[argyllcms] Re: Black turning down problem - help!
- From: Elena [service address] <1007140@xxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 10 Jan 2011 17:08:46 +0100
Hello Graeme
On 10-Jan-2011, Graeme Gill wrote:
> There is only one possible result of solving it: gamut is traded for
> smoothness.
> As to how to create a profile that achieves this trade-off, I have quite a
> few ideas, but they are non-trivial to implement.
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
http://cilab.knu.ac.kr/seminar/Seminar/2009/20090516%20Accuracy-Preserving%20Smoothing%20Of%20Color%20Transformation%20LUTs.pdf
but surely you are aware of all them already
>> These things shouldn't really happen.
> I'm not sure why you say that. Even if inks mixed in a simple way (and
> inks of various compositions don't, particularly if they have pigments in
> them), the fact that there are multiple ways of creating blacks and
> independent ink choices (CMYK) means that it is quite possible.
I meant, ideally things like CMYK being lighter than CMY, or even K being
lighter than CMY are annoying eveniences.
I tried hard-limiting ink channels in my software in order to obtain a
more predictable behavior (cmy being a dark grey and still a bit ligther than
k alone; 100% cyan being more a cyan than a deep blue, more likely to what
happens with regular offset inks, and so on) but the tradeoff is gamut, of
course: doing so I totally lose the capability of obtaining saturated
secondaries.
>> It would be interesting to know, however, how
>> other profilers work out this condition. As soon as I have additional time
>> I will try PM to see what comes out (I never made plain paper profiles with
>> PM).
> I have no specific knowledge of how or even if they tackle this problem.
I really have to test yet
>> The matter, however, is that no artifact should be visible. I will try today
>> to see what happens with a profile made out of some 3000 patches and -qh
> The nature of the test chart won't changed the fundamentals of how the
> inks behaved.
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.
So I tried, just for understanding, the following way, but without big luck.
Making a first good profile starting with 3000 patches and -qh, so the A2B
table (which is the device behavior) is precise enough.
The profile is made intentionally with -kx, which gives the abrutp K ramping
down.
Then I created and printed a second target passing the previous profile with -c
and
using the -I option (as suggested somewhere in this list) and I made a second
profile with it, always using -kx. Thus, I was expecting to see an 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.
I hope I could explain my concern with my poor English (I'm Italian).
>
>> or register problems). But when the used K is actually weaker than CMY,
>> one would have the temptation to simply drop it :-D
> Perhaps this is why some printers have "matte black" inks for matte paper ?
I never tried the matte black since I heard it actually contains some dyes
and may not be durable as pigments.
>> I noticed that -K sometimes offers smoother results, but it's not a rule
> A specifically chosen -k curve is usually a better approach.
And it's not easy. I experienced how a predicted curve looking reasonably
smooth may later appear slightly different when making the actual
profile. It's almost trial and error. Sometimes the final profile shows
discontinuities which were not predicted; 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 ?
/&
Other related posts:
