Compensation of geometrical distortion and inter-reflection effects in a BRDF imaging system
We present two approaches to decompose and neutralize the distortion generated by an imaging system when measuring BRDF data of materials. BRDF acquisition time can be considerably shortened using imaging systems, making it possible to gather detailed characterizations of multiple materials in very short times. The drawback of this acquisition strategy is that several processing steps have to be applied in order to obtain data that are comparable to the standard sampling method. Two of them are the directional mapping to spherical coordinates and the correction of the radiometric distortions due to geometry and inter-reflections that occur during the acquisition process. Here we present two approaches that address the decomposition of various interactions that occur when measuring BRDF material functions with an imaging system. The first one uses the measurement of a mirror as a Kronecker delta function input to the imaging system and focusses on the inter-reflections within the curved projection surface. The second approach addresses the three main contributions that hinder the direct comparison between sampled and imaged measurements and models the dependency to the geometry associated. The components considered are the curved projection surface, the inter-reflections, and the effects associated with the imaging system itself. We introduce the idea of considering the distorting factors as two separable components. The first one depends only on the surface geometry and camera properties. The second contains all interactions attached to the reflectance characteristic of the sample. This enables the decoupling of the distorting factors and allows us to examine them independently. Current results are presented and discussed. The implications for the radiometric quantitative accuracy of the method will be summarized.