Hi,
We've implemented a similar feature on top of lcms in our PDF
Enhancer software (http://www.pdfenhancer.com), and I'm curious what the
implementation differences are.
Great. For conversation with some people of the industry, that seems to be
a hot topic. And of course I will be glad to share my method.
Are you simply saving the K, performing the transform, and then
restoring K? Are you doing any special casing of [0 0 0 K]? What about [C
M Y 0]?
The algorithm is a little bit more complex. Here is a coarse outline:
1) A K tone curve from K-> K' is computed. That is, which amount of K'
on the output space gives same L* as which amount of K in input.
That is done by measuring ramps of (0,0,0, K) using the proof direction,
on both input and output profile. Then joining the obtained curves.
2) The tone curve is used on (0,0,0,K) -> (0,0,0,K') so pure black is
always preserved as pure black.
2) AToBXX tags on output profile are reversed and a special BKToA is
computed. This is a new LUT has a extended PCS using Lab + K.
3) Then a devicelink is built by using the original AtoB plus this new
BKToA. Original K is passed to the extended PCS across tone curve.
The method for inversion is a modified Newton-Raphson extended to 3
dimensions. The original CMY + K across tone curve is used as a seed for
the search. The algorith iterates across Jacobian as far as the error
decreases and the system is convergent.
This have some nice side effects:
- If the color cannot be matched at all, the original separation is keept as
much as possible.
and
- New amount of inks are close to original amount.
In this way, the algorithm avoids to use, for example, black over pure
yellow, and thus tries to avoid grain as much as possible.
Regards
--
Marti Maria
The littlecms project.
www.littlecms.com
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