CMYK to Lab bug (part 1)
CMYK to Lab bug (part 1)
- Subject: CMYK to Lab bug (part 1)
- From: Henrik Holmegaard <email@hidden>
- Date: Sat, 10 Mar 2001 18:31:28 +0100
The bug is isolated to Photoshop and Distiller. Linocolor and other
applications behave correctly with Distiller, Photoshop behaves
incorrectly with Distiller. The Photoshop behaviour is identical for
v5.0 and v6.0 and for Acrobat 3 / PDF 1.2 compatibility and Acrobat 4
/ PDF 1.4 compatibility. The problem has nothing to do with the PDF
1.2 and PDF 1.3 format, only with the Adobe implementations. One
could argue that the problem with ICC color management is a problem
with Adobe implementations, but I already made that point once or
twice before -:).
Also I should have tested this long ago. PostScript Color Management
is an unimplemented implemented technology that I don't spend any
time on - except the problems. But given the logical designer and
prepress behaviour this mental block was clearly a mistake.
The problem: There are two aspects to CMYK, first that the ink
densities are right for the process and second that the color is
correctly managed. If the ink densities are wrong then the physics of
the printing process are in trouble (e.g. drying, smearing), and if
the color is wrong then the customer is unhappy. The problem with
converting CMYK to Lab is that the separation (ink limit and black
generation) are lost, even if the colors may remain right. Losing the
separation parameters by converting into a three channel space is the
KEY reason the whole prepress and printing industry deeply distrusts
late binding three channel workflows (RGB and Lab) - after a decade
of problems with in-RIP color management. The current bug would have
taken an Adobe engineer with a simple matrix at most three hours to
fix in the beta phase, and now cause users problems they have to
solve on their own without a central source for a fix.
Designer behaviour: The user now knows that she always has to tag her
CMYK or she will not be able to softproof and proof print it in true
colors. Therefore, she correctly selects the available tagging
options. There are two tagging options for EPS so both tagging
options are selected, ICC and 'PostScript Color Management'. A lot of
people have asked here on the List whether they are supposed to
select PSCM. They ask that question because neither the Adobe manuals
nor the add-on manuals I've read from the US writers talk about
printing and about in-RIP color management. So what are users
supposed to believe? (Who says designers are ignorant? They aren't
ignorant in the least.)
Prepress / printer behaviour: The designer has selected the right ICC
profile in Photoshop and fully understands that it both handles the
separation parameters and the color management. The prepress or press
operator has verified that the right profile is embedded, and that
the CMYK is correct for the intended printing process. Therefore, in
Distiller she asks that the colors be left unchanged. (Who says
prepress and press operators are ignorant? They aren't ignorant in
the least.)
Adobe behaviour: In the UI and documentation there is no distinction
between ICC format color space specifications and PostScript format
color space specifications. The internal technical writers use the
same word for both, 'profiles'. This is not helpful because the
formats are different, the functionality is different and the
behaviour is different - even for the only two applications that
support PostScript format color space specifications, namely
Photoshop and Distiller.
Distiller 4.0.5, 'Acrobat 3 Compatibility' and 'Leave Colors
Unchanged' for a CMYK image using the same Adobe bundled ICC CMYK
device profile :
PS5_TagICC_TagPSCM = Converted to Lab
PS5_TagICC_UntagPSCM = DeviceCMYK
PS5_UntagICC_TagPSCM = Converted to Lab
PS5_UntagICC_UntagPSCM = DeviceCMYK
PS6_TagICC_TagPSCM = Converted to Lab
PS6_TagICC_UntagPSCM = DeviceCMYK
PS6_UntagICC_TagPSCM = Converted to Lab
PS6_UntagICC_UntagPSCM = DeviceCMYK
Distiller 4.0.5, 'Acrobat 4 Compatibility' and 'Leave Colors
Unchanged' for a CMYK image using the same bundled Adobe ICC CMYK
device profile :
PS5_TagICC_TagPSCM = Converted to Lab
PS5_TagICC_UntagPSCM = DeviceCMYK
PS5_UntagICC_TagPSCM = Converted to Lab
PS5_UntagICC_UntagPSCM = DeviceCMYK
PS6_TagICC_TagPSCM = Converted to Lab
PS6_TagICC_UntagPSCM = DeviceCMYK
PS6_UntagICC_TagPSCM = Converted to Lab
PS6_UntagICC_UntagPSCM = DeviceCMYK
Next I'll talk to the LOGO folks to see how their new Swiss army
knife of ICC color management can get us out of this.
--
------------------------------------------
Henrik Holmegaard, TechWrite
Stationay +45 3880 0721 - +45 3881 0721
Mobile +45 2178 3959
Toelloesevej 69, 2700 Broenshoej, Denmark
------------------------------------------