Hi Jan-Peter,
Thanks for your explanation. This clarifies a lot what I'm trying to accomplish.
If we have a CMM wich preserves the original separations, the internal
mechanics are much more complex.
As I talked with some developers of commercial CMMs, they described it as some
kind search strategy.
Yep, that is. See my other mail describing the algorithm. That is NOT a normal
ICC workflow. And I think the current implementation is pretty fast, it takes
only
a few seconds to precompute. And since internally is represented as a
devicelink,
there is not speed penalty.
But again, this has to be carefully tested before claiming it works :-)
Best regards,
--
Marti Maria
The littlecms project.
www.littlecms.com
----- Original Message -----
From: "Jan-Peter Homann" <[EMAIL PROTECTED]>
To: "Marti" <[EMAIL PROTECTED]>; "Leonard Rosenthol" <[EMAIL PROTECTED]>; "Lcms
Liste" <lcms-user@lists.sourceforge.net>
Sent: Sunday, May 22, 2005 5:43 PM
Subject: Re: [Lcms-user] Preview of black preserving feature on CVS
Hello Marti, hello list
Actually, I´m testing high-end solutions for doing CMYK2CMYK conversion.
Preserving the black channel or better named "preserve original separations" is a substantial step forward in the quality of
CMYK2CMYK colortransformations.
It is necessary to understand, that preserving the original seprations is
different from preserving pure gray.
Preserving pure gray is an exception for normal ICC-based CMYK2CMYK conversions. All colors are matched CMYK-Lab-CMYK, only colors
with CMY=0 (pure gray) stay the same or changed only by a 1-Bit LUT / gradation curve.
Preserving the original separations is a fundamental change in the CMM-Architecture. Imagine there are two grey tones, which have
the same Lab-value, e.g.
CMYK 50 40 40 0
CMYK 20 10 10 30
In a traditional ICC conversion CMYK-Lab-CMYK, both gray tones are converted to the same Lab-value and then converted to same
CMYK-values.
If we have a CMM wich preserves the original separations, the internal
mechanics are much more complex.
As I talked with some developers of commercial CMMs, they described it as some
kind search strategy.
For input CMYK-value with an Lab-equivalent, the CMM has to search in the output-profile a CMYK-value which has the same value for
Black and also the same Lab-equivalent.
In our expample this results to following mappings:
CMYK 50 40 40 0 -> 52 37 42 0
CMYK 20 10 10 30 -> 22 9 14 30
The Advantage of such transformations are smoother CMYK 2 CMYK conversions.
Users, which can benefit form this technology are mainly graphic arts users which can use this technology in digital proofing or
in the conversion of CMYK-Files between different printing standards.
For the last point, it it necessary to add a TAC limit (Total amount of color). So e.g. all colors with a TAC over 320 are limited
to 320.
The normal implementation in high-end applications for graphic arts is not direct into the CMM, but in solutions for creation of
devicelink-profiles.
Adding features like TAC-limits makes it longer to calculate the complete CMYK
2 CMYK transformation. This can be several minutes.
So precalculating devicelink-profiles for the tasks you need, speeds up the
workflow.
As littleCMS allows to save a colortransformation as devicelink-profile, this
should be easily to implement.
Greetings from berlin
:-) Jan-Peter
Marti schrieb:
Dear lcms list,
For those of you interested in such things, I've uploaded into CVS
latest lcms sources implementing a new cool feature:
That is black preservation.
Black preservation deals with CMYK -> CMYK transforms, and is intended to
preserve, as much as possible, the black (K) channel
whilst matching color by using CMY inks. There is a tradeoff between accuracy and black preservation, so you lost some accuracy
in order to preserve the original separation. Not to be a big problem in most cases, benefits of keeping K channel are huge!
--
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:[EMAIL PROTECTED]
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