In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled. It also refers to the distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal.
Aliasing example of the A letter in Times New Roman. Left: aliased image, right: antialiased image.
When a digital image is viewed, a reconstruction—also known as an interpolation—is performed by a display or printer device, and by the eyes and the brain. If the resolution
is too low, the reconstructed image will differ from the original image, and an alias is seen. An example of spatial aliasing
is the Moiré pattern
one can observe in a poorly pixelized image of a brick wall. Techniques that avoid such poor pixelizations are called anti-aliasing
. Aliasing can be caused either by the sampling stage or the reconstruction stage; these may be distinguished by calling sampling aliasing prealiasing
and reconstruction aliasing postaliasing.
St Fargeaux castle aliased
St Fargeaux castle antialiased
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.
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 (pixels
) per degree or per radian, or samples per mm in the focal plane of the cameraSource