[Openicc] LINUX, Gutenprint / CUPS / Color policies

[Jan-Peter Homann]

Hello List

Reading the dicussion about Gutenprint, CUPS and ICC-profiles I´m still 
confused, even if I´m a colormanagement specialist.

Actualy, we have four possibilities to apply and use profiles during the 
printing process:

- colormanagement in the application
- colormanagement with an ICC to CSA/CRD conversion during Ripping
- colormanagement as CUPS-Filter on rasterized data
- colormanagement as part of the printer-driver e.g. gutenprint

This possibilities and their combinations are making colormanagement for 
the printing process very complex and it is very hard to build an easy 
and transparent GUI, that the user really knows, what happens actually.

If we wan´t to make the process for users and GUI developers more 
transparent, we have to talk about color policies in the OS, the 
applications, CUPS, Gutenprint etc.

This color-policies should be linked with GUI recommendations. I will 
try to figur out this a little bit:

1) All sRGB colorpolicy
--------------------
This colorpolicy is dedicated to office and internet users. It is also 
dedicated to all softwares which don´t use colormanagement inside the 
application.
All RGB-data sended from the application is interpreted as sRGB-data. 
During printing, the RGB-data gets the sRGB-profile assigned and the 
data will be colortransformed to the printer-profile for printer-type / 
media / driver settings.

2) RGBdocument colorpolicy
--------------------------
All content of an document is RGB, but the application is 
colormanagement aware, can use individual RGB-profiles as working space 
for the application and makes it possible to store the actual 
working-space in the document. Opening an document results to an use of 
the document-colorspace instead of the application RGB-workingspace.
During printing, the document is transformed from the 
document-colorspace to the printer-profile for printer-type / media / 
driver settings.

3) CMYKdocument colorpolicy
-------------------------
(similar to RGBdocument policy)
All content of an document is CMYK, the application is colormanagement 
aware, can use individual CMYK-profiles as working space for the 
application and makes it possible to store the actual working-space in 
the document. Opening an document results to an use of the 
document-colorspace instead of the application CMYK-workingspace.
During printing, the document is transformed from the 
document-colorspace to the printer-profile for printer-type / media / 
driver settings.

--

With this three policies 99% of the needs of amateur and 
professional-users (office-user, graphic designers, photographers, 
prepress-people, printers) can be fullfilled, if they know, what they 
want, and if they organize their workflow proper.
But if you like colormanagement-problems, ICC-technology delivers also 
other possible colorpolicies:


mixedcolor-document colorpolicies
--------------------------------
Workflows, where every object (image, vector, text) can have its own 
profile and rendering intent seems to be cool from the view of the 
developer, but causes a lot of problems of unwanted und uncontrollable 
colortransformations during the production process.

This problems especialy occurs during printing and file-exchange of such 
documents. If you wan´t avoid such problems, it is best to convert the 
colors of individual objects to the document colorspace before printing 
or file-exchange. This should be possible for all objects inside the 
application.

If an application can do this, this results to a color policy called:

4) mixedcolor-document with application-side colormanagement
----------------------------------------------------------
In this case colormanagement for printing is the same like for RGB- or 
CMYKdocuments. The application transforms all objects to an file with 
flat color of the document workingspace.
This makes it possible to use the same GUI and colorinfrastructure for 
printing like in the standard colorpolicies

But there is also the possiblity for a color policy which make printing 
and file exchange much mor complicated:

5) mixedcolor-document without application-side colormanagement
------------------------------------------------------------
In this case, the colormanagement for individual objects is done outside 
the application, in wich the document was generated.
Things are going really complicated. E.g. if the colors of the 
individual object should transformed from their individual profile to a 
document colorspace to the printer-colorspace.
The possibilities, that somethings is not working properly, becausse of 
an  misconfiguration of e.g application, ghostscript, CUPS, gutenprint 
is very big.

For GUIs and a color-infrastructure, which are transparent and save to 
use, I definitly recommend colorpolicies 1-4.

In this case colormanagement during printing needs always one color 
transformtion from the document-colorspace to the printer-colorspace.

Where to configure profiles and do printing-colortransformations ?
-------------------------------------------------------
The profile for the document-colorspace should be sended automaticly 
fromm the application, from which the document is printed.
The configuration of the document-profile should only be done in the 
general color settings of the document/application.

In the printing-dialogue, the actual profile of the document should be 
displayed. But the user should be NOT able to change it here.
(If he want to this, he should do it in the general colorsettings for 
the application / the document.

The profile for the printer should be configured in the application, 
which is doing the colortransformation from document-colorspace to 
printer colorspace. If this is done by a filter in CUPS, then the 
printer-profile should be configured in CUPS. If this is done in the 
printerdriver (e.g. gutenprint) the printer-profile should be configured 
there.
I strongly recommend to do the colortransformation in the printer-driver 
and not in CUPS, because driver-settings, linearization and 
ICC-colortransformation should be seen as one integrated step, which 
should not be splitted over several applications.


:-) colorful greetings
Jan-Peter




-- 
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Jan-Peter Homann ------------- mobile +49 171 54 70 358
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