ATI CrossFire Technology White Paper

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.
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