Re: ColorSync and PostScript printing
Re: ColorSync and PostScript printing
- Subject: Re: ColorSync and PostScript printing
- From: Chris Murphy <email@hidden>
- Date: Fri, 4 Mar 2005 12:00:52 -0700
On Mar 4, 2005, at 11:45 AM, Uli Zappe wrote:
Am 04.03.2005 um 18:46 schrieb Chris Murphy:
Untagged RGB is tagged with Generic RGB. Untagged CMYK is left
untagged CMYK. (So you could print a CMYK target to a PostScript
printer from Mail.app, and reliably get an unmodified target from
which to build a profile. Not so for an RGB target.)
It would be interesting to know if this behavior changes for a printer
that, unlike the CLJ 5500, defaults to an RGB color space.
The reasoning could be: tag untagged images with a color space other
than the printer's default color space (because they have to be
converted), but leave alone untagged images in the printer's default
color space (because they can be sent to the printer as is).
That's not how the diagrams I've seen of how the printing system makes
its assumptions. Untagged RGB is always forced to be tagged as Generic
RGB at the time the PDF is generated by the system. So it *will* have a
source profile that the system is obligated to honor.
2. cgpdftops converts the objects in the PDF to a single CMYK space
based on their source profiles embedded in the PDF, and an unknown
destination.
So this definitely means that for the printer, the whole document is
either RGB or CMYK. Since cgpdftops will probably always convert to
the default color space of the printer (CMYK in case of the CLJ 5500),
and this default color space cannot be changed in ColorSync Utility,
it's really hard to see why the printer also registers RGB and Gray
profiles.
I concur.
You said that PostScript files print directly. That would presumably
mean that you can print to RGB if you have an RGB PostScript file, but
on the other hand, in this case the (RGB) destination profile
specified in the PPD or ColorSync Utility wouldn't be used either,
correct? If not, this case could be an explanation for the necessity
of RGB (and Gray) profiles in the PPD.
First question: yes. Second question: No.
Apps that create their own PostScript produce a PostScript file that is
untouched by the OS. That PostScript can contain RGB, gray, or LAB
data, or any combination of it and the OS will not touch it. The OS
doesn't have a means for parsing PostScript and applying color space
conversions. Once it's PostScript, it's untouched. So I don't
understand why there is a need for the RGB profile for your printer.
Gray and CMYK do make sense because any gray only content is preserved
by the printing system as gray only (it will use a gray output profile
as destination if it's registered for the device in CSU, as it is with
your printer).
is, I'm not absolutely sure what profile the system has selected as
the destination. Did it correctly select the destination profile
specified in the PPD file by the manufacturer? I'm not sure.
But the fact that changing the printer profile in ColorSync Utility
influences the result argues that the system does use the designated
destination profile, doesn't it?
It does. My next test, later this weekend is to totally bork the
manufacturers profile and rerun the tests and see if the edits I apply
affect numeric values in the PostScript file. If so, it's definitely
using the manufacturer referenced profile - but something else is also
happening because the values prior to borking are not correct. Lots of
ifs.
4. When I use what I think the destination profile SHOULD be with
Photoshop or AppleScript, I get different CMYK values (sometimes
substantially different), than the system produced PostScript that
would go to the printer.
:-(
Have you tested this with the destination profile set to something
other than the factory preset in ColorSync Utility? I mean, if this
setting influences the result (as you say it does), then you can be
quite sure that the system uses this explicitly specified profile.
Yes, and those numeric values aren't in the ballpark for the same
source-destination conversion using AppleScript or Photoshop.
Based on the PPD, which is rather straight forward, and the fact that
changing the destination profile for this printer in CSU causes a
change (but not a good one),
What do you mean by "not a good one"? That should depend on what
destination profile you select, shouldn't it?
I'm saying the conversion is bad/wrong/goofy/unexpected. I don't know
how or why, but it's jacked.
And yes it should depend on the destination profile I select. It's like
there is another conversion going on somewhere, or something.
Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
-------------------------------------------------------------
Co-author "Real World Color Management, 2nd Edition"
Published by PeachPit Press (ISBN 0-321-26722-2)
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