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.

Solar potential of rooftops in Cáceres city, Spain

“…On the other hand, in a Standard Overcast (SO) diffuse model, diffuse radiation flux varies with zenith angle. It has a uniform component plus another component that increases towards the zenith (Kennelly & Stewart, 2014).…”

Three‐dimensional terrain modeling with multiple‐source illumination

“…Lee & Steiner, 2000). However, as far as we know, some algorithms of multiple-source illumination are periodically applied for hill shading only (Mark, 1992;Hobbs, 1999;Smith & Clark, 2005;Huffman, 2014Huffman, , 2017Kennelly & Stewart, 2014;Veronesi & Hurni, 2014;Marston & Jenny, 2015;Tzvetkov, 2018), that is, for the twodimensional (2D) case of terrain modeling.…”

Visualization of Features in 3D Terrain

“…However, approaches that apply these techniques in 3D, or emphasize features in 3D terrain visualizations in general, have rarely been developed. Kenelly and Steward [28] propose using single-light illumination techniques, i.e., different sky models, to reveal features of 3D terrain. Bratkova et al [29] emphasize features to create tailored 3D landscape visualization, though they aim for artistic overview presentations.…”

“…Depending on the size and the resolution of the original height-field, triangular networks can consist of millions of triangles, making the representation computational challenging. Hence, efficient render strategies, e.g., adapted level-of-detail [6] and shading techniques [28], must be applied (see Figure 1c). Within the terrain representation, large shapes are often easily observable, since they prominently protrude off the silhouette.…”