Blinn–Phong shading model

The Blinn illumination model (also Blinn Phong model ) in the image synthesis, a local illumination model for the light reflection at surfaces. As the basis, the Phong illumination model is used. By using so-called Halfway vectors, the necessary calculations are accelerated without affecting the result in noticeable ways. The model was described in 1977 by James F. Blinn (Jim Blinn ), who also developed the bump mapping.

Application

In practice, the Blinn illumination model is used, for example, OpenGL, since it avoids the calculation of the reflection vector. Instead, the angle bisector will be used:

With

• Normed vector ... from the point to the viewer
• Normalized vector from the point ... to the point light source to be considered

With this now, the cosine of the angle are calculated between the normal and the bisector:

The above formula is of course only under the condition that is. This cosine you can now use instead of in the known from the Phong illumination model formula for calculating the specular component:

With

• ... Light intensity of the incident light beam, the point light source
• Empirically determined ... reflection factor specular component of the reflection
• Constant exponent ... for describing the surface texture ( in OpenGL " shininess " called )

Comparison with the Phong model

The angle is approximately half as large as the angle in the Phong model. ( This relationship is exact if L, V and N lie in a plane. )

In order to obtain comparable results with the Blinn - Phong model as the model, you have the exponent n in the above formula four times as great as select the exponent n in the Phong model.