Re: OS X ColorSync Utility
Re: OS X ColorSync Utility
- Subject: Re: OS X ColorSync Utility
- From: bruce fraser <email@hidden>
- Date: Tue, 9 Jul 2002 19:42:25 -0700
At 6:12 PM -0700 7/9/02, Dave Camp wrote:
And I certainly don9t doubt you are seeing what you say you are. :-) I'd
really like to figure out what is going on here as well.
Good-oh!
My assumption to date has been that existing drivers are making some
assumptions about the source space of the RGB data. For example, some
drivers (especially less ICC savvy ones) are written to assume that the RGB
data coming in is always sRGB. Or the drivers are simply using an inverted
RGB lookup to generate CMYK values.
I have it on reasonably good authority that the assumption in the
case of the Epson drivers is a space that has the sRGB primaries with
a gamma of 1.8. However, the person who told me so spoke only
slightly more English than I do Japanese, so something may well have
been lost in the translation.
On OS 9, this would have been the case for images printed from apps that did
not provide a source profile with the print job. Since only relatively high
end apps (e.g. Photoshop) would provide a source profile, a driver would
have to fall back to assuming something to perform the color conversion from
less capable applications. I assume Epson is doing something more advanced
(they have always had good color, but ultimately, with no source profile,
some sort of assumption has to be made).
When you set the Print Space in Photoshop to Same as Source, it
simply sends the numbers in the file to the driver.
Printer/Postscript Color Management is the Photoshop option that
sends a profile along with the file, a good idea if you want or
expect the driver to do color management, but not a good idea when
you're trying to print profiling targets!
For OS X, I'm assuming most vendors are simply bringing their printer color
code forward with minimal changes (partly explaining why there are no CMYK
native inkjet drivers yet). This also means that any built in color
assumptions are brought forward by default, or the drivers are modified to
give consistent results to the existing OS 9 drivers so that customers are
not surprised by suddenly getting different looking output. That sort of
thing is very important to vendors, as surprises create tech support calls,
and tech support calls cost money...
On the Epson, the conversion to CMYK (or in this case, to CcMmYKk) is
very deeply buried indeed, and is almost inextricably combined with
the dithering algorithm. Epson's own Stylus RIP feeds RGB to the
black box that does the conversion to colorant space and the
dithering, so do the EFI RIPs. I'm better than 90% sure that if you
want to use Epson's dithering, you need to feed RGB to the black box.
Certainly all the RIPs I've seen that offer meaningful CMYK control
don't use Epson's dithers.
Getting back to testing: my test scenario to validate that I could not print
a raw test target (as I was told would not work), was to create an untagged
CMYK TIFF file and print from Preview (this pre-dated Photoshop 7, maybe I
should try again). The driver was set to not give the OS any default
profiles.
Perhaps the issue is that you're sending CMYK to a driver that really
only accepts RGB input?
As long as the default Document and Output profiles in the ColorSync prefs
panel matched, my driver was sent unaltered CMYK data (because there was no
transformation happening). However, changing the default Document or Output
profiles definitely altered the CMYK values I was passed.
So, having said that, I will step back and say there is at least one way to
get raw output: make the source and dest profiles match. Of course, you
would expect that to work. However, it does not seem to explain the case of
an RGB printer driver, unless your source document is tagged with the output
profile.
My doc definitely wasn't tagged.
Have you tried printing the test target from any app other than Photoshop 7?
That might illuminate whether or not I'm completely high on crack, or maybe
Adobe is doing some behind the scenes trickery to give output consistent
with OS 9.
I'll try printing from Preview tonight, but the prints take about 24
hours to stabilize, so it'll be Thursday AM before I have anything
conclusive to report.
Bruce
--
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