Anti-aliasing

In digital signal processing, anti-aliasing is the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution signal at a lower resolution. Anti-aliasing is used in digital photography, computer graphics, digital audio, and many other domains.In the image domain, aliasing artifacts can appear as wavy lines or bands, or moiré patterns, or popping, strobing, or as unwanted sparkling; in the sound domain, as rough, inharmonic, or spurious tones, or as noise.Anti-aliasing means removing signal components that have a higher frequency than is able to be properly resolved by the recording (or sampling) device. This removal is done before (re-)sampling at a lower resolution. When sampling is performed without removing this part of the signal, it causes undesirable artifacts such as the black-and-white noise near the top of figure 1-a.
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<graphics> A technique used on a grey-scale or colour bitmap display to make diagonal edges appear smoother by setting pixels near the edge to intermediate colours according to where the edge crosses them.
The most common example is black characters on a white background. Without anti-aliasing, diagonal edges appear jagged, like staircases, which may be noticeable on a low resolution display. If the display can show intermediate greys then anti-aliasing can be applied. A pixel will be black if it is completely within the black area, or white if it is completely outside the black area, or an intermediate shade of grey according to the proportions of it which overlap the black and white areas. The technique works similarly with other foreground and background colours.
"Aliasing" refers to the fact that many points (which would differ in the real image) are mapped or "aliased" to the same pixel (with a single value) in the digital representation.
(1998-03-13)