ID Software's John Carmack has made a post at Slashdot with what does and does not constitute cheating relating to a discussion of the recent controversy and an insight into video performance in DOOM 3.
Rewriting shaders behind an application's back in a way that changes the output under non-controlled circumstances is absolutely, positively wrong and indefensible. Rewriting a shader so that it does exactly the same thing, but in a more efficient way, is generally acceptable compiler optimization, but there is a range of defensibility from completely generic instruction scheduling that helps almost everyone, to exact shader comparisons that only help one specific application. Full shader comparisons are morally grungy, but not deeply evil.
The significant issue that clouds current ATI / Nvidia comparisons is fragment shader precision. Nvidia can work at 12 bit integer, 16 bit float, and 32 bit float. ATI works only at 24 bit float. There isn't actually a mode where they can be exactly compared. DX9 and ARB_fragment_program assume 32 bit float operation, and ATI just converts everything to 24 bit. For just about any given set of operations, the Nvidia card operating at 16 bit float will be faster than the ATI, while the Nvidia operating at 32 bit float will be slower. When DOOM runs the NV30 specific fragment shader, it is faster than the ATI, while if they both run the ARB2 shader, the ATI is faster.
Driver Strategies By id Software's John Carmack
Rewriting shaders behind an application's back in a way that changes the output under non-controlled circumstances is absolutely, positively wrong and indefensible. Rewriting a shader so that it does exactly the same thing, but in a more efficient way, is generally acceptable compiler optimization, but there is a range of defensibility from completely generic instruction scheduling that helps almost everyone, to exact shader comparisons that only help one specific application. Full shader comparisons are morally grungy, but not deeply evil.
The significant issue that clouds current ATI / Nvidia comparisons is fragment shader precision. Nvidia can work at 12 bit integer, 16 bit float, and 32 bit float. ATI works only at 24 bit float. There isn't actually a mode where they can be exactly compared. DX9 and ARB_fragment_program assume 32 bit float operation, and ATI just converts everything to 24 bit. For just about any given set of operations, the Nvidia card operating at 16 bit float will be faster than the ATI, while the Nvidia operating at 32 bit float will be slower. When DOOM runs the NV30 specific fragment shader, it is faster than the ATI, while if they both run the ARB2 shader, the ATI is faster.
Driver Strategies By id Software's John Carmack
