General sky models for illuminating terrains

Abstract

Sky models are quantitative representations of natural luminance of the sky under various atmospheric conditions. They have been used extensively in studies of architectural design for nearly a century, and more recently for rendering objects in the field of computer graphics. The objectives of this paper are to (1) describe sky models, (2) demonstrate how map designers can render terrain under various sky models in a typical geographic information system (GIS), (3) illustrate potential enhancements to terrain renderings using sky models, and (4) discuss how sky models, with their well-established standards from a different discipline, might contribute to a virtual geographic environment (VGE).

Current GIS hill-shading tools use the Lambertian assumption which can be related to a simple point light source at an infinite distance to render terrain. General sky models allow the map designer to choose from a gamut of sky models standardized by the International Commission on Illumination (CIE). We present a computer application that allows the map designer to select a general sky model and to use existing GIS tools to illuminate any terrain under that model. The application determines the orientations and weights of many discrete point light sources that, in the aggregate, approximate the illumination provided by the chosen sky model. We discuss specific enhancements to terrains that are shaded and shadowed with these general sky models, including additional detail of secondary landforms with soft shadows and more realistic shading contrasts. We also illustrate how non-directional illumination models result in renderings that lack the perceptual relief effect. Additionally, we argue that this process of creating hill-shaded visualizations of terrain with sky models shows parallels to other geo-simulations, and that basing such work on standards from the computer graphics industry shows potential for its use in VGE.

Keywords:

• sky models
• daylighting
• hill-shading
• shadowing
• terrain illumination
• VGE
• CIE
• geo-simulation

Notes

1. 1. The Agarwal et al. method allocates samples to strata in proportion to the importance measure (radiance times solid angle to the 0.25 power). We found that dimmer areas of the sky got very few samples with this method and the few, isolated samples tended to cast light such as dim point sources, resulting in sometimes-perceptible sharp shadow edges. By allocating an equal number of samples per stratum, we push more samples into the dimly-lit regions, so samples in those regions become closer to each other and responsible for smaller solid angles, reducing the artifacts.