "count": {}, "type": "SCALAR"{} }}"#,
            times.len(),
            bounds(times),
        ));
        let translation_bytes: Vec<u8> = translations
            .iter()
            .flatten()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let translation = add_view(&mut bin, &mut views, &translation_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {translation}, "componentType": 5126, "count": {}, "type": "VEC3" }}"#,
            translations.len()
        ));

        let positions: Vec<u8> = [0.0f32, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0]
            .iter()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let position = add_view(&mut bin, &mut views, &positions, Some(34962));
        accessors.push(format!(
            r#"{{ "bufferView": {position}, "componentType": 5126, "count": 3, "type": "VEC3", "min": [0.0, 0.0, 0.0], "max": [1.0, 1.0, 0.0] }}"#
        ));
        let joints: Vec<u8> = (0..3)
            .flat_map(|_| [0u16, 0, 0, 0])
            .flat_map(u16::to_le_bytes)
            .collect();
        let joint = add_view(&mut bin, &mut views, &joints, Some(34962));
        accessors.push(format!(
            r#"{{ "bufferView": {joint}, "componentType": 5123, "count": 3, "type": "VEC4" }}"#
        ));
        let weights: Vec<u8> = (0..3)
            .flat_map(|_| [1.0f32, 0.0, 0.0, 0.0])
            .flat_map(f32::to_le_bytes)
            .collect();
        let weight = add_view(&mut bin, &mut views, &weights, Some(34962));
        accessors.push(format!(
            r#"{{ "bufferView": {weight}, "componentType": 5126, "count": 3, "type": "VEC4" }}"#
        ));
        let identity: Vec<u8> = [
            1.0f32, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0,
        ]
        .iter()
        .flat_map(|f| f.to_le_bytes())
        .collect();
        let bind = add_view(&mut bin, &mut views, &identity, None);
        accessors.push(format!(
            r#"{{ "bufferView": {bind}, "componentType": 5126, "count": 1, "type": "MAT4" }}"#
        ));
        let index_accessor = accessors.len();
        let indices: Vec<u8> = [0u16, 1, 2].iter().flat_map(|i| i.to_le_bytes()).collect();
        let index = add_view(&mut bin, &mut views, &indices, Some(34963));
        accessors.push(format!(
            r#"{{ "bufferView": {index}, "componentType": 5123, "count": 3, "type": "SCALAR" }}"#
        ));

        let json = format!(
            r#"{{"asset":{{"version":"2.0"}},
                "scene":0,
                "scenes":[{{"nodes":[0]}}],
                "nodes":[{{"mesh":0,"skin":0}},{{"name":"joint"}}],
                "skins":[{{"joints":[1],"inverseBindMatrices":5}}],
                "meshes":[{{"primitives":[{{"attributes":{{"POSITION":2,"JOINTS_0":3,"WEIGHTS_0":4}},
                                            "indices":{index_accessor}}}]}}],
                "accessors":[{}],
                "bufferViews":[{}],
                "buffers":[{{"byteLength":{}}}],
                "animations":[{{"name":"walk",
                    "channels":[{{"sampler":0,"target":{{"node":1,"path":"translation"}}}}],
                    "samplers":[{{"input":0,"interpolation":"LINEAR","output":1}}]}}]}}"#,
            accessors.join(", "),
            views.join(", "),
            bin.len(),
        );
        glb(json, &bin)
    }

    /// A GLB holding one triangle with a single morph target whose weight the
    /// document animates.
    fn animated_morphed_document(times: &[f32], weights: &[f32]) -> Vec<u8> {
        let mut bin = Vec::new();
        let mut views: Vec<String> = Vec::new();
        let mut accessors: Vec<String> = Vec::new();

        let input_bytes: Vec<u8> = times.iter().flat_map(|t| t.to_le_bytes()).collect();
        let input = add_view(&mut bin, &mut views, &input_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {input}, "componentType": 5126, "count": {}, "type": "SCALAR"{} }}"#,
            times.len(),
            bounds(times),
        ));
        let weight_bytes: Vec<u8> = weights.iter().flat_map(|w| w.to_le_bytes()).collect();
        let weight = add_view(&mut bin, &mut views, &weight_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {weight}, "componentType": 5126, "count": {}, "type": "SCALAR" }}"#,
            weights.len()
        ));

