ATI CrossFire Technology White Paper

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Ultimate Image Quality One of the key problems with early multi-GPU technologies is how they handle 3D applications limited by CPU performance rather than GPU performance. A similar situation holds true for applications that can execute so quickly with a single GPU that they can achieve frame rates well above the refresh rate of the attached display device. In these cases, the additional power provided by a second GPU is essentially wasted, since the CPU or display device becomes a bottleneck. These situations can be quite common on a fast system, even when running at maximum image quality settings. On systems equipped with less than cutting-edge CPUs or limited resolution displays (such as the common 1280x1024 LCD resolution), the value of adding a second GPU can be severely limited. ATI CrossFire technology addresses these problems with its new Super AA mode, which takes advantage of a second GPU to improve image quality, rather than just to increase frame rates. Super AA Mode Anti-Aliasing (AA) is a well known rendering technique designed to remove jagged edges, shimmering, and pixelation problems that are common in rendered 3D images. Rather than simply determining the color of each pixel on the screen by sampling a single location at the pixel's center, anti-aliasing works by sampling multiple locations within each pixel and blending the results together to determine the final color. Today's graphics processors employ a variety of different anti-aliasing techniques. The latest generation of ATI’s Radeon GPUs with SmoothVision HD technology use a method known as Multi-Sample Anti-Aliasing (MSAA). This method takes samples from 2, 4, or 6 programmable locations within each pixel, and uses gamma correct sample blending for high quality smoothing of polygon edges. Taking more samples per pixel increases the quality of the final image. The new CrossFire Super AA modes take advantage of the programmable sample capability of SmoothVision HD to provide even higher quality anti-aliasing on multi-GPU systems. It works by having each GPU render the same frame with anti-aliasing enabled, but uses different sample locations for each. When both versions of the frame are completed, they are blended in the CrossFire Compositing engine. The resulting image has effectively twice the number of samples, so 4x and 6x AA become 8x and 12x Super AA respectively. - 9 -
Super AA Mode 6x AA 12x SuperAA Comparison of edge smoothing with 6x AA and 12x Super AA. Note the smoother gradients along the edges in the Super AA image. While these new anti-aliasing modes deliver outstanding quality and high performance, they are still limited to polygon edges. Some types of textures, especially those with transparent portions, can exhibit aliasing that is not removed by MSAA techniques. Another form of anti-aliasing, known as Super-Sample Anti-Aliasing (SSAA), can be useful in these cases since it affects every pixel in an image. Although it normally runs quite a bit more slowly than MSAA, the power of multiple GPUs can make SSAA much more practical to use. The simplest and most commonly used form of SSAA requires a scene to be first rendered at a higher resolution than that which is output to the display. Once completed, the image is then downsampled to the display’s - 10 -
Page 1 and 2: ATI CrossFire Technology White Pap
Page 3 and 4: Background The world around us poss
Page 5 and 6: CrossFire - How it Works CrossFire
Page 7 and 8: Alternate Frame Rendering (AFR) Mod
Page 9: Scissor Mode In this mode, each fra
Page 13 and 14: Anti-Aliasing Sample Patterns An ad

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