Do you remember the result of convolving an image with a two-dimensional square wave signal in Chap. 2 ? Yes, the image was smoothed and blurred. This is what we call image filtering. Filtering is an important technique in image processing and has a wide range of applications in real life. Many people have used image processing software (such as Photoshop and Meitu XiuXiu) to retouch photos, beautify people in photos, and perform “skin smoothing”, “acne removal”, “spot removal”, etc., as shown in Fig. 3.1. These retouching operations all rely on the support of image filtering technology.