Usually, a photon is either absorbed or reflected at the surface, with a probability defined by the surface albedo. If mc_surface_reflectalways is specified, each photon is reflected and the albedo is considered by reducing the photon weight. In case of BRDF, mc_surface_reflectalways is switched on automatically because the other method is no longer implemented for non-Lambertian BRDFs, due to implementation and numerical problems. For small albedos, the computational time is increased if mc_surface_reflectalways is used; however, the accuracy of the upward radiance (reflected by the surface) is increased considerably. In case of clouds, however, computational time might be increased considerably without gaining accuracy.





Arve Kylling 2010-03-10