This might be a silly question... but in rendering a scene where the light ray is tangent to the surface, what is actually rendered? Will that light be added to every spot along that line? Or is it not rendered at all?
I could be quite wrong, but looking at Slide 26, it seems that the intersection point for a ray that is parallel to our plane lies at a t equal to positive/negative infinity. Since we are comparing this t with other objects in our scene to see whether or not that surface is blocked by another object that is closer to the camera, we will likely find that the background of our scene is "closer" to the camera than the surface, whose intersection point lies at +inf. That suggests that we would not render the surface? (or, in the case of determining whether a surface is in shadow when it is parallel to the ray of light, no light would be added to the object?). Please let me know if this reasoning makes sense/is correct.