// HDRBloom miss (runs in SHADE). Dark, sub-threshold background so the
// bloom pass only picks up the bright cubes, not the sky.
fn miss_main(ray: RayDesc, payload: ptr<function, Payload>) {
    let t = clamp(ray.direction.y * 0.5 + 0.5, 0.0, 1.0);
    rtAccumulate(mix(vec3<f32>(0.015, 0.018, 0.030),
                     vec3<f32>(0.030, 0.040, 0.070), t));
}