Marti wrote:
Please bear in mind this is still a work in progress. There is almost
nothing on that on open source, so I am still trying to figure out what
would be the "good" way to do things. Black-preserving transforms are
not well assessed by dE metrics. There are some situations where dE is
not such important, and this is one of these.
At Colorbus a couple of years back, we got all excited about using
black preserving transforms for proofing to inkjet printers, since
it seemed to address some issues with maintaining good grey matching
for CMY and K neutral patches. It also often had a beneficial effect
of improving the overall spectral matching between the target print
and the proof.
After the enthusiasm wore off, we decided it wasn't that useful
for inkjets, because of the heavy influence the black level has
on other aspects of inkjet reproduction. High black emphasizes
"dotiness" (since black has a very high visual contrast ratio),
as well as any banding. Low black levels (== high CMY) give
bumpy neutrals, that change a lot under different viewing
conditions. For consistent output of an inkjet for proofing, we ended
up going back to a carefully chosen black generation curve, balancing
off all the different aspects.
Perhaps for re-targeting CMYK separation from one printing press to
another, preserving black is more useful.
For sure you have seen prints of gray images with a huge color cast
towards magenta or green. That's very unpleasant.
Mainly, this happens because metamerism is not taken into account when
doing the profile. Black ink chromaticity changes on different
illuminants. For example, a profile is done measuring black ink under
D50, then, under D50 this black ink have tendency to magenta. Ok, the
profile captures such chroma and, when
reproducing colorimetrically, replaces the destination black with
CMY reproducing this magenta. Now, If you take the original K ink
and examine it under sunlight, it is not magenta anymore! But the
reproduction using CMY keeps going magenta. Result: a nasty color cast .
It depends if you include the lighting spectral information into
computing the PCS color values. If you know the viewing lighting
spectrum, and measure your source and destination device test
charts spectrally, then this type of effect can be taken into
account in profiling. The main complicating factor is FWA (Fluorescent
Whitener Additive) in the paper, but this can also be compensated
for to a fair degree, if you know the spectrum of the viewing
light into the U.V.
And this is just one of the reasons why keeping black ink is so
important. Maybe there is a slight discontinuity, as big as the
chromaticity of
blacks differ, but it is so small that the smoothing induced by the CLUT
is enough to compensate. And this is almost nothing when compared with
the huge cast on grays a CMYK->Lab->CMYK may create.
Good quality profiles really shouldn't introduce large errors in
grey reproduction through linked profiles. Of course, using the
B2A table in the destination profile is less accuracy than
directly inverting the A2B table.
Isn't there a risk, that the Newton method gets stuck in a local minimum?
Yes, and it happens. I am still not fully satisfied by the algorithm. It
works to some extent, but misses ink limiting (TAC) preservation. I have
to figure out
how to get the TAC from the output profile and how to avoid local minimums
as well.
I played around with a number of inversion approaches for profiles. To get
a really robust result using optimization techniques, I often had to do things
like choose a set of random offsets from the nominal starting point,
and choose the best result from multiple attempts. The alternate approach I
ended up with is far more analytical, much more complicated, isn't very fast,
but is accurate, and will return all possible solutions, as well as a
solution locus for CMYK (so for "preserve black" I could maintain the relative
black level between source and destination, as a proportion of the
possible black locus for a given PCS color).
From your description your approach seems to be a lot faster.
One of the major flaws in the ICC format I keep stumbling across, is
that there is no tag for recording the TAC! Searching the B2A table
works to some degree, but doesn't tell you what the TAC was really
intended to be. At some stage, I want to create (and document) a private
tag for storing this.
Graeme Gill.
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