Refers to a distortion that appears as patterning seen on images where two similar fine patterns overlap, for example two fine gratings can appear to produce diagonal lines. The effect can appear even when one of the patterns is normally invisible, such as the sampling frequency of the image. In a good image system this should be avoided by use of filters but, for instance, the fine detail of a grid pattern may suddenly collapse and appear as curves or diagonal lines as the camera zooms in and the pattern detail nears the digital sampling frequency.
The occurrence of Moiré patterning changes with the picture scanning system. The higher the picture’s resolution the finer the pattern that will cause Moiré. For example, a quite small dog-tooth patten worn by a news presenter could cause it in SD, but with HD it has to be twice as fine.