Editing Aliasing (section)

== Description ==
[[File:Risingstar_test_crop.png|thumb|300px|Dots in the sky due to spatial aliasing caused by [[halftone]] resized to a lower resolution]]
When a digital image is viewed, a [[Signal reconstruction|reconstruction]] is performed by a display or printer device, and by the eyes and the brain. If the image data is processed incorrectly during sampling or reconstruction, the reconstructed image will differ from the original image, and an alias is seen.

An example of spatial aliasing is the [[moiré pattern]] observed in a poorly pixelized image of a brick wall. [[Spatial anti-aliasing]] techniques avoid such poor pixelizations. Aliasing can be caused either by the sampling stage or the reconstruction stage; these may be distinguished by calling sampling aliasing {{anchor|prealiasing}} ''prealiasing'' and reconstruction aliasing {{anchor|postaliasing}} ''postaliasing.''<ref name=mitchell/>

Temporal aliasing is a major concern in the sampling of video and audio signals. Music, for instance, may contain high-frequency components that are inaudible to humans. If a piece of music is sampled at 32,000 [[Sampling rate|samples per second]] (Hz), any frequency components at or above 16,000 [[hertz|Hz]] (the [[Nyquist frequency]] for this sampling rate) will cause aliasing when the music is reproduced by a [[digital-to-analog converter]] (DAC). The high frequencies in the analog signal will appear as lower frequencies (wrong alias) in the recorded digital sample and, hence, cannot be reproduced by the DAC.  To prevent this, an [[anti-aliasing filter]] is used to remove components above the Nyquist frequency prior to sampling.

In video or cinematography, temporal aliasing results from the limited frame rate, and causes the [[wagon-wheel effect]], whereby a spoked wheel appears to rotate too slowly or even backwards. Aliasing has changed its apparent frequency of rotation. A reversal of direction can be described as a [[negative frequency]].  Temporal aliasing frequencies in video and cinematography are determined by the frame rate of the camera, but the relative intensity of the aliased frequencies is determined by the shutter timing (exposure time) or the use of a temporal aliasing reduction filter during filming.
<ref name=tessive/>{{unreliable source?|date=July 2018}}

Like the video camera, most sampling schemes are periodic; that is, they have a characteristic [[sampling frequency]] in time or in space. Digital cameras provide a certain number of samples ([[pixel]]s) per degree or per radian, or samples per mm in the focal plane of the camera. Audio signals are sampled ([[digitized]]) with an [[analog-to-digital converter]], which produces a constant number of samples per second. Some of the most dramatic and subtle examples of aliasing occur when the signal being sampled also has periodic content.