Anisotropic filtering
Encyclopedia
3D computer graphics
3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...
, anisotropic filtering (abbreviated AF) is a method of enhancing the image quality of textures
Texture filtering
In computer graphics, texture filtering or texture smoothing is the method used to determine the texture color for a texture mapped pixel, using the colors of nearby texels . Mathematically, texture filtering is a type of anti-aliasing, but it filters out high frequencies from the texture fill...
on surfaces that are at oblique viewing angles
Dutch angle
Dutch tilt, Dutch angle, Dutch shot, oblique angle, German angle, canted angle, Batman angle, or jaunty angle are terms used for one of many cinematic techniques often used to portray the psychological uneasiness or tension in the subject being filmed...
with respect to the camera where the projection of the texture (not the polygon or other primitive on which it is rendered) appears to be non-orthogonal (thus the origin of the word: "an" for not, "iso" for same, and "tropic" from tropism
Tropism
A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. In tropisms, this response is dependent on the direction of the stimulus...
, relating to direction; anisotropic filtering does not filter the same in every direction).
Like bilinear
Bilinear filtering
Bilinear filtering is a texture filtering method used to smooth textures when displayed larger or smaller than they actually are.Most of the time, when drawing a textured shape on the screen, the texture is not displayed exactly as it is stored, without any distortion...
and trilinear filtering
Trilinear filtering
Trilinear filtering is an extension of the bilinear texture filtering method, which also performs linear interpolation between mipmaps.Bilinear filtering has several weaknesses that make it an unattractive choice in many cases: using it on a full-detail texture when scaling to a very small size...
, anisotropic filtering eliminates aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
effects, but improves on these other techniques by reducing blur and preserving detail at extreme viewing angles.
Anisotropic compression is relatively intensive (primarily memory bandwidth
Memory bandwidth
Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a processor. Memory bandwidth is usually expressed in units of bytes/second, though this can vary for systems with natural data sizes that are not a multiple of the commonly used 8-bit bytes.Memory...
and to some degree computation
Computation
Computation is defined as any type of calculation. Also defined as use of computer technology in Information processing.Computation is a process following a well-defined model understood and expressed in an algorithm, protocol, network topology, etc...
ally, though the standard space-time tradeoff
Space-time tradeoff
In computer science, a space–time or time–memory tradeoff is a situation where the memory use can be reduced at the cost of slower program execution...
rules apply) and only became a standard feature of consumer-level graphics cards in the late 1990s. Anisotropic filtering is now common in modern graphics hardware (and video driver software) and is enabled either by users through driver settings or by graphics applications and video games through programming interfaces.
An improvement on isotropic MIP mapping
Hereafter, it is assumed the reader is familiar with MIP mappingMipmap
In 3D computer graphics texture filtering, MIP maps are pre-calculated, optimized collections of images that accompany a main texture, intended to increase rendering speed and reduce aliasing artifacts. They are widely used in 3D computer games, flight simulators and other 3D imaging systems. The...
.
If we were to explore a more approximate anisotropic algorithm, RIP mapping, as an extension from MIP mapping, we can understand how anisotropic filtering gains so much texture mapping quality. If we need to texture a horizontal plane which is at an oblique angle to the camera, traditional MIP map minification would give us insufficient horizontal resolution due to the reduction of image frequency in the vertical axis. This is because in MIP mapping each MIP level is isotropic, so a 256 × 256 texture is downsized to a 128 × 128 image, then a 64 × 64 image and so on, so resolution halves on each axis simultaneously, so a MIP map texture probe to an image will always sample an image that is of equal frequency in each axis. Thus, when sampling to avoid aliasing on a high-frequency axis, the other texture axes will be similarly downsampled and therefore potentially blurred.
With RIP map anisotropic filtering, in addition to downsampling to 128 × 128, images are also sampled to 256 × 128 and 32 × 128 etc. These anisotropically downsampled images can be probed when the texture-mapped image frequency is different for each texture axis and therefore one axis need not blur due to the screen frequency of another axis and aliasing is still avoided. Unlike more general anisotropic filtering, the RIP mapping described for illustration has a limitation in that it only supports anisotropic probes that are axis-aligned in texture space, so diagonal anisotropy still presents a problem even though real-use cases of anisotropic texture commonly have such screenspace mappings.
In layman's terms, anisotropic filtering retains the "sharpness" of a texture normally lost by MIP map texture's attempts to avoid aliasing. Anisotropic filtering can therefore be said to maintain crisp texture detail at all viewing orientations while providing fast anti-aliased texture filtering.
