private static Ray rayThruPixel(Camera cam, int xi, int yi) {
Ray result = new Ray();
float x = xi + .5f;
float y = yi + .5f;
final int halfW = cam.getHalfW();
float alpha = (float) (cam.getTanHalfFovX()) * ((x - halfW) / halfW);
final int halfH = cam.getHalfH();
float beta = (float) (cam.getTanHalfFovY() * (halfH - y) / halfH);
Vector3D p13 =
cam.getU().scalarMultiply(alpha).add(cam.getV().scalarMultiply(beta)).subtract(cam.getW());
result.setP0(cam.getFrom());
result.setP1(VectorUtils.toRealVector(p13));
return result;
}
