Bidirectional Texture Contrast Function

Three dimensional surface corrugations on globally smooth surfaces give rise to brightness modulations of global shading patterns. We study systematic variations of such 3D image texture as a function of illumination and viewing geometry. The 3D texture is especially noticeable near the shadow terminator (for collimated illumination) or near the dark pole (for hemispherical diffuse illumination). We find that a simple micro-facet model, assuming locally Lambertian scattering, suffices to robustly describe texture contrast gradients of a large variety (measured and rendered textures; laboratory and field conditions) in a semi-quantitative manner. Robust statistical measures of the texture allows one to draw inferences concerning the nature of the light field (collimated to diffuse) and of surface roughness parameters, which can be used as input to the simplest BRDF models.