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Anti-aliasing may refer to any of a number of techniques to combat the problems of
aliasing In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when ...
in a sampled signal such as a
digital image A digital image is an image composed of picture elements, also known as ''pixels'', each with '' finite'', '' discrete quantities'' of numeric representation for its intensity or gray level that is an output from its two-dimensional functions f ...
or digital audio recording. Specific topics in anti-aliasing include: *
Anti-aliasing filter An anti-aliasing filter (AAF) is a filter used before a signal sampler to restrict the bandwidth of a signal to satisfy the Nyquist–Shannon sampling theorem over the band of interest. Since the theorem states that unambiguous reconstruc ...
, a filter used before a signal sampler to restrict the bandwidth of a signal such as in audio applications. * Manual anti-aliasing, an artistic technique done in
pixel art Pixel art () is a form of digital art drawn with graphical software where images are built using pixels as the only building block. It is widely associated with the low-resolution graphics from 8-bit and 16-bit era computers and arcade video g ...
graphics to smooth transitions between shapes, soften lines or blur edges. *
Computer-generated imagery Computer-generated imagery (CGI) is the use of computer graphics to create or contribute to images in art, printed media, video games, simulators, and visual effects in films, television programs, shorts, commercials, and videos. The image ...
(CGI), the application of computer graphics for creating or improving images in art, printed media, simulators, videos and video games. **
Spatial anti-aliasing In digital signal processing, spatial anti-aliasing is a technique for minimizing the distortion artifacts ( aliasing) when representing a high-resolution image at a lower resolution. Anti-aliasing is used in digital photography, computer graph ...
, the technique of minimizing aliasing when representing a high-resolution image at a lower resolution **
Fast approximate anti-aliasing Fast approximate anti-aliasing (FXAA) is a screen-space anti-aliasing algorithm created by Timothy Lottes at Nvidia. FXAA 3 is released under a public domain license. A later version, FXAA 3.11, is released under a 3-clause BSD license. Algor ...
(FXAA), an anti-aliasing algorithm created by Timothy Lottes under Nvidia. May also be referred to as Fast Sample Anti-aliasing (FSAA). **
Multisample anti-aliasing Multisample anti-aliasing (MSAA) is a type of spatial anti-aliasing, a technique used in computer graphics to remove jaggies. Definition The term generally refers to a special case of supersampling. Initial implementations of full-scene anti ...
(MSAA), a type of spatial anti-aliasing method ** Super-sampling (SSAA), a type of spatial anti-aliasing method **
Morphological antialiasing Morphological antialiasing (MLAA) is a technique for minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution. Contrary to multisample anti-aliasing (MSAA), which does not work for defe ...
(MLAA), a type of spatial anti-aliasing method ** Conservative morphological anti-aliasing (CMAA), a type of spatial anti-aliasing method ** Area-based anti-aliasing (ABAA), new anti-aliasing algorithm that relies on subpixel area-sampling instead point-sampling. Unlike with N points sampling, with ABAA the pixel is divided into N subpixel areas of equal size. For each triangle edge, the covered area assigned to subpixel areas is equal to the actual covered area, to the closest digit. *
Temporal anti-aliasing Temporal anti-aliasing (TAA) is a spatial anti-aliasing technique for computer-generated video that combines information from past frames and the current frame to remove jaggies in the current frame. In TAA, each pixel is sampled once per frame bu ...
(TAA) in CGI, techniques to reduce or remove the effects of temporal aliasing in moving images **
Deep learning anti-aliasing Deep learning anti-aliasing (DLAA) is a form of spatial anti-aliasing created by Nvidia. DLAA depends on and requires Tensor Cores available in Nvidia RTX cards. DLAA is similar to deep learning super sampling (DLSS) in its anti-aliasing method, ...
(DLAA), a type of spatial and temporal anti-aliasing method relying on dedicated tensor core processors ** Deep learning super sampling (DLSS), a family of real-time deep learning image enhancement and upscaling technologies developed by Nvidia that are available in a number of video games.


