Phong shading

Phong shading is an interpolation technique for surface shading in three dimensional computer graphics, invented by the graphics pioneer Bui Tuong Phong at the University of Utah. Instead of interpolating colors across a polygon, as Gouraud shading does, it linearly interpolates the surface normal vector from the polygon's vertex normals, then normalizes it at each pixel and feeds it to a reflection model to obtain the final pixel color. This fixes a weakness of Gouraud shading, where a specular highlight in the middle of a large polygon is lost because the vertices carry no highlight. Phong published the method in his 1973 dissertation and a 1975 paper. It costs more than Gouraud shading because the reflection model runs for every pixel rather than every vertex, and modern graphics hardware implements variants of it in pixel or fragment shaders. The term can also mean the combination of Phong interpolation with the Phong reflection model.