A comparison of material and illumination discrimination performance for real rough, real smooth and computer generated smooth spheres

The appearance of objects in natural scenes is determined by their reflectance, their 3D texture, their shape and by the nature of the illumination. Results of previous experiments using computer generated images of spheres with different reflectance modes and under different canonical illuminations suggested that perception of reflectance mode and illumination are basically confounded. In the present study we investigate whether the conclusions from the experiments with simplified rendered spheres can be extended to ecologically valid images. We use two sets of photographs of real spheres, the first set is taken from the Dror Database [Dror et al. 2001], with simple reflectance modes but complex natural illumination (e.g. desklamp, lab, foodcourt). The second set is taken from the Utrecht Oranges Database [Pont and Koenderink 2003], with simple canonical illumination but material consisting of both reflectance and 3D texture differences (e.g. orange, golf ball, christmas decoration). We find that, although to a lesser extent, even in images of complex objects, perception of material and illumination are basically confounded. Overall, illumination and material are confounded most when we present rendered spheres that differ only in reflectance mode under simple canonical illumination conditions. Interestingly, adding complex natural illumination containing higher order angular frequencies helps to disambiguate this confound in material judgments, but not in illumination judgments. Most helpful was the addition of 3D texture.