Overview
Here are the technical specification from both manufacturer's website.ATI X800XL 256 MB
Core Clock : 400 MHz
Memory Clock : 500 MHz (1000 MHz DDR3)
Memory Bus Width : 256 bit
NVIDIA GeForce 6800GT 256 MB DDR3
Core Clock : 390 MHz
Memory Clock : 1000 MHz (500 MHz DDR3)
Memory Bus Width : 256 bit
Image Quality Comparison
A Little Background
Of course, when we're talking image quality in relation to graphics cards, we're talking about image quality with AA and AF. Someone once said, aliasing (or any artifact for that matter) is what you get from trying to do everything too fast. This means you made several shortcuts and compromises to get a high enough frame rate. Anti aliasing (AA) and anisotropic filtering (AF) is a 'hack' to remove these artifacts. From this simple definition, we can base our image quality comparison by seeing which card has the less aliasing.Let's look at how anti aliasing is done on the GeForce 6800GT and Radeon X800XL. While their implementation is different in some ways, they're actually quite similar. Both cards rely on multisampling anti aliasing (MSAA) by computing an anti aliased pixel from several texel / geometry samples. The main difference between ATI and NVIDIA is the sample pattern, the amount of modes each card supports and gamma correction or adjustment. You can see the sample pattern for each AA mode on both cards (the ones' selectable from the driver control panel) below.
ATI (from left to right - 2x, 4x, 6x)
NVIDIA (from left to right - 2x, 2xQ, 4x, 6xS)
You can see that for the most part, ATI's sample pattern for 2x and 4x are similar to NVIDIA's - a mirror of each other. Both vendor's 4x sample pattern use a rotated grid sample pattern. The only major difference between ATI and NVIDIA is ATI's 6x mode and NVIDIA's 8x mode sample pattern. NVIDIA uses two texture and 8 geometry samples while ATI only uses one texture and 6 geometry samples. ATI's sample pattern looks less 'ordered' than NVIDIA so this may produce better image quality. Of course, we have to see actual in game screenshots to see whether or not that's the case. There's at least two (or three modes) we can compare directly - ATI's 2x to NVIDIA's 2x and 2xQ (short for Quincunx) and 4x implementation from both vendors. From the looks of it, 4x AA screenshots should be fairly close, if not identical. As is often is the case, more samples are more effective so expect 4x AA to be 'better' at removing aliasing than 2x. We'll mainly concentrate on 4x AA for our comparison (since this mode is the one most heavily used) and touch 2x, 2xQ, 6x and 8xS along the way.
Although comparing anti aliasing implementation is fairly easy, comparing anisotropic filtering between ATI and NVIDIA (or on this case, the Radeon X800XL and GeForce 6800) prove to be more difficult. Unlike anti aliasing, there's a lot more optimization involved. Both vendors have implemented optimizations differently so getting the same exact image from both cards is close to impossible. Both vendors implementation supports up to 16 samples anisotropic filtering and of course, plain bilinear and trilinear filtering. The MIP level patterns from the X800XL and 6800GT is very similar, although you can see NVIDIA is slightly more aggressive with their MIP levels. You can see their angle optimizations more clearly (in 8x and 16x shots).
ATI's default quality (anisotropic filtering is application controlled) (from left to right - 1x, 2x, 4x, 8x and on the bottom 16x)
NVIDIA High Quality (anisotropic filtering is application controlled) (from left to right - 1x, 2x, 4x, 8x and on the bottom 16x)
By default, the image quality setting is set to 'Quality' (and not High Quality) in NVIDIA's drivers. In this settings, some optimizations are enabled (for better performance). You could still control the sample number for anisotropic filtering from the application (if it has an anisotropic filtering setting). In the MIP levels shots, you can see this setting is even more aggressive than the High Quality settings.
NVIDIA quality (anisotropic filtering is application controlled) (from left to right - 1x, 2x, 4x, 8x and on the bottom 16x)
If you were to select the number of samples for anisotropic filtering in the driver panel, the MIP levels stay fairly the same. However, there is one significant difference. Forcing the number of samples from the driver panel in Quality mode actually forces bilinear filtering on all texture stages except stage 0. You can see the difference below, the left is 16x AF High Quality and the right is 16x AF Quality.
Why is this happening? Anisotropic optimizations. Under 'Quality' image settings, whenever you change the amount of samples to filter to another option than 'Application Controlled', NVIDIA's drivers will enable all optimizations - trilinear, anisotropic sample and mip filter optimizations. In their words,
"Aniso mip filter optimization enables the NVIDIA display driver to substitute point-mipmap filtering for linear-mipmap filtering on all but the primary texture stage...Anisotropic sample optimization enables a variety of sample-relate optimizations on all but the primary texture stage...Trilinear optimization...enables the NVIDIA display driver to apply trilinear optimization for better texture filtering performance."
You can manually turn off anisotropic filter optimization to re-enable 'optimized' trilinear filtering on all texture stages for best image quality, but if you want to avoid texture shimmering, you must turn off all these optimizations. NVIDIA does have newer drivers implementing these optimizations without causing texture shimmering (the recently released 81.85), but we've already finished testing when that driver came out, so we're sticking to Forceware 78.01 for this comparison.
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