[argyllcms] Re: xicclu
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 30 Dec 2005 13:01:18 +1100
Klaus Karcher wrote:
but now to the problem I've encountered with xicclu:
xicclu -fb -ip -pj /any/cmyk/profile.icc:
10.000000 0.000000 -30.000000 [Jab] -> Lut -> nan nan nan nan [CMYK]
20.000000 0.000000 -60.000000 [Jab] -> Lut -> nan nan nan nan [CMYK]
30.000000 0.000000 -90.000000 [Jab] -> Lut -> nan nan nan nan [CMYK]
... same story with -fif, other Rendering Intents or RGB-Profiles:
as soon as J/a = -1/3 and b=0, xicclu returns nan
I'm neither a programmer nor know much about Color Appearance Models,
but I fiddled about with gdb and guess that the trigger is the
evaluation of the Helmholtz-Kohlraush effect in cam_to_XYZ() (file
cam02.c):
Actually there is a problem whenever J == 0. There are severe problems with
the
whole of CIECAM02, when applied to real world tasks like gamut mapping, and
although I have tried to work around these problems, I don't think I've
solved the issues completely. (The Helmholtz-Kohlraush correction
exacerbates things.) There were two papers on this issue
at the last CIC (See <http://www.imaging.org/store/epub.cfm?abstrid=33548>,
and <http://www.imaging.org/store/epub.cfm?abstrid=33567>), although
both of these papers use rather a different approach to address the problem
than the approach I've chosen.
A workaround seems to be to add the following at about line 512 of cam02.c:
-------------------------------------------------------
/* Achromatic response */
A = spow(J, 1.0/(s->C * s->z)) * s->Aw;
+ /* Avoid numerical issues when J == 0.0 */
+ if (fabs(J) < 1e-8)
+ J = J < 0.0 ? -1e-8 : 1e-8;
/* Preliminary Saturation - always +ve */
tt = fabs(J);
ss = pow(C/(pow(tt, 0.5) * s->nn), 1.0/0.9); /* keep +ve */
-------------------------------------------------------
I'll include this in the next release.
Graeme Gill.
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