Contributor: Aravind S.K.
In order to increase graphics performance, two or more GPU’s are used to simultaneously render the graphic. Scan-Line Interleave (SLI) from 3dfx is a method for linking two (or more) video cards or chips together to produce a single output. It is an application of parallel processing for computer graphics, meant to increase the processing power available for graphics. SLI from 3dfx was introduced in 1998. But 3dfx moved out of the scene and the two major players, NVIDIA and ATI technologies have their own multi-GPU solutions.
NVIDIA Corporation reintroduced the name SLI (renamed as Scalable Link Interface) and intends for it to be used in modern computer systems based on the PCI Express bus. SLI is, two graphics processors doing the work of one. Each graphics card is assigned 50% of the visual workload for a given scene and both GPUs render their share concurrently, effectively doubling the output. SLI offers two rendering and one anti-aliasing method for splitting the work between the video cards:
Split Frame Rendering (SFR): This analyzes the rendered image in order to split the workload 50/50 between the two GPUs. To do this, the frame is split horizontally in varying ratios depending on geometry..
Alternate Frame Rendering (AFR): Here, each GPU renders entire frames in sequence - one GPU processes even frames, and the second processes odd frames, one after the other.
SLI Antialiasing. This is a standalone rendering mode that offers up to double the antialiasing performance by splitting the antialiasing workload between the two graphics cards, offering superior image quality. One GPU performs an antialiasing pattern which is slightly offset to the usual pattern (for example, slightly up and to the right), and the second GPU uses a pattern offset by an equal amount in the opposite direction (down and to the left). Compositing both the results gives higher image quality than is normally possible.
ATI technologies have named their multi GPU solution as CrossFire. This technology also makes use of two PCI Express cards. The CrossFire system supports four different rendering modes, each offering their own specific advantages and disadvantages.
SuperTiling: It divides the screen up like a checkerboard, allocating adjacent squares ('quads') to alternate GPUs. (One card would render the white squares, and the other the black).
Scissor: Divides the screen into two rectangles, one above the other. This render mode is more commonly known as Split Frame Rendering (SFR), which is how nVidia refers to it in SLI. Using Scissor mode means that the system has to carefully choose the "cutting point" in order to balance the load.
Alternate Frame Rendering: Alternate Frame Rendering (as the name suggests) sets one GPU to render odd frames, and one the even frames. While this produces a high performance boost, it is incompatible with games using render-to-texture functions because one card doesn't have direct access to the texture buffer of the other.
CrossFire Super Anti-aliasing: It is intended to improve the quality of the frames rendered. Super AA is able to double the anti-aliasing factor without any drop in frame rate.
The scope for multi GPU systems lies in
· Higher performance gaming computers
· Better physics processing (in the field of animation)
To name a few and many more uses in better graphics rendering.
References:
· www.slizone.com
· http://www.bit-tech.net/hardware/2004/06/30/multi_gpu_tech/1
· http://en.wikipedia.org/wiki/Scalable_Link_Interface
· http://www.extremetech.com/article2/0,1697,2136956,00.asp
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