From pixel to ray
Ray generation
Reverse ray tracing begins by turning each pixel into a ray. The origin is always the camera; the direction comes from where that pixel sits on the image plane. Click a pixel in the little image on the right and watch its ray appear in the scene — and the colour it brings back.
One pixel, one ray. Click any of them. The grid is deliberately coarse so that every square you see corresponds to exactly one ray in the scene above.
Slide plane distance and watch the image: nothing changes. The image plane is a bookkeeping device, not a physical object — moving it towards or away from the camera scales the plane and the offsets on it by exactly the same factor, and the ray direction comes out identical once normalised.
Field of view is different. It changes how far across the plane a given pixel sits relative to the distance, so it genuinely changes the directions — which is why the picture gets wider or narrower.
Turn on every pixel's ray and the whole frustum fills in. This is the moment reverse ray tracing pays off: every one of those rays is guaranteed to land on a pixel, because it was built from one.