Lambert shading The simplest lighting model merely colors a surface in proportion to its angle relative to the viewer. This is commonly referred to as Lambert or cosine shading. To establish the intensity of the surface, the model uses the cosine of the angle between the ray of light hitting the surface and the surface normal. As the light source comes to be closer to perpendicular to the surface, so the angle between the light ray and the surface normal decreases and the surface lightens. When the light is at right angles to the surface, the angle is zero and the intensity is at a maximum. Lambert shading is extremely fast to calculate and produces a marked improvement in depth perception for the scene. It produces faceted models, as if each surface had been cut from glass, and tends to emphasize the edges between surfaces. Often it is preferable that the individual surfaces that comprise the model be calculated to approximate a smooth edge, but, by emphasizing the surface decomposition as much as the model itself, Lambert shading can make the model appear artificial. In 1971, Henri Gouraud published an improved shading model that generates smooth shading across surfaces—preserving the depth cues inherent in Lambert shading while not emphasizing the edges between surfaces.
