Houdini VOP raytracer part 3
Shadow Ray and Illumination Ray
Now this is where fun starts. Following a photon in our backward raytracing next step would be to test where it came from. We already know last hit position before photon hit our Image Plane, we can check if there is direct path from hit point to light source simply by shooting another ray. When a our new ray is able to reach a light source without any interruption, we call it illumination ray, otherwise, when there is any obstruct on our ray path we call it shadow ray.