[Openicc] Printing Plans GhostScript

[Hal V. Engel]

On Tuesday, March 01, 2011 02:15:32 AM Kai-Uwe Behrmann wrote:
> Hello Michael,
> 
> Am 28.02.11, 22:09 -0800 schrieb Michael Vrhel:
> > Hi Jan-Peter,
> > 
> > So yes, ghostscript does apply ICC base color transformations on
> > Postscript files and this is true even for DeviceRGB and DeviceCMYK.
> > I need to add in the interface for rendering intent and black point
> > compensation.  That will be coming shortly.  In fact, I am also adding in
> > the ability to specify different output profiles for graphics, images,
> > and text as ghostscript keeps track of these objects during rendering,
> > even through transparency blending.   The specification for these
> > options is primarily through the CLI but can be made through special
> > configurations.
> 
> Would it possible to pass those profiles along the PDF document itself?
> 
> > As far as "turning off" transformations for DeviceRGB or DeviceCMYK, that
> > will occur in cases where the source and destination profiles are the
> > same.
> 
> We came several time to the conclusion that this scheme is not realy
> robust. So we would be happy you could point us to an other mechanism as
> well.

The reason that this is not robust is that the user app currently has no way 
to specify the source profiles to be used when the GS pdftoraster filter is 
called and no way to know what profile will be used by the print system so that 
it can specify a matching output color space.

The reason that this whole DeviceXXX thing is an issue is because we have so 
many dumb apps and libraries that use DeviceRGB and DeviceGray for objects 
that really should be taged with either sRGB or an equivalent CalRGB tag for 
RGB images and generic gray ICC profile or a CalGray tag for monochrome images.  
Dealing with these malformed PDFs causes all sorts of headaches. 

> 
> > The obvious example occurs if I have a document that has an RGB image and
> > a CMYK image and I am printing to a CMYK device.  In this case, the RGB
> > image will have to be transformed in some manner.  That transformation
> > is under your control by specifying the desired default RGB source
> > profile to use. For the CMYK image,  if you did not want the data
> > touched, you should make sure that your default CMYK source profile is
> > the same as your destination profile.  The CMYK data will then pass
> > through unmolested.
> 
> Will adding a OutputIntent to a PDF/X (A/E) be honoured for DeviceXXX
> objects by Ghostscript? Leonard pointed this requirement out [1].
> It could then be used to pass through unmolested Cmyk or DeviceN to a
> according configured printer.

Why not unmolested RGB and Gray as well?  Any app that is smart enough to 
create PDF spool files with the OutputIntent set should be assumed to be smart 
enough to know what it is doing and the PDF spec should be fully honered.  The 
issue with assuming a color space for DeviceXXX objects should, if possible, 
be limited to apps that don't know better.  At this point there do not appear 
to be any CM dumb apps that can create PDF spool files with the OutputIntent 
set.  This is definitiely true for Qt apps that are using the standard Qt PDF 
printing chain.

The issue here is retargeting.  If the print system receives a spool file with 
an OutputIntent that is different from what the system would use normally that 
device and settings what should it do.  One of the reasons for using DeviceXXX 
for objects along with an OutputIntent is to allow for retargeting.  Under 
those conditions it is assumed that the DeviceXXX objects are in the 
OutputIntent color space.  This is to allow for the output to be retargeted to 
a different device if needed using an OutputIntent --> new device color space 
transform for all DeviceXXX objects.  This means that setting the OutputIntent 
is also not a robust solution unless there is some way for the app creating 
the spool file to query the system for the devices profile so that it can use 
that for the OutputIntent.   This could be done using Oyranos or colord but 
this means that the app creating the PDF needs to be not only aware of color 
but also needs to knwo what printer is being targeted and how to get it's 
default profile for the print mode being used.

The basic issue always come back to the fact that most of the apps on peoples 
machines are producing malformed PDF files that by default use DeviceXXX and do 
not set the OutputIntent.  If we were following the PDF spec. and DeviceXXX 
data would be passed through to the printer unchanged with unpredictable 
results for many apps.  In addition, if the DeviceXXX objects did not match 
the color mode of the printer (IE. DeviceRGB but printer is a CMYK only or is 
set to CMYK in CUPS) or if the PDF has more than one DeviceXXX type (like 
DeviceRGB and DeviceGray which is possible with the current Qt PDF code) then 
the print job would fail.  The problems is users expect stuff to "just work" 
and following the specification will fail for some number of print jobs with 
most software currently being used.  This means that we need to do things like 
get the Qt PDF code fixed to be inline with the PDF specification and work at 
getting good PDF code into other libraries that support printing like GTK+.  
This is going to take some effort.

> 
> kind regards
> Kai-Uwe Behrmann
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