AMD explains Radeon HD 2900XT's poor AA performance 1:16PM, Monday 14th May 2007
The R600 is finally here, and in keeping with its mysteriously long gestation, in at least in its first incarnation as the HD 2900XT, AMD's new GPU still poses a lot of questions. One of the things we noticed during our in-depth testing of the card is that compared to its principle rival, the Nvidia GeForce 8800 GTS 640MB, the HD 2900XT performs poorly in many games when anti-aliasing is enabled.
In F.E.A.R., at 1,600 x 1,200, with AA and AF disabled, the HD 2900XT easily outstripped the 640MB 8800 GTS, delivering a minimum that was 23fps higher than the latter's. However, with 4x AA, the HD 2900XT's minimum framerate dived from 82fps to 21fps, while the 640MB 8800 GTS produced a minimum of 30fps. Adding 4x AA results in a 74% drop for the Radeon, compared to only a 49% drop for the GeForce.
The Radeon's framerates suffer disproportionately with anisotropic filtering, too. Again testing in F.E.A.R. at 1,600 x 1,200, we saw the HD 2900XT's minimum FPS drop by 10 per cent with 16x anisotropic enabled, compared to 3 per cent for the GTS, although the HD 2900XT still had a faster average. It was a slightly different result at 2,560 x 1,600, as the HD 2900XT's massive bandwidth gave it a boost, although adding 16x AF still had more impact than it did on the 640MB GTS.
As most gamers will want AA and AF enabled in games, the HD 2900XT's poor performance with these processing options enabled is a serious problem for the card and ATi. We asked ATi to comment on this surprising result and the company revealed that the HD 2000-series architecture has been optimised for what it calls 'shader-based AA'. Some games, including S.T.A.L.K.E.R., already use shader-based AA, although in our tests the 640MB 8800 GTS proved to be faster than the HD 2900XT.
We asked Richard Huddy, Worldwide Developer Relations Manager of AMD's Graphics Products Group, to go into more detail about why the Radeon HD 2000-series architecture has been optimised for shader-based AA rather than traditional multi-sample AA. He told us that 'with the most recent generations of games we've seen an emphasis on shader complexity (mostly more maths) with less of the overall processing time spent on the final part of the rendering process which is "the AA resolve". The resolve still needs to happen, but it's becoming a smaller and smaller part of the overall load. Add to that the fact that HDR rendering requires a non-linear AA resolve and you can see that the old fashioned linear AA resolve hardware is becoming less and less significant.' Huddy also explained that traditional AA 'doesn't work correctly [in games with] HDR because pixel brightness is non-linear in HDR rendering.'
While many reviews of the HD 2900XT have made unflattering comparisons between it and Nvidia's GeForce 8800-series, Huddy was upbeat about AMD's new chip. 'Even at high resolutions, geometry aliasing is a growing problem that can only really be addressed by shader-based anti-aliasing. You'll see that there is a trend of reducing importance for the standard linear AA resolve operation, and growing importance for custom resolves and shader-based AA. For all these reasons we've focused our hardware efforts on shader horsepower rather than the older fixed-function operations. That's why we have so much more pure floating point horsepower in the HD 2900XT GPU than NVIDIA has in its 8800 cards... There's more value in a future-proof design such as ours because it focuses on problems of increasing importance, rather than on problems of diminishing importance."
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