Aliasing & Anti-Aliasing in CGI

The purpose of anti-aliasing is to reduce, if not eliminate, distracting aliasing artifacts in
Computer Generated Imagery Computer-generated imagery (CGI) is the use of computer graphics to create or contribute to images in art, printed media, video games, simulators, and visual effects in films, television programs, shorts, commercials, and videos. The ima ...
. Most aliasing artifacts, or
jaggies "Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing, which in turn is often caused by non-linear mixing effects producing high-frequency components, or missing or poor anti-aliasing filtering prior to sampl ...
, result from using only one sample point per pixel when computing 2D images. Images produced with single point sampling can achieve the fastest frame rate, at the expense of aliasing artifacts such as “stairsteps” and “narrow faces breakup”. These artifacts are more noticeable in real-time (RT) CGI applications, such as when simulating visual scenes for flight training or playing computer video games. In moving images, stairsteps result in “crawling“. Also, “narrow faces breakup” result in “faces popping in-and-out of scenes”. Most anti-aliasing solutions rely on multiple point sampling, or single point sampling followed by postprocessing, to reduce stairsteps and crawling. But they have limited success at correcting narrow faces breakup. Also, their performances depend on edge orientation.


ABAA

A new approach, area-based anti-aliasing (ABAA), relies on subpixel area sampling. It is the fastest and produces the best static and moving images with anti-aliasing. Currently, there is no readily available product using ABAA. But simulations have shown that ABAA is the best anti-aliasing solution.


Detecting Intersected Pixels

Most approaches use sample points to detect when a portion of the triangle covers the pixel. With ABAA, the rendering operation is optimized with 2 types of edges: Horizontal edges (HE) and Vertical edges (VE). When edges traverse the image from pixel to pixel, the intersected pixels are easily identified. A pixel is partially covered by a triangle edge, when that edge intersects a midline inside of that pixel. At the same time, the partially covered area of that pixel, that is used for color mix, is readily available in 1 measurement (no lengthy computations). ABAA can be implemented directly using pixel covered areas, or the area can be mapped into 4, 8, 16 or 32 subpixel areas. The area-based approach solves several problems. * ABAA can quickly determine the covered area inside intersected pixels and compute the mixed colors. * It produces equal steps as triangle edges move across pixels, for all edge-orientations. The covered area of pixels increases uniformly (linearly) from 0.0 to 1.0 as edges move across pixels. * With 4 (or 8) subpixels It can handle narrow face breakup for faces at least 1/4 (or 1/8) pixel wide. * It can produce high image quality without speed penalty. It is faster since is does not require multiple frame processing.


Multiple Sample Points

Two methods rely on multiple sample points anti-aliasing. It consists of computing multiple images, followed by averaging.


SSAA

The super-sampling anti-aliasing (SSAA) approach has been used in non-real-time applications. In this approach, a 512x512 image is first computed at higher resolution, such as 2048x2048, for example. It is then reduced through averaging or filtering to produce a 512x512 image. It is computation intensive and cannot be used for RT CGI applications. Since there are no time constraints, large images can be computed offline using high-speed general-purpose.


MSAA

With
multisample anti-aliasing Multisample anti-aliasing (MSAA) is a type of spatial anti-aliasing, a technique used in computer graphics to remove jaggies. Definition The term generally refers to a special case of supersampling. Initial implementations of full-scene anti ...
(MSAA), images are computed for 4 (or 8) subpixel sample points, followed by averaging. It is slow, since the frame rate is reduced by a factor of 4 (or 8). It works well for horizontal and vertical triangle edges. For other edge angles, the gaps between subpixels can cause narrow face breakups.


Eight-queens puzzle

The position of the subpixel sample points for MSAA can be derived from solutions to the “ Eight-queens puzzle”. The solutions to the 8-queens puzzle provide good results for near horizontal and vertical edges. But the anti-aliasing effectiveness is not as good for edges with angles in-between.


Other Solutions

In order to improve the frame rate, several methods rely on single point sampling followed by image post-processing, with mixed results; among these are
fast approximate anti-aliasing Fast approximate anti-aliasing (FXAA) is a screen-space anti-aliasing algorithm created by Timothy Lottes at Nvidia. FXAA 3 is released under a public domain license. A later version, FXAA 3.11, is released under a 3-clause BSD license. Algor ...
(FXAA),
temporal anti-aliasing Temporal anti-aliasing (TAA) is a spatial anti-aliasing technique for computer-generated video that combines information from past frames and the current frame to remove jaggies in the current frame. In TAA, each pixel is sampled once per frame bu ...
(TAA), and
morphological antialiasing Morphological antialiasing (MLAA) is a technique for minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution. Contrary to multisample anti-aliasing (MSAA), which does not work for defe ...
(MLAA).


See also

*
Aliasing In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when ...
* Pixel-art scaling algorithms *
Nyquist–Shannon sampling theorem The Nyquist–Shannon sampling theorem is a theorem in the field of signal processing which serves as a fundamental bridge between continuous-time signals and discrete-time signals. It establishes a sufficient condition for a sample rate that per ...


References

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