[tig] Re: Tig digest, Vol 1 #881 - 3 msgs

Thomas Carlton Blake tcb
Thu Apr 22 20:50:30 BST 2004

> Some of you may have seen this before, or other things
> like it; I just found this yesterday.  Look at box A, and
> then box B.  They look like they are different shades=85.
> but in fact they are not - they are identical.
> I am figuring it is not
> the boxes themselves, but the SURROUNDING boxes
Sort of.  You perceive the "lighter" box as being one of the truly lighter
boxes _in the shadow of the cylinder._  Block off the parts that imply a
shadow and the shades look the same.

Two processes are in effect.  One is the strong tendency of the visual
system to reject coincidence.  You infer the shadow even when the cylinder
is masked off.  The second is the fact that luminance, in the eye, is only
referred to the rest of the scene. Edges or lack of them make an enormous
difference as to what is perceived.  See _The Oxford Companion to the Mind_,
p344f. and p. 151f.

To see the boxes equally, mask off not only the cylinder but also the boxes
containing the gradient "defining" the shadow AND block off the adjacent
brightness differences, including the background; in short, block everything
except the interiors of the two boxes in question.

I think that in a real scene, the shadow would always be real and therefore
there would almost always be a real luminance difference between the two
boxes.  But I'm not sure.

>how do I get my brain to stop doing that!?!??

AFAIK, you would have to have your visual cortex removed!!!


Carlton Blake

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