From: "Hal V. Engel" <[email protected]>
Subject: Re: [poppler] Color Management
Date: Fri, 27 Feb 2009 12:45:12 -0800
Message-ID: <[email protected]>
hvengel> On Friday 27 February 2009 02:06:05 am Koji Otani wrote:
hvengel> > Hi
hvengel> >
hvengel> > Thank you for your comments.
hvengel> >
hvengel> > First, poppler is a library. So, applications using poppler should be
hvengel> > responsible for location of profiles, I think.
hvengel> > Applications (not poppler) can know
hvengel> > on which device output is displayed and which
hvengel> > profile is appropriate one.
hvengel>
hvengel> This is correct. I was mostly pointing out how end users would expect
this to
hvengel> work eventually with the intent of making sure that the poppler API
would
hvengel> allow for that to happen. It appears that the API does but there are
no
hvengel> current apps, even demo apps included with poppler, that make the
calls for
hvengel> this to occur so it is untested at this time.
hvengel>
hvengel> Apps that are not CM aware like Okular built against poppler git head
are much
hvengel> better behaved when rendering ICC based pdf objects than before.
Poppler now
hvengel> has a reasonable default behavior when it encounters ICC based objects
in
hvengel> spite of not having access to the correct output profiles. This is a
good
hvengel> thing and a big step in the right direction.
hvengel>
hvengel> >
hvengel> > I think that what we should do next about this is to add some
hvengel> > API (that set location of profiles) to the poppler external API
(glib API).
hvengel>
hvengel> Having taken a look at okular as an example of how applications are
coded I
hvengel> see that they are interfacing to poppler through the API defined in
poppler-
hvengel> qt4.h perhaps believing that this provides a complete interface. Of
course
hvengel> the API for setting the output profiles and for setting up the color
hvengel> transforms is not exposed in poppler-qt4.h. Okular basically does
things in
hvengel> about the same way as qt4/demos/poppler_qt4viewer so this appears to
be fairly
hvengel> typical of how many apps are/will be coded.
hvengel>
There are 2 external APIs for poppler. They are Qt4 and Glib.
I mentioned only Glib in the previous mail, but as you wrote, it's
needed to add the API for setting profiles to Qt4.
And it's good to change sample programs to use this API.
hvengel> Perhaps a good way to validate the API would be to incorporate basic
CM
hvengel> awareness into the demo apps. In addition, this would then give
application
hvengel> developers some example code that would help guide their work in this
area.
hvengel>
hvengel> Speaking of the API, this is a minor issue, but one that should be
fixed
hvengel> before applications start using the CM APIs. The "displayProfile" is
actually
hvengel> the output profile and it could be for any type of output device such
as a
hvengel> CMYK printer or even a N channel device. For example a program using
poppler
hvengel> to generate raster output for a printer would call either
setDisplayProfile()
hvengel> or setDiplayProfileName() before calling setupColorProfiles() to setup
the
hvengel> correct transforms for getting output for the printer. This is
somewhat
hvengel> counter intuitive. The code looks like it handles this correctly so I
am
hvengel> pointing this out mostly to make the API more accessible since it
would make
hvengel> things clearer if this where named outputProfile rather than
displayProfile.
hvengel> The same thing applies to other parts of the API that use
displayProfile or
hvengel> display or Display as part of their name. These should all be changed
to
hvengel> output* or Output* for clarity.
hvengel>
I see.
hvengel> >
hvengel> > Though current code looks
/usr/share/poppler/ColorProfiles/display.icc
hvengel> > and ~/.xpdf/ColorProfiles/display.icc, this is only default
behavior.
hvengel> > It seems OK to change these default to other files
hvengel> > such as /usr/share/color/icc/poppler.icc and
~/.color/icc/poppler.icc.
hvengel>
hvengel> Looking at the code it appears that it will try to load display.icc by
hvengel> default. I don't find any references to poppler.icc in the code. Of
course
hvengel> neither of these (display.icc or poppler.icc) exist in any location on
most
hvengel> machines including mine. So I am not sure what profile is actually
being
hvengel> used by the current code.
hvengel>
hvengel> In addition, it is accepted practice that if no display or output
profile is
hvengel> specified by the user or is available through system calls that this
should
hvengel> default to sRGB. I have attached a patch that changes the section of
hvengel> GfxColorSpace::setupColorProfiles() that sets up the output (display)
profile
hvengel> so that it defaults to sRGB. This patch also changes references to
hvengel> displayProfile and displayTransform and other things like this to use
output
hvengel> instead of display. The patch does not address the profile path issue
since
hvengel> this should probably be handled using Oyranos since this is a more
generalized
hvengel> solution.
hvengel>
If no display or output profile is specified, current code uses
littele cms's built in sRGB profile.(see GfxState.cc: line 305)
hvengel> I tested the above code with okular and poppler_qt4viewer and it
appears to
hvengel> be working correctly. At least it builds and the apps run without
apparent
hvengel> errors.
hvengel>
hvengel> Again I would like to thank Koji for his hard work on this since it is
a
hvengel> significant improvement to poppler even at this early stage. In fact
so much
hvengel> so that I hope it is included in a formal release sometime soon. I
will be
hvengel> looking this over as time permits and will make additional comments as
hvengel> necessary and perhaps even supply a few patches. I will also try to
take a
hvengel> look at qt4/demos/poppler_qt4viewer to see if perhaps I can find some
time to
hvengel> create a CM awareness patch for it.
hvengel>
hvengel> Hal
hvengel>
hvengel>
hvengel> >
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