I've updated the code to be even more readable, and as an aside, made it more optimal.

sin(A) and sin(B) were being computed repeatedly in the most inner of inner loops, which I pulled out, because A and B don't change in the middle. sin(j) and cos(j) was being computed for every i, even though j wasn't changing. And the intensity was being computed even if the pixel was going to appear beyond the buffer (or wasn't going to appear at all because of a closer pixel in the z buffer).

I also refactored parts of it, and gave names to some constants which you can tweak for fun. DELTA_I can be increased without apparent detriment to image quality. I can't think of good names for the two constants in the middle of the code though: 2 and 5. Are those the minor and major radius of the donut?