Improving the Reliability/Cost Ratio of Goniophotometric Comparisons

Many scattering models have been presented in the graphics literature. Few of them, however, have been evaluated through comparisons with real measured data. As the demand for plausible and predictable scattering models increases, more attention is given to performing such comparisons. In this paper we examine the implementation of virtual goniphotometers used to obtain BRDF (bidirectional reflectance distribution function) and BTDF (bidirectional transmittance distribution function) records from algorithmic scattering models. These records can be compared to data from actual experiments in order to validate the models. Our discussion focuses on practical issues, namely the subdivision of the devices’ collector sphere and the ray density required to obtain reliable BRDF and BTDF estimates. The subdivision techniques examined in this paper have been used before in publications, but the details of their computation are not readily available in the literature. Although the mathematical bound presented to determine appropriate ray densities for virtual goniphotometers is a direct generalization of a bound used for virtual spectrophotometers, it has not been published before. Our discussion of these issues is supported by practical experiments whose results are also provided in this paper.