These new video cards absolutely scream in today’s games, and will likely run all new games at very high frame rates for some time to come. They are meant to run games with more advanced features like anti-aliasing and anisotropic filtering enabled. We’ll touch on anti-aliasing in a bit, but for now let’s discuss anisotropic filtering.
A full explanation of anisotropic filtering would be quite long and complicated. You can find a bit about it in our earlier 3D Pipeline feature, but the basic gist of it is this: it uses more texture samples to produce sharper, more accurate texturing. In essence, it’s like selectively using higher-detail mipmaps throughout the scene. Both the GeForce 6800 and Radeon X800 cards offer 2X, 4X, 8X, and 16X anisotropic filtering. 1X AF would be just like trilinear, with 8 texture samples per pixel. So 2X AF is 16 samples per pixel, 4X is 32 samples, and so on up to 128 samples per pixel for 16X AF. The picture below demonstrates the difference between trilinear and anisotropic texture filtering. Notice how much sharper and more detailed the AF side is, especially as you move into the distance?
Reading in that much texture data for every single pixel would require massive amounts of memory bandwidth, so both NVIDIA and ATI instead perform “adaptive” anisotropic filtering.
If a surface is facing straight at the camera, it won’t really benefit from anisotropic filtering. 16X AF, with all its texture samples, won’t really look different than plain trilinear filtering. Therefore, both graphics card companies use an adaptive algorithm that analyzes the angle and distance of each surface in a 3D scene and applies more or less filtering as needed. The result is that if you set the video card to perform 16X anisotropic filtering, you get anywhere from 4 (bilinear) to 128 (16X aniso) filtering. What you get depends on how badly the video card driver thinks it needs more texture samples. It’s a trade-off between visual quality and speed that ultimately benefits the consumer. If they tried to perform a full 16X AF on every pixel, even today’s super-fast $500 video cards would slow to a crawl. In a way, the trilinear optimizations introduced in newer video cards is just like this adaptive AF optimization, only less necessary, as the performance difference is much smaller.