        let positions: Vec<u8> = [0.0f32, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0, 0.0]
            .iter()
            .flat_map(|f| f.to_le_bytes())
            .collect();
        let position = add_view(&mut bin, &mut views, &positions, Some(34962));
        accessors.push(format!(
            r#"{{ "bufferView": {position}, "componentType": 5126, "count": 3, "type": "VEC3", "min": [0.0, 0.0, 0.0], "max": [1.0, 1.0, 0.0] }}"#
        ));
        let mut deltas = [0.0f32; 9];
        deltas[4] = 1.0;
        let delta_bytes: Vec<u8> = deltas.iter().flat_map(|f| f.to_le_bytes()).collect();
        let delta = add_view(&mut bin, &mut views, &delta_bytes, None);
        accessors.push(format!(
            r#"{{ "bufferView": {delta}, "componentType": 5126, "count": 3, "type": "VEC3" }}"#
        ));
        let index_accessor = accessors.len();
        let indices: Vec<u8> = [0u16, 1, 2].iter().flat_map(|i| i.to_le_bytes()).collect();
        let index = add_view(&mut bin, &mut views, &indices, Some(34963));
        accessors.push(format!(
            r#"{{ "bufferView": {index}, "componentType": 5123, "count": 3, "type": "SCALAR" }}"#
        ));

        let json = format!(
            r#"{{"asset":{{"version":"2.0"}},
                "scene":0,
                "scenes":[{{"nodes":[0]}}],
                "nodes":[{{"mesh":0}}],
                "meshes":[{{"primitives":[{{"attributes":{{"POSITION":2}},"indices":{index_accessor},
                                            "targets":[{{"POSITION":3}}]}}]}}],
                "accessors":[{}],
                "bufferViews":[{}],
                "buffers":[{{"byteLength":{}}}],
                "animations":[{{"name":"walk",
                    "channels":[{{"sampler":0,"target":{{"node":0,"path":"weights"}}}}],
                    "samplers":[{{"input":0,"interpolation":"LINEAR","output":1}}]}}]}}"#,
            accessors.join(", "),
            views.join(", "),
            bin.len(),
        );
        glb(json, &bin)
    }

    #[test]
    fn a_keyframe_clamps_past_either_end_and_splits_the_span() {
        let times = [0.0, 1.0, 3.0];
        assert_keyframe(keyframe(&times, -1.0), (0, 0.0));
        assert_keyframe(keyframe(&times, 0.0), (0, 0.0));
        assert_keyframe(keyframe(&times, 0.5), (0, 0.5));
        assert_keyframe(keyframe(&times, 2.0), (1, 0.5));
        // The end of the range sits at the end of the last segment: the
        // returned index names the segment, not the keyframe.
        assert_keyframe(keyframe(&times, 3.0), (1, 1.0));
        assert_keyframe(keyframe(&times, 4.0), (1, 1.0));
    }

    #[test]
    fn a_step_holds_the_keyframe_before_the_time() {
        let times = [0.0, 1.0, 3.0];
        assert_eq!(step_frame(&times, -1.0), 0);
        assert_eq!(step_frame(&times, 0.0), 0);
        assert_eq!(step_frame(&times, 0.9), 0);
        assert_eq!(step_frame(&times, 1.0), 1);
        assert_eq!(step_frame(&times, 2.9), 1);
        assert_eq!(step_frame(&times, 3.0), 2);
        assert_eq!(step_frame(&times, 9.0), 2);
    }

    #[test]
    fn a_linear_translation_blends_between_its_keyframes() {
        let times = [0.0, 2.0];
        let values = [[0.0, 0.0, 0.0], [4.0, 2.0, 0.0]];
        let sample = |time| sample_vec3(Interpolation::Linear, &times, &values, time);
        assert_eq!(sample(1.0), [2.0, 1.0, 0.0]);
        // Outside the keyframe range the value clamps to the nearest end.
        assert_eq!(sample(-5.0), [0.0, 0.0, 0.0]);
        assert_eq!(sample(99.0), [4.0, 2.0, 0.0]);
    }

    #[test]
    fn a_step_translation_jumps_at_the_next_keyframe() {
        let times = [0.0, 1.0];
        let values = [[0.0, 0.0, 0.0], [5.0, 0.0, 0.0]];
        assert_eq!(
            sample_vec3(Interpolation::Step, &times, &values, 0.999),
            [0.0, 0.0, 0.0]
        );
        assert_eq!(
            sample_vec3(Interpolation::Step, &times, &values, 1.0),
            [5.0, 0.0, 0.0]
        );
    }

