bevy/crates/bevy_pbr/src/render/skinning.wgsl

// If using this WGSL snippet as an #import, a dedicated
// "joint_matricies" uniform of type SkinnedMesh must be added in the
// main shader.

#define_import_path bevy_pbr::skinning

/// HACK: This works around naga not supporting matrix addition in SPIR-V
// translations. See https://github.com/gfx-rs/naga/issues/1527
fn add_matrix(
    a: mat4x4<f32>,
    b: mat4x4<f32>,
) -> mat4x4<f32> {
    return mat4x4<f32>(
        a.x + b.x,
        a.y + b.y,
        a.z + b.z,
        a.w + b.w,
    );
}

fn skin_model(
    indexes: vec4<u32>,
    weights: vec4<f32>,
) -> mat4x4<f32> {
    var matrix = weights.x * joint_matrices.data[indexes.x];
    matrix = add_matrix(matrix, weights.y * joint_matrices.data[indexes.y]);
    matrix = add_matrix(matrix, weights.z * joint_matrices.data[indexes.z]);
    return add_matrix(matrix, weights.w * joint_matrices.data[indexes.w]);
}

fn inverse_transpose_3x3(in: mat3x3<f32>) -> mat3x3<f32> {
    let x = cross(in.y, in.z);
    let y = cross(in.z, in.x);
    let z = cross(in.x, in.y);
    let det = dot(in.z, z);
    return mat3x3<f32>(
        x / det,
        y / det,
        z / det
    );
}

fn skin_normals(
    model: mat4x4<f32>,
    normal: vec3<f32>,
) -> vec3<f32> {
    return inverse_transpose_3x3(mat3x3<f32>(
        model[0].xyz,
        model[1].xyz,
        model[2].xyz
    )) * normal;
}