[argyllcms] profile -S -g
- From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 24 Apr 2008 17:22:31 +0200
Hi Graeme, Hi list,
being back from the "temple of the muses" of fine art reproduction, I
have to find pragmatic solutions for much more coarse problems right now:
I have to build profiles for a large format solvent printer and a large
number of different media. The goal is not the "perfect" reproduction,
but the maximum likelihood for acceptable, pleasing result. Most of the
input data is conditioned for Offset printing (ISO Coated v2), some of
the media have larger gamuts and some smaller than ISO Coated v2 (iC2).
The ICC differentiates between to possible objectives in this context:
re-targeting (where the offset print is the reference) and re-purposing
(where the original is the reference) and says:
"In support of re-purposing, the ICC Version 4 specification places a
new emphasis on perceptual rendering intent transformations:
* In ICC Version 4 compliant [actual-output-referred] output profiles,
the actual-output-referred-to-reference-output-referred Device-to-PCS
perceptual rendering intent transform should invert (i.e., undo) that
profile's own PCS-to-Device perceptual rendering intent transform, to
allow for re-purposing from the ICC perceptual intent reference medium."
(<http://color.org/ICC_white_paper_2_Perceptual_rendering_use_cases.pdf>)
Graeme, you demonstrated the pitfalls of this approach in your document
"About ICC profiles and Gamut Mapping"
(<http://argyllcms.com/doc8/iccgamutmapping.html>) and you gave good
reasons for using a different one: gamut mapping in argyll takes place
exclusively in the B2A0 table and the A2B0 table is *not* the inverse of
the B2A1 table, but identical with the A2B1 table (i.e. the A2B
transform is always colorimetric).
This approach works smooth and seamless with argyll profiles, but in my
case, it results in a "clash of cultures": even though "ISO Coated v2"
is no ICCv4 profile and doesn't use the PRMG, it is "ICC V4 compliant"
in terms of "symmetrical" A2B0/B2A0 tables.
When I build a profile with
profile -S 'ISOcoated_v2_eci.icc' -cpp -dpp solventLFP
I get *heavy* clipping when using the perceptual intent.
One workaroud would be telling the CMM to use the relative colorimetric
intent for the iC2 to PCS transform and the perceptual intent for the
PCS to LFP transform.
Another way would be using a device link profile with the for the
aforementioned conversion -- and, indeed, the device link profile
created with
icclink -ir -op ISOcoated_v2_eci.icc solventLFP.icc iC2r_to_solventLFPp.icc
causes no clipping and leads to excellent results.
Unfortunately, the LFP's RIP neither permits selecting different
rendering intents for forward- and backward transforms nor using device
link profiles.
One could outwit the CMM by manipulating the tag table of the output
profile, but this "dirty trick" has unwanted side-effects.
The attempt to build a profile that uses the *expanded*, "virtual"
iC2-Gamut to the solventLFP device gamut fails:
iccgamut -ff -ip -cpp ISOcoated_v2_eci.icc
mv ISOcoated_v2_eci.gam iC2pf.gam
profile -S ProPhoto.icm -g iC2pf.gam -cpp -dpp solventLFP
gamut map: Gamuts aren't compatible
Creating smoothed near points failed
profile: Error - Failed to make perceptual gamut map transform
profile -S ProPhoto.icm -g iC2pf.gam -cpp -dpp solventLFP
fails with te same error message and
profile -g iC2pf.gam -cpp -dpp solventLFP
does not create a perceptual table.
What else can I do for the short term to negotiate the obstacle?
Do you think a "play nice with Heidelberg" switch for profile makes
sense in the long run?
TIA,
Klaus
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