Blinn–Phong reflection model

Shading algorithm in computer graphics

The Blinn–Phong reflection model, also called the modified Phong reflection model, is a modification developed by Jim Blinn to the Phong reflection model. Blinn–Phong is a shading model used in OpenGL and Direct3D's fixed function pipeline (before Direct3D 10 and OpenGL 3.1), and is carried out on each vertex as it passes down the graphics pipeline; pixel values between vertices are interpolated by Gouraud shading by default, rather than the more computationally expensive Phong shading.

Blinn Phong will be faster than Phong in the case where the viewer and light are treated to be very remote, such as approaching or at infinity. This is the case for directional lights and orthographic/isometric cameras. In this case, the halfway vector is independent of position and surface curvature simply because the halfway vector is dependent on the direction to viewer's position and the direction to the light's position, which individually converge at this remote distance, hence the halfway vector can be thought of as constant in this case. therefore can be computed once for each light and then used for the entire frame, or indeed while light and viewpoint remain in the same relative position. The same is not true with Phong's method of using the reflection vector which depends on the surface curvature and must be recalculated for each pixel of the image (or for each vertex of the model in the case of vertex lighting). In 3D scenes with perspective cameras, this optimization is not possible.

This sample in the OpenGL Shading Language consists of two code files, or shaders. The first one is a so called vertex shader and implements Phong shading, which is used to interpolate the surface normal between vertices. The second shader is a so called fragment shader and implements the Blinn–Phong shading model in order to determine the diffuse and specular light from a point light source.