A visual comparison is to compare two or more things by eye. This might be done by placing them side by side; by overlaying them; by alternating an image or by presenting each image to a separate eye.
Such comparisons are the first stage in a child's development of an understanding of geometry and measurement, before they move to an understanding of measuring devices such as a ruler. [cite book|title=The Child's Conception of Geometry|author=Jean Piaget, Bärbel Inhelder, Alina Szeminska|pages=33|url=http://books.google.com/books?id=A-yQR6DxTkUC|isbn=0415209994|year=1999|publisher=Routledge]
People with good control of the
parallaxof their eyeballs (e.g. those who can easily view random-dot stereograms), can hold up images and go cross-eyed to superimpose them. This invokes the same visual processes responsible for depth perceptionand differences stand out almost immediately. This technique is good for finding edits in graphical images, or for comparing an image with a compressed version to spot artifacts.
Visual comparison with a standard chart or reference is often used as a means of measuring complex phenomena such as the
weather, sea states or the roughness of a river. [cite book|title=Roughness Characteristics of New Zealand Rivers|author=D. Murray Hicks, Peter D. Mason|url=http://books.google.co.uk/books?id=Oez427pUSB0C|isbn=4770260873|year=1991] A colour chart is used for this purpose in many contexts such as chemistry, cosmetics, medical testing and photography.
Comparison by eye may also be used as a source of amusement or
intelligence testing, as in the popular puzzle of spot the difference.
policing, the technique is used for analysis of fingerprintsand identity parades.
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