In the Blinn-Phong shading model, the light reaching the pixel depends on the angle between the normal and the bisector of the and . If , then ideal reflection is obtained, as in the case of a mirror.
Now suppose that the object is ``shiny''. If it were a perfect mirror, then all of the light from the source would be reflected to the pixel only if they are perfectly aligned; otherwise, no light would reflect at all. Such full reflection would occur if and form the same angle with respect to . What if the two angles are close, but do not quite match? The Blinn-Phong shading model proposes that some amount of light is reflected, depending on the amount of surface shininess and the difference between and . See Figure 7.3. The bisector is the vector obtained by averaging and :
Using the compressed vector notation, the Blinn-Phong shading model sets the RGB pixel values as
This additively takes into account shading due to both diffuse and specular components. The first term is just the Lambertian shading model, (7.2). The second component causes increasing amounts of light to be reflected as becomes closer to . The exponent is a material property that expresses the amount of surface shininess. A lower value, such as , results in a mild amount of shininess, whereas would make the surface almost like a mirror. This shading model does not correspond directly to the physics of the interaction between light and surfaces. It is merely a convenient and efficient heuristic, but widely used in computer graphics.
Steven M LaValle