Smoothly shaded renderings of polyhedral objects on raster displays

Abstract

The appearance of raster-scan renderings of polyhedral approximations to curved surfaces can be enhanced greatly by shading them in a manner that varies smoothly across each polygon and which matches the correct shade at each vertex. Henri Gouraud and Bui Tuong-Phong have previously described two methods of computing such shading functions. While the computations in Gouraud's method are quite quick to perform, the renderings produced often exhibit pronounced Mach bands, and animated sequences tend to have annoying fluctuations in intensity. Phong's method tries to cure these problems, but is much more expensive computationally. We will show a new, faster method of computing Phong shading, and discuss the relationship of this method to Gouraud shading. We will also exhibit representatives of a class of surfaces for which Phong shading surprisingly produces worse Mach bands than Gouraud shading.