From: accwai@math.uwaterloo.ca (Andy Wai)
Subject: Re: Posting scanned pictures, Macbeth ColorChecker
Organization: Math Faculty Computing Facility, University of Waterloo
Date: Fri, 9 Oct 1992 01:20:29 GMT

In article <1992Oct8.224045.12863@odin.corp.sgi.com> donl@glass.esd.sgi.com (donl mathis) writes:
>
>In article <1992Oct6.030506.1@cc.curtin.edu.au>,
>zrepachol@cc.curtin.edu.au (Paul Repacholi) writes:
>
>|> The first image I would like too see is the wonderfull, dynamic, exciting...
>|> McBeth colour chart... sorry, couldn't resist. [...]
>
>I don't think this is possible, because the numbers used to represent
>red, green, and blue can only be "correct" with respect to specific
>spectral definitions of those colors.  

I don't have a McBeth color chart (nobody around here wants to carry it
because it's outrageously expensive and the distributor insists on shipping
big lots only), but didn't somebody mention eariler in this thread that a
sheet with CIE tristimulus values for the patches is packaged with the
chart?  If two colors have the same tri-stimulus values, they will look
the same to an average observer.  No spectral definition is needed.

>There is no one, true definition
>of "blue" that will apply to the MacBeth chart, and to any monitor, or
>anything else, for that matter.

It's actually quite simple to find the tristimulus values for any given
color.  Just take the spectrum of the color and run it through the three
weighting functions and you come up with the X, Y and Z values.  Of course,
to sample the spectrum of a color in the real world, a regular densitometer
won't do.  You'll need a spectral radiometer.

Now to display tristimulus values on a monitor, you'll need a mapping
from XYZ to RGB for that monitor.  The RGB to XYZ mapping can be
determined experimentally and the reverse is than derived.  (Just hand
waving here.  It's been a while since I looked at these things)  This
procedure has to be repeated for every monitor you'll use because no
two of them in the world are exactly the same.  And if you're meticulous,
you'll recalibration each monitor once in a while because monitors do
drift and some of them quite wildly at that.

>  You can define blue in terms of the
>values reported by your densitometer, but they may very well be a
>little bit different than the values reported by mine.

That's a big problem with scanning techniques in common use right now.
Consider how much more work is actually involved in converting an image
to XYZ rather than RGB, it is not surprised that RGB scanning is far
more prevalent.

>You might be
>adventurous enough to define them in terms of a balance of wavelengths,
>[...]

Actually, it is possible to look at all spectra from a given image,
extract a set dominant basis spectra from them, and than express your
input as linear combinations of these basis.  Turns out that for a
typical image, 4 to 6 basis spectra is enough to give an excellent
characterization of the whole image's spectral space.  So it isn't
quite as bad as you might think (storagewise as least).

Now, since Alan Paeth from the Graphics Lab here is the real expert
on the subject and he's a rec.photo guy too, I better not embarrass
myself any further here...

Andy Wai