Function surface_from_hit

tools/bloom-reference/src/main.rs:1120–1167

Build the per-pixel surface sample from a BVH hit. Interpolates barycentric attributes, samples material textures, and perturbs the shading normal via the tangent-space normal map (if present).

(scene: &Scene, ray: &Ray, hit: &Hit)

/// barycentric attributes, samples material textures, and perturbs the
/// shading normal via the tangent-space normal map (if present).
fn surface_from_hit(scene: &Scene, ray: &Ray, hit: &Hit) -> SurfaceSample {
    let tri = &scene.triangles[hit.triangle_index as usize];
    let u = hit.barycentric.x;
    let v = hit.barycentric.y;
    let w = 1.0 - u - v;

    let geom_normal = (tri.n0 * w + tri.n1 * u + tri.n2 * v).normalize_or_zero();
    let uv = tri.uv0 * w + tri.uv1 * u + tri.uv2 * v;
    let position = ray.origin + ray.direction * hit.t;

    let material = &scene.materials[tri.material_index as usize];
    let base_color = scene.sample_base_color(material, uv);
    let (metallic, roughness) = scene.sample_metallic_roughness(material, uv);
    let emissive = scene.sample_emissive(material, uv);
    let occlusion = scene.sample_occlusion(material, uv);

    // Build the per-hit TBN from the interpolated tangent+normal. The
    // bitangent sign comes from the glTF tangent.w (±1) — if the mesh
    // has no tangents (length 0), skip normal mapping entirely.
    let tangent_interp = tri.t0 * w + tri.t1 * u + tri.t2 * v;
    let tangent_xyz = Vec3::new(tangent_interp.x, tangent_interp.y, tangent_interp.z);
    let shading_normal = if material.normal_texture.is_some()
        && tangent_xyz.length_squared() > 1e-8
    {
        let t = tangent_xyz.normalize();
        // Re-orthogonalize the tangent against the normal (Gram-Schmidt)
        // so numerical drift from interpolation doesn't skew the basis.
        let t = (t - geom_normal * geom_normal.dot(t)).normalize_or_zero();
        let bitangent_sign = tangent_interp.w.signum().max(-1.0).min(1.0);
        let b = geom_normal.cross(t) * bitangent_sign;

        let n_tangent = scene.sample_tangent_normal(material, uv);
        // Compose tangent-space normal into world space.
        (t * n_tangent.x + b * n_tangent.y + geom_normal * n_tangent.z).normalize_or_zero()
    } else {
        geom_normal
    };

    SurfaceSample {
        position,
        normal: shading_normal,
        base_color,
        metallic,
        roughness: roughness.max(0.04), // clamp to avoid div-by-zero
        emissive,
        occlusion,
    }
}

/// Build an orthonormal basis (tangent, bitangent) around `n`.
/// Branchless method from Frisvad 2012 — more stable than cross(n, up)