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From: alanr@rd.bbc.co.uk (Alan Roberts)
Subject: Re: FAQ purpose/colorspace-faq
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Date: Wed, 10 May 1995 10:30:58 GMT
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Alain Fontaine (fontaine@sri.ucl.ac.be) wrote:
: In article (Dans l'article) <D8B4n9.Csy@westminster.ac.uk>,
: ajoec1@westminster.ac.uk (Adrian Ford) wrote (crivait):

: > Coloureq.txt, a document containing equations relating to colour spaces
: > and colour transforms, available as ASCII text from;

: I did look at this document. I still have problems with the various uses
: of the word 'linear', when not further qualified. For example, here:

: > RGB (Red Green Blue)
: > 
: > Additive colour system based on trichromatic theory, used by CRT 
: > displays where proportions of excitation of red, green and blue emitting 
: > phosphors produce colours when visually fused. Easy to implement, non linear, 

: it seems (to me) that 'non linear' means 'non perceptually linear', while
: here:

Non-linear means gamma-corrected here, voltages that drive the CRT to
produce linear light signals.

: > [4.4] CIE L*u*v*
: > 
: > This is based on CIE Yu'v' (1976) and is a further attempt to linearise 
: > the perceptibility of unit vector colour differences. It is a non-linear 
: > colour space, but the conversions are reversible.

: it seems (to me also) that 'non-linear' means that it is a non-linear
: transformation from linear-light. Or is the confusion entirely in my mind ?
:                                                         /AF

"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. CRTs have a transfer characteristic that is (almost) the
inverse of that of the eye, so the electric voltage drives for a CRT can
be thought of as being "perceptually linear", but they are non-linear in
measureable voltage terms. This "perceptual linearity" of CRT drive
signals is very useful since it allows us to use the minimum quantisation
levels (8-bit), whereas we would need at least 12-bit to quantise linear
(that is linear light) signals for the same visibility of quantisation
artifacts.

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.

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