Contrast Enhancement

Contrast in an image is the difference between pixels values. Without contrast we can't see anything because everything will be white or black, or just gray. If an image has a low contrast, it means that different objects in the image have low luminosity difference, and and it'll be difficult to separate them in our perception. Contrast enhancement will be helpful in increasing the visibility of an image. Figure 1 show two images with different contrast and their histogram.

Figure 1. Histograms of An Image Before Contrast Enhancement (upper image) and After Contrast Enhancement (lower image)

Contrast enhancement can be done using various gray scale transformation (GST) formulas. The following is one of contrast enhancement GST formula:

Po=G(Pi-C)+C

Which G is contrast gain, and C is the center for contrasting reference. At point C the pixel value is not modified, at above C point the pixels values is increased, and below C they're decreased.