My question is not about the code, but it's about the approach used by the author Peter Shirley (and apparently common practice in ray tracers) of handling Instances. To understand the question it's necessary to be familiar with the book, because posting the code for every class involved is not feasible. Something I don't understand is how rays behave.
Let's consider the traslation. The idea is to offset the origin, and check if the new ray - which is now shot from the new origin with the same direction of the original ray - hits the object. If so, we shift the hitting point by an equal and opposite offset.
1) Why is it not sufficient to shoot rays from the "standard" origin, and offset the hitting point without actually shifting the origin first? Example: the ray is shot, if it hits the object in point (x,y) I move the hitting point by 2 (x+2, y+2) and store it.
2) The hit function of the class
translate is only called when the original ray hits the object? Because in that case, the translation would not result in the whole object, since it would only store those points that are first hit by the original ray, and for which the new ray hits the object again.