[argyllcms] Re: Custom Illuminant

  • From: <robert@xxxxxxxxxxxxxxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Sat, 5 Jul 2014 12:41:53 +0100

Hi Graeme,

I'm not quite sticking to my promise of doing some studying before getting
back to questions!

I read your note "About ICC Profiles and Gamut Mapping" and I would like to
clarify a couple of things:
- You say: "The colorimetric intent is meant to convey the exact device
color behaviour, without any gamut mapping". Clearly there is clipping that
occurs on output to a device like a printer.  Are you saying that the
clipping is effectively left to the printer, so the CMM will just output
colors without caring if these colors can be printed or not?
- You say in relation to ICC V2 behaviour: "The main disadvantage is that
the gamut mapping will only operate exactly as intended when the profile is
linked with the source profile it was setup for". (This only relates to
Perceptual and Saturation as Colorimetric does not do a gamut mapping). I
take it that for a print profile, that the source profile should normally be
the working space?  I had put the monitor profile in the -S flag in colprof,
but this would seem to be incorrect.  Do I understand correctly that if the
profile is made with -S AdobeRGB1998.icc but the working space is ProPhoto
RGB that the Perceptual and Saturation gamut mappings will not operate as
intended (I was going to say 'wrong', but I'm sure you would tell me that
there is no 'right' or 'wrong' in these matters :)).
- You say in relation to ICC V4 behaviour: "The chief drawback, is that only
one (non colorimetric) intent can really be supported, that of saturation".
If I understand you correctly (that if you stretch and compress to and from
the RMG in Perceptual that some horrible things are likely to happen) that
we then we use Perceptual to our peril in V4 LUT-based profiles.  Is that
correct?  If so, that's a very useful bit of information!!



-----Original Message-----
From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:argyllcms-bounce@xxxxxxxxxxxxx]
On Behalf Of Graeme Gill
Sent: 05 July 2014 06:37
To: argyllcms@xxxxxxxxxxxxx
Subject: [argyllcms] Re: Custom Illuminant

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
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
> 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
> 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.

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