ATI Radeon X1900 XTX review - Page 4

Look clearly at the overexposed lighting effects, that's HDR. It'll bring a nice cinematic effect to gaming. Mind you that these screenshots did not have AA disabled while with an upcoming Far Cry patch that will be possible.

What about Fetch4 then?

Yes I noted Shadow map acceleration and Fetch4 in the introduction already. You are on Guru3D, let's go a little deeper. For those that do not like tech chat, skip this part for sure. Texture lookups have long been a widespread procedure in 3D rendering. One widely used class of techniques that is placing a heavier weight on the importance of texture filtering is shadow mapping.

This method of rendering shadows works by first rendering the scene from the point of view of a shadow-casting light source. The results are not displayed, but instead stored in a special shadow map texture where each value represents the distance of the nearest object to the light source. The scene is then rendered from the standard viewpoint, and each pixel is checked against the shadow map to determine if there are any objects between it and the light source. If the result is true, the pixel is in shadow and can be darkened, otherwise it is lit normally. One limitation of shadow maps is that they normally create hard-edged shadows. In the real world, shadows tend to have softer edges. Techniques that create soft-edged shadows often work by filtering the shadow map. This can be done by taking a number of samples and then combining them in a pixel shader. Using a larger number of samples can result in higher quality shadows, but also requires a large number of texture lookups, which can hurt performance. Dynamic branching can be used to improve the performance of this shadow rendering technique by detecting pixels that lie on or near shadow edges. These pixels can then use a high quality filter with many texture samples, while other pixels use just a single texture lookup to determine if they are in or out of shadow.

To further facilitate this technique, the Radeon X1900 includes a new texture sampling feature known as Fetch4. It works by exploiting the fact that most textures are composed of color values, each consisting of four components (Red, Green, Blue, and Alpha or transparency). The texture units are designed to sample and filter all four components from one texture address simultaneously. However, when looking up different types of textures with single-component values (such as shadow maps), Fetch4 instead allows four values from adjacent addresses to be sampled simultaneously. This effectively increases the texture sampling rate by a factor of 4. With Ultra-Threading technology providing fast flow control and Fetch4 providing fast texture lookups, the Radeon X1900 can render attractive soft shadows at speeds approaching those of traditional hard-edged shadow mapping techniques.

* courtesy ATI X1900 Whitepaper