Contrast stretching is a process that expands the range of intensity levels in an image so that it spans the full intensity range of the recording medium or display device 。Fig.1 shows a typical transformation used for contrast stretching.stretching.The locations of points (r1, s1) and (r2, s2) control the shape of the transformation function. with ( ), the contrast stretched image Fig.3 was obtained from the original image Fig.2. Fig.1 Fig.2 Fig.3