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Complement Image
Human vision detects detail and differences based on a local percentage difference (a minimum of a few percent under most conditions). A difference between brightness values of 20 and 40 is 100 percent, but between 220 and 240 is less than 10 percent. Consequently, it is more difficult to see detail in the bright areas of images than in the dark areas. Photographers have long known that some details are easier to see in negatives. Reversing the contrast in a digital image is accomplished by subtracting the pixel value from 255 (white). This is usually done for the red, green and blue values individually, but in some cases it is more useful to reverse the intensity and leave the colors unchanged. Many programs call this operation “inverse” but of course it is not the arithmetic inverse (= 1/value). This interactive tutorial illustrates the effects of reversing or complementing an image.
The tutorial initializes with a randomly selected specimen appearing in the Specimen Image window. The Choose A Specimen pull-down menu provides a selection of specimen images, in addition to the initial randomly chosen one. The Original Image and Complement Image buttons display either the original or the result of reversing the intensity values. For a color image, clicking the RGB Complement button reverses each color channel individually, while the LAB Complement button reverses the brightness values only leaving the colors unchanged.
Contributing Authors
John C. Russ - Materials Science and Engineering Dept., North Carolina State University, Raleigh, North Carolina, 27695.
Matthew Parry-Hill, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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