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From: "John D'Errico" <woodchips@rochester.rr.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: How to extract yellow color under different iluminations
Date: Mon, 19 Oct 2009 11:34:02 +0000 (UTC)
Organization: John D'Errico (1-3LEW5R)
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"Prasanna " <pmgunawardana@gmail.com> wrote in message <hbhgam$3ok$1@fred.mathworks.com>...
> Hi,
> 
> How can I extract the yellow color regions from a .Jpg file under different illumination conditions? Please help me with this. Send me the codes or the method.
> 

The "yellow" color region alone of color space is
non-simple to describe, that will depend on the
color space you might work in. A blob is the best
description.

Now, you want to change some nominal illuminant,
then to recover from the original scene or from a
print, which pixels fall into that nondescript blob?
This is a non-simple problem, made worse because
of your less than complete description of the
problem. Which illuminant must be changed? That
of the original scene? That of the environment in
which we will view a print, or the monitor? Or
maybe change the monitor white-point? Any of
these can be difficult problems to solve, depending
on how well you wish to do the job.

For example, if it is a question of print viewing, then
we need an accurate characterization of the printer.
We will need to know the device gamut for how this
device prints yellows. Which pixels were out of gamut,
and exactly what strategy is used to move those
colors to the device gamut. (There is more than one
such strategy to be found.) Alternatively, you need
a mapping that will predict where each color in
the original color space ends up after it is printed.
Finally, we will need to have a good specification of
the region of a color space where the color yellow
lives.

Sp yeah, right, we'll get right on it. Stop work on
all else and send you code for this immediately.
If you really expect an answer, then it will at most
be in the form of some ideas on what to do. Even
for that, you MUST give an adequate description
of the problem you are trying to solve.

John