- Superimposition
In
graphics , superimposition is the placement of animage 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 aphotograph ).This technique is used in
cartography to produce photomaps by superimposing grid lines,contour line s and other linear or textual mapping features overaerial photograph s.Superimposition of 2D images containing correlated periodic grid structures may produce
moiré pattern s. Superimposition of two correlated layers comprising parallel lines or curves may give riseline moiré patterns. The movement of one of the layers results in a faster movement of theline moiré superimposition image. Such optical acceleration is known asmoiré speedup (check for the formulas of optical speedup for curved patterns). When superimposing two identical layers comprising randomly spaced parallel lines, at a small angle or with a small scaling difference [http://switzernet.com/people/emin-gabrielyan/070227-random-line-moire/ random line moiré] patterns, namely line Glass patterns (after Leon Glass, 1969) appear. Similarly, when superimposing two identical layers of randomly scattered dots at a small angle or with a small scaling difference random dot Glass patterns, namely [http://switzernet.com/people/emin-gabrielyan/070212-random-moire/ random dot moiré] , appears. When one of the layers embeds complex shapes, such as sequences of symbols forming a text, and another layer contains parallel lines or curves, the superimposition image may gives rise to magnified shapes, calledshape moiré patterns.References
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