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FSAA Quality and Performance

ATI uses the same method of FSAA that 3DFX did. A hardware accumulation buffer is used to combine successive frames together and producing a sharper image. Both 2 sample and 4 sample FSAA capabilities to the Radeon family with the 4.13.7041 driver release. To get an idea of the image improvements FSAA offers, these screenshots are from the Q3DM1 Arena Gate map.


Click on the images for the high quality version.

At the top we see a portion of a screenshot from Quake 3 running with FSAA disabled. The Jagged edges are quite noticeable, and certainly detract from the overall quality. On the bottom is the same frame but with 2 sample FSAA enabled. The difference is immediately apparent, FSAA produces a much nicer looking image. For those wondering about 4X FSAA, the largest improvement in image quality is from 0-2X, the noticeable improvements from 2-4X are marginal. The cost for FSAA is performance, 2 sample FSAA requires the video card to render twice the number of frames per second to obtain an equivalent frame rate.

The one problem we ran into was using 4 sample FSAA at resolutions above 800x600. After enabling 4X in the drivers and rebooting, both OpenGL and Direct X applications displayed no video. Switching back to 2X FSAA or 800x600 fixed the problems, yet another bug in the drivers.

On the performance side of things, we see the enormous frame rate hit when using FSAA, well over 60% at the higher resolutions. The performance drop is nearly linear using both 2X and 4X, 800x600 being about the highest resolution playable. The large drops in performance would seem to indicate that the memory is the bottleneck rather than the core. Radeon users have reported modest gains in FSAA performance when overclocking so that may be an option for purely performance oriented users.



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