Bugged GeForce Drivers Improve Image Quality


One of the fun things about being a hardware analyst is that every now and then, research and comparison between products turns up something both interesting and unexpected. In this case, we were testing and comparing image quality between ATI's 5970 and the GeForce GTX 480 as part of a system review when we stumbled across an officially confirmed antialiasing bug in NVIDIA's 197.41 drivers. Unlike some "bugs" that artificially inflate benchmark performance or lower detail levels, this one improves image quality by performing supersampling AA (SSAA) across the entire image.

Antialiasing 101
Antialiasing is a process that removes jagged edges (jaggies) from displayed content.  Supersampling, the type of AA we're primarily discussing here, functions by rendering an entire image at a very high resolution internally, then sampling an area around each pixel. The advantage of supersampling is that the entire image is antialiased, rather than just the edges or intersection points. As a result, it's arguably the best of the antialiasing methods, provided you don't mind the blurring it tends to create. The disadvantage of standard supersampling (other than the blur) is that it scales with the grace of Rosie O'Donnell running 30 flights of stairs. 4xSSAA when gaming at 1900x1200 instructs the video card to internally render four pixels for every single pixel of displayed data. Texture data and lighting calculations are performed on each of the pixels at this high resolution, and the output is then downsampled to 1900x1200. On an NVIDIA card, using nHancer to render in 4x4 mode requires 16x the computational power required to render the original image.

Multisampling streamlines this process by only processing texture data and lighting effects once per group of pixels. Both ATI and NVIDIA have proprietary algorithms that further evaluate what data needs to be processed. Both companies have introduced their own techniques for improving visual quality or lowering the performance hit when enabling AA including ATI's Edge Detect filters and NVIDIA's Coverage Sampling (CSAA).

The Bug: Transparency AA = FSAA
The bug we're going to be discussing affects NVIDIA's Transparency AA (TSAA) settings. Normally, changing this option under the NVIDIA Control Panel adjusts the amount of antialiasing performed on objects that are transparent/translucent, including chain link fences or telephone wires. The images below are from our original discussion of transparency AA in Half Life 2 and demonstrate how Transparency AA functions under normal circumstances.

No Transparency AA

TRMS AA - Multi-Sample

TRSS AA - Super Sample

The 197.41 Forceware drivers allow the end-user to select between 2x, 4x, and 8x supersampled transparency AA. We compared image quality between multisampled TSAA and supersampled TSAA and then contacted NVIDIA. The company confirmed our suspicions: at present, choosing 2x, 4x, or 8x supersampled TSAA applies supersampling to the entire frame. ATI added support for supersampled AA when it launched the HD 5000 series, but only when games are running in DX9 mode. That's not the case with NVIDIA—we were able to use supersampling in DX11 games, although performance took a beating.

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