10.6 FUZZY METHODS OF CONTRAST ENHANCEMENT The human eye does not respond to subtle differences in illumination. The purpose of contrast enhancement is to improve the overall visibility of the image gray levels. This may be achieved by transforming the image gray levels in such a way that the dark pixels appear darker and light pixels appear lighter. Such a transform increases the differences in gray level intensity and thus enables our vision system to discern these differences. The contrast stretching algorithms may employ single-level or multilevel thresholding. For example, a threshold can be selected at a gray level XI and any gray level falling below that is still reduced and any gray level falling above it is increased. Care is taken that the stretching saturates at the extreme ends. For example, if we employ linear stretching across a single threshold xt, mathematically it can be represented as