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Definition: anti-aliasing


(1) Smoothing a distorted communications signal by applying techniques that add data or filter out unwanted noise.

(2) Smoothing the jagged appearance of diagonal lines in a bitmapped image. The pixels that surround the edges of the line are changed to varying shades of gray or color in order to blend the sharp edge into the background. Also called "dithering," the technique is usually known as anti-aliasing when applied to diagonal and curved lines. See bitmap.






Anti-aliasing
This teapot from the University of Utah is a famous early example of anti-aliasing. (Images courtesy of Computer Sciences Department, University of Utah.)




TRADITIONAL ANTI-ALIASING
Anti-aliasing techniques have been around for many years; however, new approaches are being developed all the time for modern graphics processors in order to make fast-moving video games more lifelike. Following are several techniques used to anti-alias an image. See GPU.

SSAA (super-sampling AA) and MSAA (multi-sampling AA) render the image several times higher (as much as 16x) and then scale it down to fit the target display resolution. SSAA and MSAA are similar, except that MSAA samples only along the edges of the triangles. MSAA cannot be used with the popular "deferred lighting," which applies light and shadows at the output resolution.

CSAA (coverage-sampling AA) and NVIDIA's QSAA (quincunx super AA), used in GeForce 8000 series and higher, are designed to reduce the added stress of MSAA. A quincunx is a five-point geometric pattern, four of which form a square or rectangle, and the fifth is at the center.

MFAA (multi-frame AA) is the successor to CSAA and is used in GeForce GTX 900 processors and higher. MFAA is used with MSAA to improve performance but usage below 40 fps degrades results.

POST-PROCESSING ANTI-ALIASING
Post processing is applied after the image is rendered, which is typically less taxing than other methods, and techniques are compatible with most games and videos.

FXAA (fast-approximate AA) is often the default setting in a GPU. FXAA analyzes every pixel but tends to add blur to patterns with multiple colors because it looks for a specific luma differences between pixels.

MLAA (morphological AA) looks for discontinuities in the last pipeline stage. However, it does not work as well when there many thin lines and elements in the image. MLAA has a greater performance impact than FXAA.

SRAA (subpixel reconstruction AA) is similar to MLAA for the UniEngine.

DLAA (directionally localized AA) provides better temporal stability than MLAA. Used in Star Wars: The Force Unleashed II and other games.

SMAA (subpixel morphological AA) is an evolution of FXAA and MLAA. SMAA is an edge-detection AA method that looks for curves and object boundaries. SMAA blends colors at the top and left edges of a pixel with the bottom and right edges from the neighboring pixel.

CMAA (conservative morphological AA) provides better image quality and temporal stability than FXAA and sits between FXAA and SMAA in processing cost.

TEMPORAL ANTI-ALIASING
Temporal anti-aliasing deals with time-based, frame-to-frame techniques.

TAA (temporal AA) is an umbrella term for techniques such as the following:

TxAA (temporal AA) was designed by NVIDIA for action scenes and adds filters used in CGI. TxAA reduces motion crawling and flickering.

TSSAA (temporal super-sampling AA) compares the frame with the previous frame and blends them together. Like FXAA and MLAA, TAA methods are post-processing techniques.