Separating Lambertian and Specular Reflectance Components using Iterated Conditional Modes

In this paper we address the problem of separating Lambertian and specular reflection components in order to improve the quality of surface normal information recoverable using shape-from-shading. The framework for our study is provided by the iterated conditional modes algorithm. We develop a maximum a posteriori probability (MAP) estimation method for estimating the mixing proportions for Lambertian and specular reflectance, and also, for recovering local surface normals. The MAP estimation scheme has two model ingredients. Firstly, there are separate conditional measurement densities which describe the distributions of surface normal directions for the Lambertian and specular reflectance components. The second ingredient is a smoothness prior which models the distribution of surface normal directions over local image regions. We experiment with the method on real-world data. This reveals not only that the method accurately estimates the proportion of specular reflection, but that it also results in good surface normal reconstruction in the proximity of specular highlights.