ATI's Optimized Texture Filtering Called Into Question

Linear Filtering

To attain a better result, the texture must be additionally filtered. Anyone who has worked with image editing programs will be familiar with the effect of pixelation. If you take a picture and reduce it by an uneven factor, many details are lost. To prevent this from happening, good image editing programs allow various filtering methods to keep losses from scaling to a minimum.

Here we scale down a bitmap showing a part of a website. Without filtering, the text becomes unreadable. On the filtered image, the text is a bit blurred but still readable.

A texture is nothing but a bitmap, that is, an image consisting of a horizontal and vertical amassing of pixels. If you reduce this picture, fewer pixels - and thus fewer details - are available. But back to our texture. It is spatially arranged and thus is also scaled asymmetrically. In the case of our floor texture, it is therefore considerably more reduced in the horizontal dimension than it is in the vertical.

With textures, today's graphics cards, like image editing programs, use a special filtering that is called linear filtering, sometimes called linear sampling. As a result, when the image is reduced, color values are calculated from several texels to represent the new pixel. The value of the final pixel is the result of four neighboring texels (a texel is a pixel in a texture; in the final spatial representation, i.e. in the game, they are referred to as pixels).