    #[test]
    fn a_cubic_translation_follows_its_tangents() {
        // Two keyframes, each written as (in-tangent, value, out-tangent). The
        // first in-tangent and the last out-tangent never enter the formula.
        let times = [0.0, 1.0];
        let values = [
            [9.0, 9.0, 9.0],
            [0.0, 0.0, 0.0],
            [2.0, 0.0, 0.0],
            [3.0, 0.0, 0.0],
            [4.0, 0.0, 0.0],
            [9.0, 9.0, 9.0],
        ];
        let sample = |time| sample_vec3(Interpolation::CubicSpline, &times, &values, time);
        assert_eq!(sample(0.0), [0.0, 0.0, 0.0]);
        assert_eq!(sample(1.0), [4.0, 0.0, 0.0]);
        assert_eq!(sample(0.5), [1.875, 0.0, 0.0]);
    }

    #[test]
    fn a_linear_rotation_slerps_toward_its_keyframe() {
        let quarter = glam::Quat::from_rotation_z(std::f32::consts::FRAC_PI_2);
        let values = [
            [0.0, 0.0, 0.0, 1.0],
            [quarter.x, quarter.y, quarter.z, quarter.w],
        ];
        let half = sample_rotation(Interpolation::Linear, &[0.0, 1.0], &values, 0.5);
        let eighth = glam::Quat::from_rotation_z(std::f32::consts::FRAC_PI_4);
        assert!(half.dot(eighth).abs() > 1.0 - 1e-6, "{half:?}");
    }

    #[test]
    fn a_linear_rotation_takes_the_short_way_around() {
        // +170° to -170° is a 20° step through 180°, not a 340° one.
        let a = glam::Quat::from_rotation_z(170f32.to_radians());
        let b = glam::Quat::from_rotation_z((-170f32).to_radians());
        let values = [[a.x, a.y, a.z, a.w], [b.x, b.y, b.z, b.w]];
        let mid = sample_rotation(Interpolation::Linear, &[0.0, 1.0], &values, 0.5);
        let expected = glam::Quat::from_rotation_z(180f32.to_radians());
        assert!(mid.dot(expected).abs() > 1.0 - 1e-5, "{mid:?}");
    }

    #[test]
    fn a_cubic_rotation_stays_a_unit_quaternion() {
        let times = [0.0, 1.0];
        let values = [
            [0.0, 0.0, 0.0, 0.0],
            [0.0, 0.0, 0.0, 1.0],
            [0.0, 0.0, 0.5, 0.0],
            [0.0, 0.0, -0.5, 0.0],
            [
                0.0,
                0.0,
                std::f32::consts::FRAC_1_SQRT_2,
                std::f32::consts::FRAC_1_SQRT_2,
            ],
            [0.0, 0.0, 0.0, 0.0],
        ];
        let value = sample_rotation(Interpolation::CubicSpline, &times, &values, 0.5);
        assert!((value.length() - 1.0).abs() < 1e-5, "{value:?}");
    }

    #[test]
    fn a_linear_weight_blend_is_per_target() {
        let times = [0.0, 2.0];
        // Two targets per keyframe: 0, 10 then 2, 20.
        let values = [0.0, 10.0, 2.0, 20.0];
        assert_eq!(
            sample_weights(Interpolation::Linear, &times, &values, 2, 1.0),
            vec![1.0, 15.0]
        );
    }

    #[test]
    fn a_cubic_weight_blend_reads_its_kind_first_layout() {
        // One target, two keyframes. A cubic keyframe groups every in-tangent,
        // then every value, then every out-tangent, so the second keyframe's
        // value sits after both keyframes' in-tangent and value blocks.
        let times = [0.0, 1.0];
        let values = [0.0, 1.0, 2.0, 3.0, 9.0, 4.0];
        assert_eq!(
            sample_weights(Interpolation::CubicSpline, &times, &values, 1, 0.5),
            vec![4.875]
        );
    }

    #[test]
    fn an_animation_reports_its_name_and_duration() {
        let gltf = UnlitGltf::from_bytes(&parented_animated_document(
            "LINEAR",
            &[0.0, 1.5, 4.0],
            &[[0.0; 3]; 3],
            &[[0.0, 0.0, 0.0, 1.0]; 3],
        ))
        .expect("the animated document parses");
        assert_eq!(gltf.animation_count(), 1);
        assert_eq!(gltf.animation_name(0), Some("walk"));
        assert!((gltf.animation_duration(0) - 4.0).abs() < 1e-6);
    }

