Illusory light and dark bands that are seen at the transitions between uniformly illuminated regions of uniform density. Mach bands arise because the human visual system is good at recognizing edges and thus enhances the intensity change between the regions so that the lighter side appears lighter and the darker side appears darker. This is a particular problem with shading algorithms, such as Gouraud shading, which cause a change in intensity to appear at the edge of an object between two polygons. Phong shading reduces Mach banding.