[argyllcms] Re: Black turning down problem - help!

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 11 Jan 2011 10:17:08 +1100

Elena [service address] wrote:

Hi,

I saw there're many research papers around on this topic. For example:
http://ee.washington.edu/research/guptalab/publications/2009-CIC-Smooth-LUTsWithAnimation.pdf

Hmm. I don't think this article makes any reference to the topic at hand. It 
instead
seems to be dealing with setting up the A2B table. The approach has many 
similarities
to what Argyll already uses for forward profile creation, the regular spline, 
which
is based on much earlier research articles.

http://cilab.knu.ac.kr/seminar/Seminar/2009/20090516%20Accuracy-Preserving%20Smoothing%20Of%20Color%20Transformation%20LUTs.pdf

Interesting work, but it is in the area of post smoothing the lookup table.

The problem at hand (I think) is not colorimetric smoothness of the table, but 
the
lack of smoothness of the separation (K channel).

This is likely to be a result of topological inconsistencies due to the extra
degrees of freedom when there are more than 3 colorants. I've not often come
across any articles that touch on this. I suspect that this is either because
few people are aware of the issue, or that those who are aware of it regard
any solutions as proprietary information. A lot of academic articles (ie. 
student
papers) fall into the first category. CMYK is seen as a messy detail, and it's 
only
people dealing directly with devices that are faced with actually coping with
such issues !

This is how I see the problem, definitely. Assume the simple case, when I for 
testing
started with -kx and I looked at what happened. The K channel is turning down 
at about
90% density, an that's is fine, since I understood why and that made a sense at 
the
end. The problem is, I shouldn't being noticing it visually as a bluish bump or 
other
irregularities. If I notice it, I assume that colprof has not enough sample 
points
(patches) around that zone to work out a reliable result, i.e to resolve this 
even
abrupt transition in a way which is colorimetrically acceptable and without 
high errors
(to the eye, at least). At the end, I could well be WANTING such a K curve, and 
I
assume that if colprof had enough patches in that zone, it could make out an 
abrupt
transition which is however visually precise and without visible 
discontinuities. I
just can't figure out how to accomplish that in a simple fashion. I would have,
perhaps, to add more patches in that zone with xicclu, but it would require a 
dos
script at least and I can't play with scripts.

The level of accuracy of the forward (A2B) characterisation of the device
is quite important of course, but it comes down to judging where the
color errors are being introduced. "Bumpiness" in the darker areas, particularly
near the gamut edges, with a visibility that is heavily influenced by
the B2A grid resolution and black generation curve level, are a hint that
there are black topology problems. Currently, grid point accuracy and maximum
gamut size have priority over the black curve, and it is only the black curve
smoothness that results in B2A table smoothness of K value. So nothing prevents
sudden transitions in the black level at grid points at the gamut edge,
and the resulting device value interpolations of the B2A table can reveal that
the device value interpolated color is very far from the color of the two grid
points that are being interpolated between. Currently the best that can be done
is to choose a black generation curve that minimises sudden transitions at the
gamut edges. Ink limits (total and black) may also have an influence.

improvement in handling that area, i.e less noticeable visual discontinuity.
Unfortunately that was not the case, or if so, the improving is very very 
little.

Yes. That is a way of proving that the problem must lie elsewhere !

I hope I could explain my concern with my poor English (I'm Italian).

Your English is very good, and infinitely better than any attempt
of mine at Italian !

I never tried the matte black since I heard it actually contains some dyes and 
may not
be durable as pigments.

I'm not sure what the difference is, since I have not investigated such details.
Certainly on the inkjet I have (an Epson R1800), the claim is that all the
inks are long lasting.

sometimes else the opposite happens: the final profile has curves smoother than
predicted. But the issue I see is always the same: the problem is not so much 
how
"good" the K curve looks, rather what the visual result is. If, let's say, the
transition in the source space from red to black leads to one graysh bump 
around 80%,
how is it that colprof doesn't notice it and try to fix it ? Perhaps those bumps
actually originates at interpolation level, i.e between actual B2A grid points ?

Yes. K smoothness issues result in poor color accuracy between B2A grid points, 
where
it is the device values that are being interpolated between.

Graeme Gill.



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