Degree of anisotropy supported
Different degrees or ratios of anisotropic filtering can be applied during rendering and current hardware rendering implementations set an upper bound on this ratio. This degree refers to the maximum ratio of anisotropy supported by the filtering process. So, for example 4:1 anisotropic filtering will continue to sharpen more oblique textures beyond the range sharpened by 2:1.In practice what this means is that in highly oblique texturing situations a 4:1 filter will be twice as sharp as a 2:1 filter (it will display frequencies double that of the 2:1 filter). However, most of the scene will not require the 4:1 filter; only the more oblique and usually more distant pixels will require the sharper filtering. This means that as the degree of anisotropic filtering continues to double there are diminishing returns in terms of visible quality with fewer and fewer rendered pixels affected, and the results become less obvious to the viewer.
When one compares the rendered results of an 8:1 anisotropically filtered scene to a 16:1 filtered scene, only a relatively few highly oblique pixels, mostly on more distant geometry, will display visibly sharper textures in the scene with the higher degree of anisotropic filtering, and the frequency information on these few 16:1 filtered pixels will only be double that of the 8:1 filter. The performance penalty also diminishes because fewer pixels require the data fetches of greater anisotropy.
In the end it is the additional hardware complexity vs. these diminishing returns, which causes an upper bound to be set on the anisotropic quality in a hardware design. Applications and users are then free to adjust this trade-off through driver and software settings up to this threshold.
Implementation
True anisotropic filtering probes the texture anisotropically on the fly on a per-pixel basis for any orientation of anisotropy.In graphics hardware, typically when the texture is sampled anisotropically, several probes (texel
Texel (graphics)
A texel, or texture element is the fundamental unit of texture space, used in computer graphics. Textures are represented by arrays of texels, just as pictures are represented by arrays of pixels....
samples) of the texture around the center point are taken, but on a sample pattern mapped according to the projected shape of the texture at that pixel.
Each anisotropic filtering probe is often in itself a filtered MIP map sample, which adds more sampling to the process. Sixteen trilinear anisotropic samples might require 128 samples from the stored texture, as trilinear MIP map filtering needs to take four samples times two MIP levels and then anisotropic sampling (at 16-tap) needs to take sixteen of these trilinear filtered probes.
However, this level of filtering complexity is not required all the time. There are commonly available methods to reduce the amount of work the video rendering hardware must do.
Performance and optimization
The sample count required can make anisotropic filtering extremely bandwidthMemory bandwidth
Memory bandwidth is the rate at which data can be read from or stored into a semiconductor memory by a processor. Memory bandwidth is usually expressed in units of bytes/second, though this can vary for systems with natural data sizes that are not a multiple of the commonly used 8-bit bytes.Memory...
-intensive. Multiple textures are common; each texture sample could be four bytes or more, so each anisotropic pixel could require 512 bytes from texture memory, although texture compression
Texture compression
Texture compression is a specialized form of image compression designed for storing texture maps in 3D computer graphics rendering systems. Unlike conventional image compression algorithms, texture compression algorithms are optimized for random access....
is commonly used to reduce this.
As a video display device can easily contain over a million pixels, and as the desired frame rate can be as high as 30–60 frames per second (or more) the texture memory bandwidth can become very high very quickly. Ranges of hundreds of gigabytes per second of pipeline bandwidth for texture rendering operations is not unusual where anisotropic filtering operations are involved.
Fortunately, several factors mitigate in favor of better performance:
- The probes themselves share cacheCacheIn computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...
d texture samples, both inter-pixel and intra-pixel. - Even with 16-tap anisotropic filtering, not all 16 taps are always needed because only distant highly oblique pixel fills tend to be highly anisotropic.
- Highly Anisotropic pixel fill tends to cover small regions of the screen (i.e. generally under 10%)
- Texture magnification filters (as a general rule) require no anisotropic filtering.
See also
- Bilinear filteringBilinear filteringBilinear filtering is a texture filtering method used to smooth textures when displayed larger or smaller than they actually are.Most of the time, when drawing a textured shape on the screen, the texture is not displayed exactly as it is stored, without any distortion...
- Trilinear filteringTrilinear filteringTrilinear filtering is an extension of the bilinear texture filtering method, which also performs linear interpolation between mipmaps.Bilinear filtering has several weaknesses that make it an unattractive choice in many cases: using it on a full-detail texture when scaling to a very small size...
- Anti-aliasingAnti-aliasingIn digital signal processing, spatial anti-aliasing is the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution...
- Digital artifactDigital artifactA digital artifact is any undesired alteration in data introduced in a digital process by an involved technique and/or technology.-Possible causes:...