The H-Test, a Method of High Speed Interpolative Shading

Abstract

In computer graphics curved surfaces are commonly represented by a mesh of polygons. When these objects are rendered, the ‘visibility’ of the polygonal definition is diminished by using an interpolative shading algorithm. The two algorithms used for this are Phong shading (Phong 1975) and Gouraud shading (Gouraud 1971). Phong shading is capable of producing more realistic specular highlights, but is more expensive than Gouraud shading.

In general for an object, the number of specularly highlighted polygons is small compared to the number of non-highlighted polygons, especially for a single light source. We describe a reliable and consistent method which allows objects to be Gouraud shaded, with Phong shading used only to add the specular highlights. Thus, images of the quality associated with Phong shading can be produced with little more computation than that needed for an equivalent Gouraud shaded image.