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.
People with sufficient control over the parallax of their eyeballs (e.g. those who can easily view random-dot stereograms), is to hold up two paper printouts and go cross-eyed to superimpose them. This invokes deep, fast, built-in image comparison wetware (the same machinery responsible for depth perception) and 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. 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.
In policing, the technique is used for analysis of fingerprints and identity parades.
Read more about Visual Comparison: Computer Jargon
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