In some paper was said, that Doom 3 does at first one pass with color/textures etc. disabled, just to fill the z-buffer for speeding up lighting calculations later.
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What if i want to do stuff like fences. In this case i have a mask in the alpha-channel, which indicates, whether a texel is visible or not, so that the alpha-test will discard it. But to fill the z-buffer correctly, i will have to have texturing enabled in the first pass, so that alpha-testing works. However nvidia says, that operations like filling the z-buffer are now twice as fast as some time ago. Is this because no colors are written, or is this only true, when texturing is disabled?
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A lot of diagrams show, that register-combiners and texture-shaders are combuted BEFORE depth and alpha-testing. I know that it is true for the alpha-test (because i can change the alpha-value in a combiner and then the pixel will be discarded). But i wonder why the depth-test should not be BEFORE the register-combiners, because this way a lot of computations could be saved (and in this case doing a z-only pass does make sense).
So is it possible, that the depth-test is done earlier, if reg-comb. are enabled? Or is this not possible on current chips. Or are reg-comb. so fast, that doing those calculations is done in no time, so doing depth-test first wouldn´t save any time?
Jan.