Shading is one of the generic "monocular depth (and shape) cues". It is of conceptual interest because it apparently implies "causal relations' between the geometry of the scene in front of the observer, the formal description of brain activity, and the visual awareness of the observer. These are three disjunct ontological levels, so the very notion of "causal connections' is problematic. Some silent assumptions in current accounts indeed invoke "magic", we identify internal and external local sign as instances.We attempt an account of the shading cue that avoids at least some of these pitfalls. We conclude that (for the human observer, machine vision has different objectives) the shading cue allows "direct perception" of surface shape.
Koenderink, J. , van Doorn, A. (2014)., Shape, shading, brain and awareness, in G. Citti & A. Sarti (eds.), Neuromathematics of vision, Dordrecht, Springer, pp. 87-106.
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