Illuminance texture due to surface mesostructure

We study the illuminance texture due to three-dimensional surface mesostructure. In the conventional rendering algorithms various methods of approximate photometry are used whose domains of validity have been ill defined. We introduce the necessary photometric concepts and do a comparative study of rendering algorithms. In the computer graphics/vision literature the physically distinct causes of illuminance variation, namely, the obliqueness of surface elements and the effects of vignetting of extended sources, are thoroughly confused. This leads to serious errors when expressions are used in contexts outside their original realm. The treatment of the texture that is due to the illumination of surface mesostructure is an example. Erroneous results are obtained when simple expressions for the illuminance of convex bodies are interpreted in terms of equivalent sources and when these again are used in calculations involving different geometrical structures. We classify current methods in terms of the approximations (often implicitly) made and compare results for a large class of applications: that of surface mesostructure on convex objects. Large differences among the various approximative methods are found. We discuss simple methods that might be used to produce more realistic results at reasonable computational cost. (C) 1996 Optical Society of America