[Openicc] Print-colormanagement, application->CUPS->gutenprint

[Jan-Peter Homann]

Hello list

It seems, that a lot of people understand the word "printer-driver" as 
an software, which is generating some kind of PDL from the appplication.

As it seems, that we have no special term for a software, which drives 
the printer directly, I will use the term gutenprint in future.

So lets look, where colormanagement can take part during printing:

A. application based colormanagement
B. colormanagement in the RIP based on CSA/CRD instead of ICC
C. colormanagement as CUPS-Filter
D. colormanagement in gutenprint

Looking at the color-structure of document during printing, we have 
mainly three types of documents:
1. flat-color RGB-documents
2. flat-color CMYK-ducuments
3. mixed-color documents
(every image, vector-object or text-object can have his own profile and 
his own rendering intent.

Most documents yet are flat-color documents. If mixed-colored documents 
are processed for output on monitor or printer, the colors of the 
individual objects have to be flattend according the profiles and 
rendering-intents of the individual objects.

This color-flattening can be done direct to:
3.1 colorspace of the monitor
3.2 colorspace of the printer
3.3 main-colorspace of the document, called document-colorspace

Most applications and workflow-recommendation according mixed-color 
documents are concentrated on version 3.c.
Typical documents are e.g. CMYK-DTP-documents for offset-printing with 
placed RGB-content or PDF/X-3 files, where the Output-Intent specifies 
the document colorspace.

In this mail, I will concentrate on print-colormanagement for following 
types of documents:
1. flat-color RGB-documents
2. flat-color CMYK-ducuments
3.3 mixed-color documents with a defined document-colorspace

Printing documents of type 1 or 2, we need only one colortransformation 
from the document-colorspace to the printer-colorspace. As I wrote in 
mails before, I strongly recommend, to do this colortransformation in

D. gutenprint

For "3.3 mixed-color documents", we have to flatten the color of 
individual objcets to the document-colorspace before doing the 
colortransformtion to the printer-space.
The color-falttening can be done in:
A. Colormanagement in the application
B. colormanagement in the RIP based on CSA/CRD instead ICC

If we try do B. "colormanagement in the RIP" we have to take in 
consideration, that PostScript don´t supports ICC-profiles and 
rendering-intents. Instead of this, the profiles of the individual 
objects and their rendering-intents have to be converted to CSA.
Every combination of profile and rendering intents results to an 
individaul CSA.
The document-colorspace must be converted to an CRD. If individual 
objects are specified with differnt rendering intents, individual CRDs 
have to be calculated from the document-colorspace in combination with 
the rendering intents.
At the end of this processm every object in the mixed-color document is 
specified with his individual pairs of CSA and CRD which have to 
processed in the RIP.

In daily practice, this process is extreme unstable. Thats, why ALL 
colormanagement-experts recommend NOT to use
"B. colormanagement in the RIP"
The best solution for mixed color documents is
"A. Colormanagement in the application"

Conclusion
----------
We have now three cases
1. flat-color RGB-documents
2. flat-color CMYK-documents
3. mixed-color documents with conversion to document-colorspace inside 
the application

In all cases, the CUPS, the PS-RIP and gutenprint get flat-color 
printing-data, which has to be converted from the document-colorspace to 
printer-colorspace, which should be done in gutenprint.

For me, ripping is similar to the CUPS-filter ps2raster. So we have the 
main chain:
application->CUPS->gutenprint

If we want easy and transparent color management for printing, the 
application should be able to deliver its document-colorspace via CUPS 
to gutenprint.

As I´m not a developer I have following questions:

- Has CUPS a possibility to get the document-colorspace automaticly from 
an application ?
- Has CUPS the possibillity to specify a rendering intent for printing ?
- Has CUPS the possibility to transport the document-colorspace and the 
rendering-intent to gutenprint ?
- Has gutenprint the possibility to get the document-colorspace and 
rendering-intent from CUPS automaticly ?


greetings
:-) Jan-Peter




--

homann colormanagement ------ fon/fax +49 30 611 075 18
Jan-Peter Homann ------------- mobile +49 171 54 70 358
Kastanienallee 71 ------- http://www.colormanagement.de
10435 Berlin --------- mailto:homann at colormanagement.de