A Practical Analytic Model for Daylight

A. J. Preetham

Peter Shirley

Brian Smits

Abstract

Sunlight and skylight are rarely rendered correctly in computer graphics. A major reason for this is high computational expense. Another is that precise atmospheric data is rarely available. We present an inexpensive analytic model that approximates full spectrum daylight for various atmospheric conditions. These conditions are parameterized using terms that users can either measure or estimate. We also present an inexpensive analytic model that approximates the effects of atmosphere (aerial perspective). These models are fielded in a number of conditions and intermediate results verified against standard literature from atmospheric science. Our goal is to achieve as much accuracy as possible without sacrificing usability.

Both compressed postscript and pdf files of the paper are available.

Example code is available.

Color images from the paper are shown below.

Left: A rendered image of an outdoor scene with a constant colored sky and no aerial perspective. Right: The same image with a physically-based sky model and physically-based aerial perspective.

The new model looking west at different times (left morning and right evening) and different turbidities (2, 3, and 6 top to bottom).

Overcast skies with turbidity 10 (left) and turbidity 30 (right).

Left: the CIE clear sky model using constant chromaticity coordinates and Ward's aerial perspective approximation for west and east viewing directions and the same viewpoint. Right: the new model. Note the change in hue for different parts of the sky for the new model.