This technique is similar to what teachers do when they grade tests on a curve. A few students typically get extremely high or low test scores, while the majority fall in the middle of the distribution, giving the resulting histogram of test scores a distinctive bell-shaped curve. The teacher then assigns the top student on the list 100 percent, the next 99 percent, and so on producing an even distribution of marks. The final mark depends on a student's rank in the class, not on the actual test score. By adjusting the grading curve, the distribution of marks can be adjusted at will.
Histogram equalization has the same effect on pixel values.
Stretching is the most basic and important image-processing function. Another popular set of techniques, known collectively as filtering, can be used to modify the content of the image. Canadian amateur astronomer Douglas George is an electrical engineer involved with new sensor technologies at GasTOPS, Ltd., in Ontario. This article first appeared in the Fall 1994 issue of CCD Astronomy. Sky Publishing Corp. All rights reserved.