Histograms

Another useful tool when performing mathematical calculations on images is to use a histogram. In a non-vision sense, histograms are commonly used in statistics to plot the values in a list. They are used to graphically identify frequently or infrequently occurring items. In the world of computer vision, histograms are typically used for listing the colors from each color channel in an image. This is sometimes known as a color channel histogram. This is helpful when manipulating the color and brightness of an image.

The simplest color channel histogram is with a grayscale image. Each pixel in a grayscale image only has one value representing the brightness of that pixel (or what shade of gray it is)—as opposed to the three values representing the different color values in an RGB image. Each of these grayscale values will be in the range from 0 to 255, with 0 being black and 255 being white. To view the brightness histogram of The Starry Night: