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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