The theoretical picture now 
looks very different than the 
one outlined earlier as 
characterizing a stimulus 
theory. If the perceptual system 
makes use of depth information 
or the like in arriving at 
"decisions" about whether 
regions of particular 
luminance values result from 
reflectance properties of 
surfaces or from illumination, 
a process of computation must 
occur that takes account of the 
whole array of relevant edges 
in leading to "assignment" of 
lightness values. This process 
is very different from the direct 
determination of a color 
percept by a specific luminance 
ratio between two regions. The 
entire process depends much 
more on cognitive decision and 
inference, although admittedly 
unconscious, than has been 
thought to be the case. While 
constancy is still to be 
understood in terms of 
unchanging ratios of 
luminance with changing 
overall illumination, it is now 
clear that a good deal of mental 
processing lies behind this 
seductively simple formulation.