Surface illuminance flow
Sylvia Cornelia Pont, Jan J. Koenderink · International Symposium on 3D Data Processing, Visualization and Transmission · 2004
We introduce the concept of in two steps. First we reiterate the notion of the vector, and then proceed to decompose the light vector into a scalar illuminance and a vector flow. The scalar normal generates the familiar pattern that is often known as the shading. The vector surface flow is irrelevant for smooth surfaces (which probably explains why it is conventionally ignored) but it generates the texture due to surface mesorelief. We show how observation of the induced texture leads to robust inferences of the surface flow direction modulo 180/spl deg/. This makes surface flow a property that is observable in natural scenes. Much like optical flow, the surface flow is largely due to a global property that exists independent of local structure, namely the direction of the general illuminating beam (sunlight or light from the overcast sky in a typical outdoors scene). However, due to the fact that there exists true screening in radiometry (thus removing any hope for a field theory of radiometry) the local scene structure has an influence on the local light field. Such vignetting is a main cause of the complexity of radiometry, the other cause being the ever present multiple scatterings. This complicates matters but also introduces additional sources of information. We illustrate instances of the observation of flow in scenes. We also present a database of roughly spherical objects, illuminated in a number of standard ways that we have used to study flow over actual rough objects.