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Message-ID: <457741DD.4020606@argyllcms.com>
Date: Thu, 07 Dec 2006 09:19:09 +1100
From: Graeme Gill <graeme2@argyllcms.com>
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To: tiff@lists.maptools.org
Subject: Re: [Tiff] Re: Tiff Digest, Vol 31, Issue 3
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Status:   

Joris wrote:

> If you colour model goes as far as to be based on actual physical
> properties of light, it's extremely complicated to convert RGB to
> "greyscale". You'll have to dig into spectra and whatnot, for all I
> know.

I can't agree with you on this. Taking the weighted sum
of an RGB output in linear light is not an approximation,
it's actually correct, equivalent to CIE Y value if the
right weightings are used. The weightings simply depend
on the primary locations and the white point.

This is because any additive space is a 3x3 transform from
CIE XYZ space, so it only takes one row of the inverse
matrix to compute Y again.

See <http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html>

Graeme Gill.
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