    #[test]
    fn apply_animation_moves_a_node_and_carries_its_child() {
        let gltf = UnlitGltf::from_bytes(&parented_animated_document(
            "LINEAR",
            &[0.0, 2.0],
            &[[0.0, 0.0, 0.0], [4.0, 0.0, 0.0]],
            &[
                [0.0, 0.0, 0.0, 1.0],
                [
                    0.0,
                    0.0,
                    std::f32::consts::FRAC_1_SQRT_2,
                    std::f32::consts::FRAC_1_SQRT_2,
                ],
            ],
        ))
        .expect("the animated document parses");
        let mut world = World::new();
        let parent = world.spawn((Transform::default(),));
        let child = world.spawn((Transform::default(),));
        let nodes = [
            GltfNode {
                node: 0,
                entities: vec![parent],
                pose: None,
                weights: None,
            },
            GltfNode {
                node: 1,
                entities: vec![child],
                pose: None,
                weights: None,
            },
        ];
        gltf.apply_animation(&world, 0, 1.0, &nodes);
        // The parent's own channel moves it...
        assert_eq!(
            world
                .get::<Transform>(parent)
                .expect("the parent carries a transform")
                .translation,
            glam::Vec3::new(2.0, 0.0, 0.0)
        );
        // ...and the child, which no translation channel names, inherits it
        // along with the quarter turn its own channel reaches at the midpoint.
        let child = world
            .get::<Transform>(child)
            .expect("the child carries a transform");
        assert_eq!(child.translation, glam::Vec3::new(2.0, 0.0, 0.0));
        let quarter = glam::Quat::from_rotation_z(std::f32::consts::FRAC_PI_4);
        assert!(child.rotation.dot(quarter).abs() > 1.0 - 1e-5, "{child:?}");
    }

    #[test]
    fn apply_animation_holds_a_step_keyframe() {
        let gltf = UnlitGltf::from_bytes(&parented_animated_document(
            "STEP",
            &[0.0, 2.0],
            &[[0.0, 0.0, 0.0], [4.0, 0.0, 0.0]],
            &[[0.0, 0.0, 0.0, 1.0]; 2],
        ))
        .expect("the animated document parses");
        let mut world = World::new();
        let parent = world.spawn((Transform::default(),));
        let nodes = [GltfNode {
            node: 0,
            entities: vec![parent],
            pose: None,
            weights: None,
        }];
        gltf.apply_animation(&world, 0, 1.999, &nodes);
        assert_eq!(
            world
                .get::<Transform>(parent)
                .expect("the parent carries a transform")
                .translation,
            glam::Vec3::ZERO
        );
    }

    #[test]
    fn apply_animation_rebuilds_a_skin_pose() {
        let gltf = UnlitGltf::from_bytes(&animated_skinned_document(
            &[0.0, 2.0],
            &[[0.0, 0.0, 0.0], [4.0, 0.0, 0.0]],
        ))
        .expect("the animated skinned document parses");
        let mut world = World::new();
        let mesh = world.spawn((Transform::default(),));
        let pose = world.spawn((SkinPose::default(),));
        let nodes = [GltfNode {
            node: 0,
            entities: vec![mesh],
            pose: Some(pose),
            weights: None,
        }];
        gltf.apply_animation(&world, 0, 1.0, &nodes);
        let pose = world
            .get::<SkinPose>(pose)
            .expect("the pose entity carries a pose");
        assert!(
            pose.matrices[0].abs_diff_eq(
                glam::Mat4::from_translation(glam::Vec3::new(2.0, 0.0, 0.0)),
                1e-6
            ),
            "{}",
            pose.matrices[0]
        );
    }

    #[test]
    fn apply_animation_writes_morph_weights() {
        let gltf = UnlitGltf::from_bytes(&animated_morphed_document(&[0.0, 2.0], &[0.0, 2.0]))
            .expect("the animated morphed document parses");
        let mut world = World::new();
        let mesh = world.spawn((Transform::default(),));
        let weights = world.spawn((MorphWeights::new(vec![0.0]),));
        let nodes = [GltfNode {
            node: 0,
            entities: vec![mesh],
            pose: None,
            weights: Some(weights),
        }];
        gltf.apply_animation(&world, 0, 1.0, &nodes);
        let weights = world
            .get::<MorphWeights>(weights)
            .expect("the weights entity carries weights");
        assert_eq!(weights.weights, vec![1.0]);
    }
}