// RTVolume any-hit shader (runs in TRACE on every candidate sphere hit,
// because the geometry is registered non-opaque). Punches a spherical
// checkerboard of holes: for half the cells it returns RT_ANYHIT_IGNORE,
// so the ray passes straight through and the background / spheres behind
// show through. Returning RT_ANYHIT_ACCEPT keeps the hit. This is the
// visible proof the any-hit path runs — with it the spheres are perforated,
// without it they would be solid.
fn anyhit_main(ray: RayDesc, hit: HitInfo, payload: ptr<function, Payload>) -> u32 {
    // Object-space hit point on the unit sphere → its normal/direction.
    let posObj = hit.objectRayOrigin + hit.objectRayDirection * hit.t;
    let n = normalize(posObj);

    let PI = 3.14159265;
    let longitude = atan2(n.z, n.x);                 // [-PI, PI]
    let latitude  = asin(clamp(n.y, -1.0, 1.0));     // [-PI/2, PI/2]

    let cu = i32(floor((longitude + PI) / PI * 6.0));
    let cv = i32(floor((latitude + PI * 0.5) / PI * 6.0));

    if (((cu + cv) & 1) == 0) {
        return RT_ANYHIT_IGNORE;   // cut-out cell — see through
    }
    return RT_ANYHIT_ACCEPT;
}