[argyllcms] Re: Custom Illuminant

  • From: Roger Breton <graxx@xxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 03 Jul 2014 13:49:35 -0400

Robert,

The shift will be the same throughout. 

Think of the way that the raw colorimetric measurements for an output
profile are first encoded into the PCS: there is XYZ (linear) normalization
to the raw XYZ measurements applied so that L=100 a=0 b=0 on the device
paper. That's what gets encoded into the RelCol tag (A2B0).

Roger

-----Original Message-----
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of robert@xxxxxxxxxxxxxxxxxx
Sent: 3 juillet 2014 09:46
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Custom Illuminant


> Nothing useful shifts colors by a fixed offset in either XYZ or L*a*b* 
> or any other typical color space. Typically shifting a white point 
> involves moving all the other colors in a way that has most effect for 
> colors with a similar Y/L*, and proportionally less as you get close 
> to black, the details depending on what space it is done in, and 
> whether there is a chromatic adaption matrix involved, etc.

Hi Graeme,

My English is sloppy.  What I really meant is that the gray lines are
brought into line (so to speak) ... so (normally) there will be a greater
shift at the white point, less at the middle gray and least at the black
point.  But at a particular luminance level, are not all colors shifted by
the same amount in an Absolute Colorimetric transform?  I take it that this
is what you said above?

I don't know what a chromatic adaptation matrix does, or how it is produced.
Are you talking about the illuminant and FWA compensations?  If so, is there
a separate mapping matrix (as you imply) or is the compensation built in to
the table?

If you know of any really good literature on this whole subject (which would
be comprehensible to someone without a background in color theory) I would
be very grateful for the reference.  I do have an engineering degree so I
can cope with a certain amount of maths, but preferably not too much!  I
really hate asking silly questions that take up your time and the time of
others, but I've found it very difficult to find fairly in-depth and at the
same time correct information.  Going to the ICC is just way too much for
me.

Thanks

Robert 



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