> In sample(x, y), we only care about the ray of light that passes through (x, y, camera_z). So why bother with all the other rays? We'll shoot the ray backwards! We'll start it at (x, y, camera_z), and at each step, query the SDF to get the distance from the ray's current point to the scene (in any direction). If the distance is less than some threshold, we hit the scene! Otherwise, we can safely "march" the ray forward by whatever distance is returned, since we know the scene is at least that distance away in the forward direction.
Doesn't this assume all the rays are only in the z direction? We are assuming the light source is directly in front of the camera? The code actually only increments z so it's basically only capable of determining occlusion, right?