In histogram endpoint scaling, the user selects the fraction of pixels that can satu rate black and the fraction that can saturate white, as well as parameters describing the scaling between the endpoints. The software evaluates the histogram, finds pixel values corresponding to the low and high endpoints, computes a transfer function, and alters the image.
Other than the method of finding the endpoints, linear and nonlinear stretches with histogram endpoint specification work exactly the same as direct endpoint specification. Although specifying them differs, computing a result does not.
From the user's point of view, the principle advantage of histogram endpoint specification over direct endpoint specification is that it is result-oriented. Because you specify the result you want, results are predictable. In addition, the size of the image and the number of gray levels in the image do not influence the output. The same fraction of pixels is saturated black or white, regardless of the image itself. For each type of image—deep-sky, lunar, or planetary—well-chosen histogram endpoint specifications produce dependable and repeatable results.
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