[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 05 Jul 2014 15:36:49 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:


> The Argyll printer profiles I've looked at use B-curves, which is why I
> wondered what they are for.

They are the inverse of the A2B curves, which are chosen to improve
the profile fit, as well as shift the white point to be on a grid point,
so that it will be mapped with precision.

> The
> matrix is in XYZ as you say. I would have expected an Argyll Illuminant A
> profile, for example, to have an A to D50 matrix ... but maybe Graeme does
> it some other way?

No, because that would defeat the purpose. The point of the chad tag
is to say "and by the way, the instrument measured XYZ values using some
other illuminant, but we converted it to be as if it had been measured under 
D50 by
applying a chromatic transform". This is not as accurate as doing this 
spectrally using
the spectral reflectance, or, best of all, using actual D50 measurement 
illuminant, but
is the best that can be done in this situation to make it conform to the PCS 

The point of creating a (non-standard) profile with a non-standard illuminant
is to actually represent the color appearance under that illuminant, not to
make it as if this wasn't the case undoing it and using a chad tag.

> All the printer profiles I've looked at have a gamt tag.  These have 3
> curves and one LUT.  I had a guess at the purpose of the LUT data (3 inputs
> one output) but I can't even begin to guess at the purpose of the curves!

The gamt tag is meant to represent the gamut boundary in a binary way. So
typically the 3D Lut maps the PCS to some value that tries to represent
the "in gamutness" of the color in an continuous way, and the 1D lookup
at the end quantizes it. The result is a not-very accurate gamut boundary
representation, which is why no-one uses it.

> I should just forget it and move on ... but I'm one of these people who feel
> a need to have some understanding of the underlying technology. I'll do a
> bit more reading and see if I can find something that sheds some light on
> these mysteries!

Phil Green's book goes into ICC in more detail, but given the flexibility
of the format and the different possible approaches, won't cover everything,
and certainly won't cover the unique things that ArgyllCMS does.

Graeme Gill.

Other related posts: