[argyllcms] Re: profile -S -g

Klaus Karcher wrote:


profile -S 'ISOcoated_v2_eci.icc' -cpp -dpp solventLFP

I get *heavy* clipping when using the perceptual intent.


That's most surprising. The source gamut is obtained from
the colorimetric A2B table. It should not be subject to any
gamut mapping, and should represent the true gamut of the device.

Are you sure you are subsequently linking the ISOcoated_v2_eci.icc
source to the Argyll destination with the source intent colorimetric,
and the destination perceptual or saturation ? (Or doesn't the
CMM/RIP you're using allow such "mixed" intents ?).
Unfortunately, the LFP's RIP neither permits selecting different rendering intents for forward- and backward transforms nor using device link profiles. 

Right.
How about rebuilding the ISOcoated_v2_eci.icc using Argyll ?
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


Hmm. Yes, that won't work, by default profile uses CIECAM02
space (Jab) internally, and the gamut needs to be compatible
with that. And CIECAM02 starts with absolute colorimetric
data, not perceptual. (I would like to fix this so that it automatically
translates between the colorspaces, but haven't thought it that
important, and in this situation you'll get the ICCV4 problem
of the result having a saturation intent bias.)


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? 

I had a look at the colorimetric and perceptual gamuts and
they are quite a different, although the perceptual is
not the PRMG, it's a similar shape, but substantially smaller.

The source data is supposed to be on the FOGRA site
(FOGRA39L I think). You should be able to convert it
with logo2cgats and go from there.

I'm not sure there is any way of dealing with this type of issue
without side effects. The most transparent "fix" would be to grab
the gamut from the profile supplied with the -S options from the
perceptual intent, although this would need some deep changes
(ie. I need to extract "absolute" values from the perceptual table,
to get the XYZ to feed into CIECAM02), but maybe it's not too hard.
Translating the colorspace would give another path too.

Graeme Gill.

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