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nVidia SLI Technology - Introduction and 6800 Ultra Performance - What is SLI? AFR and SFR

By: Shawn Baker | NVIDIA SLI Articles in Video Cards | Posted: Jan 27, 2005 5:00 am

What is SLI?


SLI, AFR, SFR - So many acronyms, all with different meanings. While just about everyone who shows a slight interest in computers knows about SLI, their knowledge on the subject doesn't go much further then the fact that they know you can run two graphics cards at the same time giving almost twice as much performance. While that is the bulk of it, there is more then just inserting two cards into your motherboard, hitting enable, and getting the most out of your games.


For starters there are two drawing modes of SLI when it comes to rendering the picture that is being outputted to your monitor. AFR and SFR are the two particular modes we are talking about and we will look at these in more details now.




Alternate Frame Rendering or AFR is the first mode that we will be looking at. What seems to be the most common mode used, the idea behind AFR is that the two GPUs take one frame each. We would see our slot 1 graphics card taking our uneven frames like 1 - 3 - 5 - 7 and etc while the graphics card in slot 2 takes 2 - 4 - 6 - 8. In turn this means that both graphics cards are sharing the work load to what most people would think the most efficient way possible.



Photo courtesy of The Tech Report


While AFR does seem to be the most effective way there are issues with particular games which can cause blurring. This is where the other form of rendering comes into play.




Split-Frame Rendering or SFR is the other mode available and works by splitting up the frame and let's each graphics card evenly share the work load of each frame. While you may instantly think that the frame is split evenly in half you are assuming wrong as it works by evenly spreading the work load on the complete frame - for example, if you have a picture that had little detail in the bottom half but a lot in the top second half, you would find that the majority of graphics processing power from both graphics cards would be working on the top half of the screen while one is also rendering the bottom of the screen that doesn't require as much processing power to render the particular scene.



Photo courtesy of The Tech Report


Right now in current nVidia SLI drivers you have no way of choosing which form of SLI drawing method you wish to use and this is a negative thing as each game performs better in different SLI modes - given nVidia do test games in-house to work out which SLI drawing method works best for each game, it would be nicer to have the option of choosing a method for yourself. nVidia do plan on allowing end-users to be able to adjust game driver profiles for which SLI mode you wish to use but there is no word on when this feature will arrive.


A website called 3DChipset does have a free tool available for download by author Andrew Poison which allows you to create new profiles for your games which do or do not support SLI where you can choose which type of SLI drawing method you wish. However, by the time most users have SLI systems, we'd hope nVidia has addressed this issue in upcoming drivers themselves.


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