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7.2 Segmentation by Thresholding

Thresholding is a well-known technique for image segmentation [Har92, Sah88]. Thresholding is the operation of converting a multilevel image into a binary image. In a binary image, each pixel value is represented by a single binary digit. In its simplest form, thresholding is a point-based operation that assigns the value of 0 or 1 to each pixel of an image based on a comparison with some global threshold value T.

Threshold functions.

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The central question in threshold segmentation is the selection of a threshold value. This is typically done interactively, based on a visual inspection of the result. However, given some appropriate measure of the quality of the segmentation, it is possible to derive automatic threshold selection algorithms. Over the years many methods have been proposed based on a large number of possible optimization criteria [Sah88, Har92]. The function OptimumThreshold returns a threshold value based on one of three user-selectable minimization criteria. The available choices are: minimum weighted sum of group variances, where the groups are formed from the pixels that fall above and below some chosen threshold [Ots79], minimum average pixel classification error [Kit86], and minimum entropy [Kap85].
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Function Where.

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