The contrast stretching operation takes a low-contrast image as input and stretches the narrower range of the intensity values to span a desired wider range of values in order to output a high-contrast output image, thereby enhancing the image's contrast. It is just a linear scaling function that is applied to image pixel values, and hence the image enhancement is less drastic (than its more sophisticated counterpart histogram equalization, to be described shortly). The following screenshot shows the point transformation function for contrast stretching:

As can be seen from the previous screenshot, the upper and lower pixel value limits (over which the image is to be normalized), need to be specified before the stretching can be performed (for example, for a gray-level image, the limits are often set to 0 and 255, in order for the output image to span the entire range of available pixel values). All we need to find is a suitable value of m from the CDF of the original image. The contrast stretching transform produces higher contrast than the original by darkening the levels below the value m (in other words, stretching the values toward the lower limit) in the original image and brightening the levels previous to value m (stretching the values toward the upper limit) in the original image. The following sections describe how to implement contrast-stretching using the PIL library.