Superimposition

Superimposition is a graphics term meaning the placement of an image or video on top of an already-existing image or video, usually to add to the overall image effect, but also sometimes to conceal something (such as when a different face is superimposed over the original face in a photograph). This technique is used to produce photomaps by superimposing aerial photographs over grid lines, contours and other normal mapping features.

Superimposition of 2D images containing correlated periodic grid structures may produce moiré patterns. Superimposition of two correlated layers comprising parallel lines or curves may give rise line moiré patterns. The movement of one of the layers resultis in a faster movement of the line moiré superimposition image. Such optical acceleration is known as moiré speedup. When superimposing two identical layers comprising randomly spaced parallel lines, at a small angle or with a small scaling difference [random line moiré] patterns or Glass patterns (after Leon Glass, 1969) appear.