[argyllcms] Re: Custom Illuminant

  • From: Knut Inge <knutinh@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 2 Jul 2014 17:57:37 +0200

K hokkkcyyyopkjykykykpo jo kanskje i ikke
2. juli 2014 13:29 skrev <robert@xxxxxxxxxxxxxxxxxx> følgende:

> Hi,
>
> I think I have a somewhat better understanding of what's going on now.
>
> What Photoshop does when simulating paper color is to do a normal transform
> from the PCS to the destination using the chosen intent and then it does an
> Absolute transform from destination back to the PCS.  I guess that this
> will
> work as the absolute AtoB transform will only affect the white point (or
> gray line).
>
> Certainly modifying the wtpt (and bkpt) values in the profile to match the
> paper under the particular illuminant does give a very accurate soft-proof
> when the paper and monitor image are viewed side-by-side.
>
> I'm confused about what happens if we do a round-trip conversion from, say
> ProPhoto to Destination and back to ProPhoto using a Perceptual intent
> (say).
>
> What I would expect would be:
> 1. ProPhoto->PCS:       Relative (since the working space uses a
> matrix-based profile). Uses ProPhoto Profile.
> 2. PCS->Destination:    Perceptual. Uses Destination profile BtoA.
> 3. Destination->PCS:    Perceptual. Uses Destination profile AtoB.
> 4. PCS->ProPhoto:       Relative. Uses ProPhoto profile.
>
> If that was the case then I would expect to see a change at 3, which I do
> if
> I use a profile made using i1Profiler or using the canned paper profile.
> However I see no difference (or can measure no difference using an i1Pro)
> using an Argyll-generated profile. This is affecting the soft-proofing,
> which (I presume) uses a round-trip as above.
>
> Robert
>
> -----Original Message-----
> From: argyllcms-bounce@xxxxxxxxxxxxx [mailto:
> argyllcms-bounce@xxxxxxxxxxxxx]
> On Behalf Of Graeme Gill
> Sent: 25 June 2014 07:37
> To: argyllcms@xxxxxxxxxxxxx
> Subject: [argyllcms] Re: Custom Illuminant
>
> 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.
>
> Hi,
>
> 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
> exactly
> what's happening.
>
> Graeme Gill.
>
>
>
>
>

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