Function alpha_cutoff_from_material

native/shared/src/models.rs:1767–1773

Convert a KHR_materials_pbrSpecularGlossiness (diffuse + specular + glossiness) material to the metallic-roughness model. Uses the reference Khronos two-path formula so materials authored in Substance/3ds Max/FBX pipelines (Lumberyard Bistro, many ORCA assets) render correctly on a metal-rough pipeline. High-level idea: assume a 0.04 dielectric reflectance baseline, solve for the metallic factor

(mat: &gltf::Material)

///          foliage + fabric as fully opaque — an alpha-cutout leaf
///          card is at least the right *shape*.
fn alpha_cutoff_from_material(mat: &gltf::Material) -> f32 {
    match mat.alpha_mode() {
        gltf::material::AlphaMode::Opaque => 0.0,
        gltf::material::AlphaMode::Mask => mat.alpha_cutoff().unwrap_or(0.5),
        gltf::material::AlphaMode::Blend => 0.5,
    }
}

/// Fake KHR_materials_transmission as a near-mirror dielectric.
///