Human observers are able to successfully infer direction and intensity of light from photographed scenes despite complex interactions between light, shape, and material. We investigate how well they are able to distinguish other low-level aspects of illumination, such as the diffuseness and the number of light sources. We use photographs of a teapot, an orange, and a tennis ball from the ALOI database (Geusebroek, Burghouts, & Smeulders, 2005) to create different illumination conditions, varying either in diffuseness of a single light source or in separation angle between two distinct light sources. Our observers were presented with all three objects; they indicated which object was illuminated differently from the other two. We record discrimination performance, reaction times, and eye fixations. We compare the data to a model that uses differences in image structure in same-object comparisons, and outcomes suggest that participants mostly rely on the information contained in cast shadows and highlights. The pattern of eye fixations confirms this, showing that after the first fixation, observers mostly fixate cast shadow areas. However, information in the highlights is rather salient, so it might be available from first fixation, making separate fixations are unnecessary.
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