Reflectance is the fancy
vision term that means basically color. It is an intrinsic
property of an object and does not change depending on external
conditions such as lighting.
Given a 2-D shaded image, it is possible to either
interpret it as a 3-D pattern that is of one reflectance (color) or as
a flat pattern with patches of different reflectances.
We conducted a series of experiments that uses stereo goggles to enforce 3-D shape perception*. These results have led us to the conclusion that the "3-D pop-out" we observe with shaded patterns isn't so much based on 3-D shape per se, but is based primarily on reflectance instead.
We suggest that upright shaded cubes are preferentially interpreted as 3-D shapes of single reflectance by early vision mechanisms, while upside-down cubes are interpreted as flat patterns with multiple reflectances.
When one upside-down cube is embedded among many
upright cubes, the one multi-reflectant pattern would pop out among
the background that is composed of a single reflectance. This scheme
also explains why one upright cube among many upside-down ones is more
difficult to find (See 3-D Shape
Pop-out).
Reference: Sun, J.Y. & Perona, P. (1996).
Early computation of shape and reflectance in the visual system.
Nature, 379, 165-168.
