When Shadows Become Interreflections

Shadows and interreflections are present in all real scenes and provide a rich set of photometric cues for vision. In this paper, we show how shadows and interreflections are intrinsically related. Shadows tend to occur in those parts of a scene in which interreflections have the largest gain. We provide several basic results concerning this relationship in terms of the interreflection modes of a scene. We show that for a given scene, the interreflection mode having the largest gain is a physically realizable radiance function. We derive bounds on the gain of this mode and discuss how this mode is related to shadows. We analyze how well an n-bounce model of interreflections approximates an infinite-bounce model and how shadows affect this approximation. Finally, we introduce a novel method for inferring surface color in a uni-chromatic scene. The method is based on the relative contrast of the scene in different color channels.

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