[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 05 Jul 2014 15:07:43 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:

> 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 guess my response was a round about way of saying "I don't understand
what you were trying to say". Absolute and Relative colorimetric only differ
by the application of a chromatic adaptation matrix. Relative colorimetric
maps white to the PCS white point, Absolute colorimetric returns the instrument
readings for white.

> I don't know what a chromatic adaptation matrix does, or how it is produced.

Typically (ie. 99.9 % of the time) a chromatic adaptation matrix uses the
Von Kries principle, which basically means scaling the 3 components 
to map one white value to another white value. What makes it a useful
transform is to do it in the right linear light space, typically a "sharpened
cone" type space. You do that by taking the XYZ and multiplying by the 3x3 
that converts to the cone space, apply the per component scaling, then undo
the 3x3 matrix to take you back to XYZ space. All three steps can be
combined into a single 3x3 chromatic adaptation matrix.

> 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?

FWA is nothing to do with the white point, it is to do with estimating
the reflectance of a surface under a different illuminant to the one
the instrument is actually using.

> 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'm not sure that I can give you a good reference. There are certainly
well known reference books such as Hunt or Wyszecki & Stiles or
Judd & Wyszecki etc., but if you want a concise introduction, I actually
like the first few chapters of Mark Fairchilds's "Color Appearance Models",
as well as Ján Morovič's "Color Gamut Mapping". The meat of both of these
books is about more specialized subjects, but the introductions cover
color science basics pretty nicely.

Graeme Gill.




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