This paper describes an algorithm used to produce realistic computer-generated images of landscapes with cloudy skies. The results are approximate and use a simplified model of the world; nevertheless, the algorithm gives convincing images. The algorithm is suitable for vectorization.

The paper reports on work done at Lawrence Livermore Laboratory in the early 1980s. After reviewing related work, the author defines his mathematical model of simplified clouds using a base height and two single-valued surfaces. He then describes his algorithm and model of light scattering. Two simplifying assumptions are that the clouds have constant density and that the sun is directly overhead. The model includes factors for light attenuation as the ray passes through clouds and for the proportion of light scattered. The value of each pixel in the scene is calculated based on a set of rays from the viewpoint, taking scattering into account. The whole scene is assembled by a scanning technique.

The exponential form of the algorithm is approximated by a polynomial, yielding an efficient algorithm that reuses terms when calculating each pixel in a vertical sight-line. The method is extended to cases where multiple clouds lie on a ray and to hazy conditions. An impressive series of still pictures from the 1983 16mm film Ethereal Flight concludes the paper.

The paper is very readable and has clear illustrations. This subject probably has more relevance now than when the work was taking place because graphics devices with increased performance to exploit the algorithms are available at more accessible prices.