Re: a link to some interesting illusions

[Larry Woolf <larry.woolf@GAT.COM>]

It is clear that color constancy is a fascinating and complex subject,
currently the focus of significant research.

Some final (from me) quotes on this issue:

1. From "Human Color Vision, Second edition" by Peter K. Kaiser and Robert
M. Boynton:
"Color constancy is not only imperfect, but in the limit when the source of
light is monochromatic, it fails completely regardless of context." (p. 521)
"Color constancy. which is the tendency for surface colors to resist changes
in appearance under differing illuminants and surrounds, is an enduring and
important problem for color theory to interpret.  Somehow, the visual system
must the illuminant into account, a task that is impossible in the limit,
but which is simplified to some extent by physical restriction in the kinds
of illuminants and surrounds that are usually encountered in the natural
world." (p.523)

2. From:
http://color.psych.upenn.edu/lab/conferences/PBD_OSA2001_Final.pdf
"Across our experiments, constancy varied systematically with the color
direction of the illuminant
change, suggesting that indeed the visual system embodies assumptions about
the relative
likelihood of different illuminant changes. With respect to CIE daylight
D65, constancy was
best for an illuminant change in a ‘blue’ direction along the blackbody
locus. Such a change
might commonly occur in natural viewing when the sun is occluded and
illumination comes
mainly from skylight. Constancy was not as good in the 'yellow' direction.
Although this is a
naturally occurring illuminant, it is possible that we have less experience
with changes in this
direction. Although the data set shown in Figure 2 may not represent a truly
random sample of
daylights, examination of the figure indicates fewer daylights near the
'yellow' illuminant
(4000K) than near the ‘blue’ illuminant. Constancy was also good in the
‘green’ direction.
Although the ‘green’ illuminant is not typical of daylights, such
illuminants do occur under
dense forest canopies (Endler, 1993). Much of our evolutionary history may
have been spent
in such dense forest environments."

Again,
http://www.kodak.com/US/en/digital/dlc/book3/chapter2/digColorM1_1.shtml
is an understandable resource that touches on all of the previous issues.

Larry Woolf;General Atomics;San Diego CA
92121;Ph:858-526-8575;FAX:858-526-8568; www.ga.com; www.sci-ed-ga.org

> -----Original Message-----
> From: John Clement
> Sent: Friday, February 07, 2003 3:35 PM
> Subject: Re: a link to some interesting illusions

> The key to color constancy is that we do not determine the color of an
> object in isolation; rather, the object's color derives from a
> comparison of
> the wavelengths reflected from the object and its surround. In the rosy
> light of dawn, for instance, a yellow lemon will reflect more long-wave
> light and therefore might appear orange; but its surrounding leaves also
> reflect more long-wave light. The brain compares the two and
> cancels out the
> increases.