Re: Color constancy

[John Denker <jsd@MONMOUTH.COM>]

At 9:15 AM -0700 11/1/00, Jim Green wrote:

> >2) The assumption that each of us "see" the same color -- we all receive
> >the same wavelengths but likely we do not all perceive the same colors.

Then at 09:47 AM 11/1/00 -0700, Larry Smith wrote:

> Jim, why do you say that #2 is "likely"?  Is there reason to believe we
> don't perceive the same colors (especially given that eye and brain
> chemistry is similar from person to person, and discounting the exception
> Herb raised of color-blindness)?

The variable perception that JG mentioned is not just "likely" -- it is
100% guaranteed.  There is abundant evidence that color perception is
nonlinear and nonlocal;  perception differs from wavelength-measurement in
interesting and important ways.

There are whole books on the subject.  See
  http://wad.www.media.mit.edu/people/wad/color/constancy.html
  http://www.research.ibm.com/journal/sj/mit/sectiond/jacobref.html
for the tip of the iceberg.

One of the big effects is called "color constancy, discounting the
illuminant".  A lemon in mid-day shadow, illuminated by bluish skylight,
looks lemon-colored.  A lemon directly illuminated by reddish evening
sunlight looks lemon-colored.  The ability to judge the pigmentation of
fruits, discounting the illuminant, has obvious Darwinian implications.

Obviously a camera does not perform this nonlocal nonlinear
processing;  the film just responds wavelength by wavelength, point by
point.  It is a minor miracle that satisfactory color photography is
possible;  there is treeeemendous complexity in the color-darkroom
equipment to achieve some semblance of color constancy, discounting the
illuminant.