Aliasing

Aliasing is the name for an effect that occurs in digital signal processing, when the sample rate is lower than the Nyquist rate.The Nyquist–Shannon sampling theorem says that it is possible to exactly reconstruct a signal that is made of samples of a given frequency, if the sampling rate is at least double that frequency. If the sampling rate is too low, effects such as aliasing will occur. Low-pass filters can be used to reduce the effect of aliasing. Moiré patterns are examples of aliasing.

An example of aliasing

This full-sized image shows what a properly sampled image of a brick wall should look like with a screen of sufficient resolution.

When the resolution is reduced, aliasing appears in the form of a moiré pattern.

The motion of the 'camera' at a constant shutter speed creates temporal aliasing known as the wagon wheel effect. The speed of the "camera", moving towards the right, constantly increases at the same rate with the objects sliding to the left. Halfway through the 24-second loop, the objects appear to suddenly shift and head in the reverse direction, towards the right.