[argyllcms] Re: B2A mapping of out-of-gamut colors
- From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Wed, 02 Jan 2008 20:18:35 +0100
Thomas Baumann wrote:
This corresponds to the inverted A2B table with CAM Jab mapping (switch
-b): [...]
$ xicclu -fif -ir -a -b SP3800_PSPP_2880_ARG2.icc
I think you confused the meaning of the b switch, see
<http://www.argyllcms.com/doc7/xicclu.html#b>
Thomas Baumann wrote:
according to the command line help the meaning has changed since the last
official release - I am using Beta 7 because it supports the DTP20. The
former -b switch is now -m (merge).
You are right. Seems like the documentation does not reflect the latest
changes yet.
BTW: there's another difference I'm excited to discover:
doc7:
-S Use internal optimized separation for inverse 4d [NOT IMPLEMENTED]
usage:
-S Use internal optimised separation for inverse 4d
:-)
To figure out the influence of the Helmholtz-Kohlrausch-Effect in this
case I compiled a xicclu version with XICC_USE_HK set to 0 (xicc.h line
49), but it didn't change a lot:
xicclu -fif -ir -a -b SP3800_PSPP_2880_ARG2.icc
9.000000 0.000000 80.000000 [Lab] -> Lut -> 0.982441 1.000000 0.000000
[RGB] (clip)
[Actual 90.854331 -3.834409 115.588589, deltaE 89.338580]
xicclu_wo_hk -fif -ir -a -b SP3800_PSPP_2880_ARG2.icc
9.000000 0.000000 80.000000 [Lab] -> Lut -> 0.963003 1.000000 0.000000
[RGB] (clip)
[Actual 89.124108 -6.217807 113.343536, deltaE 87.007616]
As to unreal colour, I know that Lab 9/0/80 is not a "real" colour. But
you get there easily if you change the brightness of a real photograph in
Lab mode.
That's one of the reasons why I think Lab is a bad editing space (at
least in Photoshop).
Actually, I noticed this tone reversal effect in a shadow part
of a real photograph. I only chose the test image to better demonstrate
the effect.
Strange or unpleasant clipping artifacts are in no way unusual with the
colorimetric intent, see e.g. this screenshot (left: sRGB, middle:
softproof rel without BPC, right: softproof rel. with BPC -- both
softproofs with ECI ISOuncoated):
<http://digitalproof.info/colorsync-users/ISOuncoated-BPC/screenshot.jpg>
If I convert from Lab to ProPhotoRGB in Photoshop (Adobe ACE) before
printing, all is well. It would be just nice, if Argyll's B2A1 table would
provide a similar mapping directly from Lab...
Thomas
I'm not sure if I would prefer that, but I think you can tweak profile's
clipping behavior when you change line 49 of profout.c:
#define USE_CAM_CLIP_OPT /* Clip out of gamut in CAM space rather than
XYZ or L*a*b* */
and use the -ax flag for profile
(no waranty -- zu Risiken und Nebenwirkungen befragen Sie Ihren
Colormanagement-Consultant oder Programmierer ;-)
Klaus
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