[argyllcms] Re: Custom Illuminant

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 25 Jun 2014 16:37:10 +1000

robert@xxxxxxxxxxxxxxxxxx wrote:

> I've checked various documents, including this one
> http://www.computer-darkroom.com/softproof/softproof_1.htm and Photoshop
> does simulate paper color with all the rendering intents, as I can see when
> I do a soft-proof.


Right, but is that a full simulation (i.e. representing the absolute paper white
on the display), or a partial simulation (i.e. representing the paper white
adapted to the display D65 white point ?).

The document you refer to mentions "Relative Colorimetric", hinting
at the latter. Photoshop having a "Simulate Paper Color" button
doesn't help clarify what it's actually doing either.

> I would really like to understand what Photoshop is doing. I have a contact
> in Adobe who should be able to point me in the right direction. However
> before doing that I would like to understand what is happening at the
> profile level.

> A print made from a profile that uses an sp file is identical to a print
> made with a profile that does not use the sp file.  So I assume that the
> BtoA (PCS to Printer) is at D50 and is not affected by the custom
> illuminant.  Is that correct?

No, that's not correct. Using a custom illuminant and/or observer changes
the XYZ numbers and possibly the relationship between the XYZ numbers, but
ICC color profiles are all normalized (i.e. chromatically transformed)
to have the white point be exactly D50. So using any non-absolute
colorimetric B2A will (superficially) seem much like the B2A from
a default D50 illuminant and 1931 standard observer XYZ values.
It's only when you examine the profiles in some more detail
that you will see differences, and these depend on the spectral
characteristics of the inks.

> If the profile made with the sp file is used for soft-proofing, the paper
> white is adjusted correctly (I've also tried different illuminants like A,
> F5 etc, all appearing correct).  So I assume that the AtoB (Printer to PCS)
> does take into account the custom illuminant.  Is that correct?

No, see above. Both the A2B and B2A tables will be different if a different
illuminant and/or observer are used. The B2A table is created by inverting
the device characterization A2B table.

> Does this apply to all the rendering intents, or only to Absolute
> Colorimetric (as I think you said)?

See above - all the tables will be affected, because they are all based
on the same measurement values. The differences between default and custom
illuminant and/or observer are likely to be larger and more obvious when
comparing the absolute colorimetric intent though.

> Is this (or whatever method you use) also applied to FWA compensation (if -i
> and -f both specify the custom .sp file)?

Yes. FWA compensation more accurately simulates the effect of U.V. in the
illuminant on the FWA/OBE in the paper, so naturally this is affected by
the spectrum (ie. the level of U.V.) in the custom illuminant.

> I've read your documentation and purchased your paper on FWA compensation,
> but I still can't make much sense of what's happening.

To have a precise understanding means comprehending the basic color science,
understanding what the profiling process and profiles are doing, and (the
hard part), figuring out what the applications that use the profiles
are actually doing. The latter is the hard part because typically the
software vendors give you no clue, and hide behind inscrutable "user friendly"
buttons such as "Simulate Paper Color". That's why often the process
is one of comparing what software does with reference implementations such
as the ArgyllCMS tools (cctiff, xicclu) or Lcms tools, where you can know 
what's happening.

Graeme Gill.

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