In most image-processing tasks, you need to scan all pixels of the image in order to perform a computation. Considering the large number of pixels that will need to be visited, it is essential that you perform this task in an efficient way. This recipe, and the next one, will show you different ways of implementing efficient scanning loops. This recipe uses the pointer arithmetic.
We will illustrate the image-scanning process by accomplishing a simple task: reducing the number of colors in an image.
Color images are composed of 3-channel pixels. Each of these channels corresponds to the intensity value of one of the three primary colors, red, green, and blue. Since each of these values is an 8-bit unsigned character, the total number of colors is 256x256x256, which is more than 16 million colors. Consequently, to reduce the complexity of an analysis, it is sometimes useful to reduce the number of colors in an image. One way to achieve this goal...