An edge in an image is a location where local brightness, texture, or color changes dramatically. As shown in Fig. 5.1, these changes often occur at the boundaries between objects and backgrounds, between objects, or between regions. Edges contain shape information of objects and structural information of scenes in the image, which play a crucial role in the visual system. Jitendra Malik, a member of the National Academy of Engineering in the USA, pointed out that shape provides more robust information than other features (Borenstein and Malik, in Shape guided object segmentation, 2006).