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ATI X1900 CrossFire

In the previous generation of top end cards the pixel pipelines tended to have a straight through path in that the number of pixel processors and texture units corresponded. E.g. the 7800 GTX with 24 pixel processors, and 24 texture units, and the X1800 XT with 16/16. However, now ATI has decided to use its transistor spend in a more efficient and forward looking method.

As you can see from the graph, since their introduction, the use of pixel shaders has increased dramatically, as cards become more and more capable of dealing with longer instructions at speed, and as game developers become more inventive using them. The ratio of shader usage to texturing in games such as F.E.A.R is 7:1 and the average is 5:1.

Therefore, it makes sense tofocus on pixel shader processing power rather than having a 50/50 split with texturing. What ATI has done then is to lay out 12 blocks of quad pixel shaders, giving a total of 48 pixel shader processors. There are still 16 texture units, eight vertex shaders and significantly, still 16 ROPs. Rather than simply increasing the ROPs, what ATI is trying to do is keep them as busy as much of the time as possible. ATI is claiming that by increasing the transistor count by 20 per cent, it has increased shader processing power by 200 per cent. Sounds like a good value deal to me.

This sort of arrangement, shouldn’t come as too much of a surprise however, as the ATI powered Xbox 360 also features 48 shader processors and 16 texture units. The difference is that the shaders in the Xbox 360 are unified and also have to do vertex processing. The similarities are such though, that porting games should be easier.

Other changes include improvements to the card ability to deal with soft shadows. Shadows are often created with Shadow Maps, but these create shadows with hard edges, which don’t look realistic. The edges can be softened with filtering but is expensive in terms of performance. The X1900 offers Fetch4 to analyse textures to determine which ones are near the edges of shadow maps, and then just work on softening those, speeding up the process of creating soft shadows.

One feature that ATI talked about on the X1800 was Parallax Occlusion Mapping. This is a method of creating depth on surfaces that it used to great affect in its Toy Shop demo. With the X1900 it can no run this demo even faster while applying Anti-Aliasing.

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