[argyllcms] Re: Custom Illuminant

  • From: <robert@xxxxxxxxxxxxxxxxxx>
  • To: <argyllcms@xxxxxxxxxxxxx>
  • Date: Sat, 5 Jul 2014 11:28:29 +0100

Hi Graeme,

Many thanks for taking the time to answer my questions / correct my

Just following on from your answers:
- Why does Argyll provide the gamt tag? It appears to be a standard gamt
tag, but you say that no-one uses it.  Do you put it there for the odd CMM
that might?  It won't do any harm if it isn't used, of course ... and I
assume that if it is used and the gamut mapping has already been done in the
cLUT (I assume that any XYZ value of 65535 indicates OOG?) that it won't do
any harm (or much harm for values close to but inside the gamut).
- Regarding the AtoB mapping: I've had another look at the i1Profiler
generated profiles and even though it has data in all three tags, the data
is the same - so it appears to be using the Colorimetric mapping for all
three, as in Argyll ... just wasting space.
- I'm not sure if I mentioned this, but Soft-Proofing with Simulate Paper in
Photoshop does the equivalent of: Convert to Profile with selected intent;
Convert back to working space with Absolute Colorimetric. That means that if
the wtpt tag is set to the paper white under the chosen illuminant (using a
.sp file in Argyll, for example) then the print and monitor image match when
viewed side by side.

Thank you for the suggested reading!  I'll do some studying before asking
any further questions :).


-----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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