X-bit labs

NVIDIA GeForce 6800 Ultra and GeForce 6800: NV40 Enters the Scene (page 14)

Category: Video

by Tim Tscheblockov , Alexey Stepin , Anton Shilov

[ 04/14/2004 | 01:48 AM ]

Now, let’s examine a portion of the scene with near-vertical polygon edges. As before, the images we got with the GeForce 6800 are to the left; the GeForce FX 5950 Ultra shows its work in the middle; the RADEON 9800 XT outputs its images to the right:

GeForce 6800 Ultra	GeForce FX 5950 Ultra	RADEON 9800 XT
2x
4x
8x/6x

Everything we said about the previous case (with nearly-horizontal polygon edges) can be repeated once again. The only difference from the previous situation is that both leaders, the GeForce 6800 and RADEON 9800 XT, are doing much like each other in the 4x mode – it’s hard to prefer any of them.

So:

• The three cards provide the same-quality 2x FSAA on nearly-vertical lines;
• The GeForce 6800 and the RADEON 9800 XT are much better in the 4x FSAA mode than the GeForce FX 5950 Ultra;
• The RADEON 9800 XT is unrivalled in the 8x/6x mode, using multisampling on 6 sub-pixels placed on a rotated grid, rather than a combination of supersampling and multisampling on ordered-grid sub-pixels.

Let’s test some intermediary angle of the polygon edge. As always, the results of the GeForce 6800 are to the left; those of the GeForce FX 5950 Ultra are in the middle; and images of the RADEON 9800 XT are to the right.

GeForce 6800 Ultra	GeForce FX 5950 Ultra	RADEON 9800 XT
2x
4x
8x/6x

• The three graphics cards all produce a disgusting picture in the 2x mode – the picture doesn’t smell of any full-screen antialiasing. The image produced by the RADEON 9800 XT is slightly less degusting than the others, though, as this GPU has the sub-pixels placed on another diagonal of the pixel;
• The 4x FSAA method helps the GeForce FX 5950 Ultra show a more or less satisfying result, while the GeForce 6800 and the RADEON 9800 XT output excellent images;
• There’s no great improvement in the visual quality in the 8x/6x mode compared to the 4x one with any of the graphics cards.

Now, we’ve got only one case left, with a transparent texture. That’s a good indicator of supersampling and of its role in the hybrid 8x mode.

The indicator works simply: with multisampling, sub-pixels are not processed for pixels that don’t fall onto polygon edges and the “transparency borders” won’t be smoothed out. If supersampling is used in any way, the “transparency borders” will be antialiased as some sub-pixels will be processed for each pixel in the scene.

Again, the GeForce 6800 Ultra is to the left; the GeForce FX 5950 Ultra is in the middle; the RADEON 9800 XT is to the right.

GeForce 6800 Ultra	GeForce FX 5950 Ultra	RADEON 9800 XT
2x
4x
8x/6x

• Clearly, the RADEON 9800 XT uses “pure” multisampling in all cases;
• The GeForce FX 5950 Ultra and the GeForce 6800 Ultra use a hybrid operational mode for 8x FSAA, a combination of multisampling and supersampling, in a perfectly identical realization. Each resulting pixel is a product of 2x2 supersampling on 4 blocks, each of which is a result of 2x multisampling.

The hybrid mode has both strong and weak points. On the one hand, supersampling in this method smoothes out transparent textures and serves like an additional method of anisotropic filtering (when sub-pixels are processed for each pixel, the projection or “footprint” of the pixel on the texture is calculated with more precision). Multisampling cannot do that. On the other hand, the additional computations make the hybrid FSAA method a resource-consuming solution, much more so than multisampling with the same number of samples.

Now, winding up the section about full-screen antialiasing, we should acknowledge the improvement of 4x FSAA in the GeForce 6800 Ultra thanks to placing sub-pixels on a rotated grid. The new chip from NVIDIA is no inferior to the RADEON 9800 XT in this FSAA mode.

Other FSAA modes came over to the NV40 from the NV38 without any changes, so you shouldn’t wait for anything new from the GeForce 6800 Ultra in this respect. On the other hand, the performance of the NV40 is definitely better than that of NVIDIA’s previous-generation GPUs. That’s why the high-quality, but heavy on the graphics card, “hybrid” 8x FSAA mode may become quite acceptable in situations where the performance of the NV38 was not enough.