[argyllcms] Re: Black turning down problem - help!
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 07 Jan 2011 09:57:08 +1100
Elena [service address] wrote:
After careful investigation, I discovered that on this paper and with these
inks (MISPro K4, only Photo K and Light K used to simulate littler dots)
the CMY combination (100 100 100 0) is actually DARKER than CMYK (100 100 100
100)!
Adding black to the mix lightens up things! Oh well, one never stops learning...
Yes, this sort of thing does happen. The CMYK 4 dimensional hyper-cube gets
folded into 3 dimensions. If the device is well behaved, the CMY+100%K cube
completely contains the CMY cube. If it does not (and in your device at least
the CMY corner pokes through the CMY+100%K cube), then the topology
is quite difficult, and Argyll's current code doesn't attempt to
solve this problem automatically. Even with the best possible
setting of the black curve, there will be problems where the sides of
the CMY cube penetrate the CMY+100%K curve, and the maximal gamut
surface suddenly changes K value.
Given that you've discovered this, it might be best now to take
Rogers advice and move on to better behaved media (assuming that
the current paper is not your real aim).
I never understood well the actual aim of the -K parameter
("Same as -k, but target is K locus rather than K value itself")
but I suspect that the relative behaviors of -k and -K are swapped...
it perhaps should be
-k = obey blindly the user choice
-K = follow the user choice as a guide line, but give priority to
what actually are the measured lightest and darkest device values
No, that's not how it works. -k sets a target K ink value. -K sets
a value between the minimum and maximum possible K ink values (the "locus")
at each and every point. That was my first approach, but in practice
I found it was very difficult means of setting a usable black curve,
hence it relegation to a test option.
Graeme Gill.
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