How to boost your Doom3 performance by 40% on ATI hardware

I don't want to complain about Carmack's work, I still consider him to be the industry leader in graphics engines. Though when I read the shader it striked me how many texture accesses it did compared to the relatively short shader, even for stuff that could just as well be done with math for a small cost in instructions. Using a dependent texture lookup for POW evaluation makes a lot of sense for R200 level hardware due to instruction set limits, but for R300 and up it's much better to just spend the three cycles it takes to evaluate POW with math instead of risking texture cache trashing with a dependent texture read, which may be much more costly, especially since the access pattern in this case will be far from linear. Also, using math improves the quality too, even though it may not be very noticable in this game.

I should point out though that I'm not sure if the constant specular factor 16 that I chose is the one that the game uses, so output may be slightly different, but if this solution will be worked into the game in a future patch, then this is easily configurable by the game so that there won't be a difference, except a lot faster.

An interesting follow-up discussion may be why this dependent texture lookup is much slower on our hardware than on nVidia. Maybe there's an architectural difference that's to blame, or maybe something else? The main point here though is that this should be good enough proof that ATI hardware can run Doom3 just as good if not better than nVidia, and that we can pass on all the "ATI suck in OpenGL", "ATI's drivers suck" etc. into the trashcan where it belongs.