If I understand correctly, performance-wise, flipping the sign of Z and the depth test is nothing but changing a < comparison to a > comparison. So, if I understand correctly and the author isn’t lying or making things up, then changing < to > used to be a vital optimization for many games.
I didn’t explain that particularly well, because it wasn’t important. I just felt it was an interesting bit of trivia to add. I didn’t intend to go over the algorithm specifically.
However, context is key. I never said that a < comparison was faster than a > comparison. Remember: we’re talking about graphics hardware depth tests, not your CPU. Not operator<.
What I was referring to was a specific old optimization where one frame you would use GL_LESS with a range of [0, 0.5]. Next frame, you render with GL_GREATER with a range of [1.0, 0.5]. You go back and forth, literally “flipping the sign of Z and the depth test” every frame.
This loses one bit of depth precision, but you didn’t have to clear the depth buffer, which once upon a time was a rather slow operation. Since depth clearing is not only free these days but actually faster than this technique, people don’t do it anymore.