[argyllcms] Re: Black generation and black ink limiting

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 30 Mar 2010 09:10:36 +1100

Fabrizio Levati wrote:

I decided to start experimenting a bit with colprof. I have been able to find 
my way
through the tricky -kp black shape definition but there is something I am not 
able to
understand regarding black ink limiting. Using always the same source data and 
parameters and then adding a -L92, I end up with roller coaster shaped curves 
as shown
in the attached pictures. Even if we are in the "far black" region I'm not so 
sure this
kind of curve won't have any consequence on the resulting separations. Graeme 
told me,

        you can be pretty sure that the total CMYK will result in the target
L*a*b* value (at least up until it clips, when the total ink limit is reached),
no matter what the curves look like. You can always check this by running
the resulting CMYK numbers back thought the forward (A2B) conversion.

The shape you see is just the consequence of meeting the black curve aim and
limits you've set. To hit the deeper blacks while not going over the
total ink limit, more black needs to be used, and less CMY. You might be
able to flatten the CMY off a bit by attempting to make the K curve
more concave.

The problem with non-smooth curves is that they demand too much precision from
the color conversion and printing workflow, and increase the danger of 
or inaccurate color reproduction in the deep blacks. On the other hand, in
deep blacks the resulting color is often rather insensitive to changes in
each colorant value, which is part of the reason that the curves often end
up a bit rough in argyll generated separations - it is having to swing the
colorant values quite significantly in order to hit the desired color.

Graeme Gill.

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