Clouds present serious problems to standard computer image generation techniques because clouds do not have well-defined surfaces and boundaries. In addition, clouds contain varying degrees of translucence, and their amorphous structure can change with time. Although several approaches to cloud simulation have produced impressive results, they have relied on complex mathematical models which produce high computation costs for a single image.This paper describes a new approach to cloud simulation using simple planar and curved surfaces whose surface shading and translucence are modulated by a mathematical texturing function. This approach represents the appearance of clouds with enough realism for a wide range of visual simulation but does so at a reasonable computational cost, allowing the generation of sequences of images on small minicomputers. In addition, the cloud model can be constructed in a straightforward manner.

Because of the high complexity of the real world, realistic simulation of natural scenes is very costly in computation. The topographical subtlety of common natural features such as trees and clouds remains a stumbling block to cost-effective computer modeling. A new scene model, composed of quadric surfaces bounded with planes and overlaid with texturing, provides an efficient and effective means of representing a wide range, of natural features. The new model provides a compact and functional data base which minimizes the number of scene elements. Efficient hidden surface algorithms for quadric surfaces bounded by planes are included. A mathematical texturing function represents natural surface detail in a statistical manner. Techniques have been developed to simulate natural scenes with the artistic efficiency of an impressionist painter.