Invisible wrote:

> It's used for CSG. (E.g., if you "difference" the sphere against
> something else, the "back" intersection might actually be where the
> solid surface begins.)


Hmm. I still don't get it. Why should I ever need to trace the ray in
the "wrong direction"?

The ray starts at camera and goes forward. If anything interesting is
found, it is in front of the ray, right? And the only exception is if
the camera is inside an object. But even then: shouldn't it be enough to
trace forward. Please enlighten me :)

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