Anisotropic filtering is a technique used in the 3-D computing field to enhance the image quality of textures on rendered surfaces which are sloped relative to the viewer. This is achieved by eliminating aliasing, which is responsible for the jagged or pixelated quality of some graphics. In addition to its anti-aliasing qualities, this filtering also reduces blurring of sloped textures, an improvement over previous types of filtering known as bilinear and trilinear filtering. An important distinction of anisotropic filtering as compared to other anti-aliasing methods is that it affects only the textures on a shape, but not the shape itself.
Developed in the mid and late 1990s, most modern graphics cards now support anisotropic filtering, which can typically be enabled or disabled within a given application. A common feature in computer games, anisotropic filtering is fairly hardware intensive, and can be set at varying levels to improve either graphics quality or computing performance. The degree of filtering is measured as a ratio, with a level of 4:1 being twice as sharp as 2:1. Diminishing returns are generally experienced the greater the ratio gets, with 16:1 only marginally sharper than 8:1, and so on. The performance trade-off diminishes in the same way, as fewer additional pixels being filtered means less additional strain is placed on the hardware.
Anisotropic filtering works by monitoring a given texture on a pixel-by-pixel basis, and mapping a pattern based on the projected shape of the texture at each pixel. At extreme angles, a single pixel can contain the area used by a great amount of texture data. In this way, the filtering process can become highly data-intensive, and despite advances such as caching texture samples, large amounts of memory bandwidth can be needed, depending on the game or application in use, and the particular scene being rendered. Given the extremely taxing nature of anisotropic filtering on hardware, some graphics card manufacturers have optimized the filtering for common geometric angles seen in gaming, such as walls, floors, and the sky.
Given its strengths in improving the quality of angled textures, the effects of anisotropic filtering are most apparent in games that feature terrain that stretches out a great distance, such as first-person shooters and flight or racing simulators. Games less suited to exploit anisotropic filtering include genres such as real-time strategy and sports simulations, in which the majority of the screen is often taken up by a static background.
