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

  • From: Elena [service address] <1007140@xxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 07 Jan 2011 17:26:26 +0100

Hello Nikolay

On  7-Jan-2011, Nikolay Pokhilchenko wrote:
> Hello!
>
> I've often struck with similar problems while CMYK profiling. There is one 
> way with Argyll - to play with ink limits and -Kp or -kp parameters.
> I've played with Elena's ti3 data and have built rather smooth profile with 
> heavy black generation. My command lines was:

Nikolay, again thanks for taking care of my problem! The profile you sent to me
privately is performing very finely, or at least, much better than I could 
obtain
to date. Before receiving your message, I just finished reading a 3000+ patches
target, made the profile again, but it shows very little improvement versus the
1300 patches one: the problem in the darks is always there. You seem to instead
have the key for success! So let's try to understand HOW !?

> spec2cie -f -i D50_1.0.sp -o 1931_2 -n 2xOFPS.ti3 2xOFPScie.ti3

I see you dropped all spectral data. Is there a particular reason why you did 
that ?

> colprof -v -al

-al should be the default, right ?

> -qh

well, I tried once -qh (at least 1 hour on my machine) but it didn't seem a so 
big key for
my black problems

> -Kp 1 0 1 0.6 0.55

Ok, you have to explain me well how you came with these parameters.
Why -K and not -k ? Also, why start level = 1 ????

> -L90 -l285

...and how you came to chose these limits. Are you using some special trick or 
tool?

> -r 0.85

I assume this is some "good habit" of yours to give an arbitrarily higher 
tolerancy
of 0.85 and not justified by some specific reason...

> -s sRGB.icm -dpp 2xOFPScie
> See attached graphs, relative.png and perceptual.png.

Well. However, you will admit there's a bug in xicclu. The K plot for 
perceptual shows
no black ramp down, which is not the actual case: a picture converted from srgb 
to
CMYK with Photoshop, Perceptual intent, by using your profile, shows clearly 
that
the K is indeed ramping down after about 50%. Just measure the CMYK numbers.
After all, xicclu was showing the same with my profiles also: an always 
increasing
black in perceptual, everything very very smooth (but the visual result not 
being so).
So I please ask Graeme again, if he's reading, to investigate this bug...

Bye!
/&


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