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From: alanr@rd.bbc.co.uk (Alan Roberts)
Subject: Re: FAQ purpose/colorspace-faq
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References: <3oamvn$32d@larch.cc.swarthmore.edu> <D8449q.DGy@hpcvsnz.cv.hp.com> <D8B4n9.Csy@westminster.ac.uk> <fontaine-1005951056590001@macaf.sri.ucl.ac.be> <D8CyJn.EwL@bbc.co.uk> <fontaine-1005951611030001@macaf.sri.ucl.ac.be>
Date: Wed, 10 May 1995 17:26:28 GMT
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Alain Fontaine (fontaine@sri.ucl.ac.be) wrote:
: In article (Dans l'article) <D8CyJn.EwL@bbc.co.uk>, alanr@rd.bbc.co.uk
: (Alan Roberts) wrote (crivait):

: > "Linear" means linear in light terms. That means as measured by a normal
: > light meter. The eye responds (almost) logarithmically and so is
: > non-linear. Etc..

: I do understand this (I am an fan of the Poynton FAQ's). I did not see
: which meaning of linear one should take when seeing the word in your
: document.

I'll try again. "Linear" means linear light, watts, power etc. As measured
by a light meter. All else is non-linear.

: > Does that make it clear? We tried to be wholly consistent in writing
: > Coloureq.txt, linear always means linear light, anything else is non-
: > linear.
: OK, I'll read the document again with this in mind. IMO, the question of the
: meaning of 'linear' in the document is a crucial point, and there should
: be a section 1.something (1.0 ?) titled 'What is the meaning of 'linear'
: in this document'....

"Linear" can only mean linear light. Nothing else in colorimetry is
linear, we habitually distort signals to make them non-linear in TV,
in CIE colour spaces etc.

: Linear is also used to qualify transformations. Is it correct to say that
: a transformation that cannot be represented by a matrix is 'linear' ?
:                                                         /AF

A colour transformation can _only_ take place on linear signals, that
means signals that are proportional to linear light. Matrix algebra
performed on non-linear (gamma-corrected, CIELab etc) does not give
accurate results, although it can be useful in some applications.

Don't confuse colour transformation with signal transformation.

Colour transformation happens between (say) CIE XYZ and CIE UVW or
TV RGB (that's linear light RGB amounts, not gamma-corrected drive
signals).

Signal transformations generally happen in gamma-corrected environments,
such as TV R'G'B' to Y' Pr Pb. That is not a colour transformation since
you can't calculate the colour from the signal voltages without knowing
the transfer characteristics of the non-linearities.

--
************* Alan Roberts **************
* BBC Research & Development Department *
* My views, not necessarily Auntie's    *
*    but they might be, you never know. *
*****************************************
