Diffuse shading, visibility fields, and the geometry of ambient light

A dominating sky principle is introduced. It states that under diffuse lighting conditions, ambient light has a bimodal structure: the sky is typically much brighter than the ground. The dominating sky principle suggests a general model of image formation under diffuse lighting conditions. The idea behind this model is that the luminous intensity of a surface is determined by the visible region of the diffuse source, or surface visibility function. The model is presented. A novel concept in shading analysis, surface aperture, is discussed. Surface aperture is the solid angle of the visible diffuse source. Although the unit surface normal is a factor under diffuse lighting conditions, in most situations surface aperture dominates. The surface visibility and aperture functions are embedded into visibility and aperture fields defined in the free space above the surfaces. The visibility field is useful because it satisfies simple local constraints. A fast, parallel computation is described for the forward problem of rendering a surface which is illuminated by a diffuse source and for the inverse problem, namely, shape from shading on a cloudy day. The forward and inverse solutions are quite similar. Experimental results are presented.<>