[argyllcms] Re: xicclu -g predictability issue

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 31 Jan 2011 14:57:18 +1100

Gerhard Fuernkranz wrote:

I was furthermore curious about the "kh" and "kz" locus in this small region 
attached plot).

While the "kz" locus is indeed smooth, the "kx" locus is discontinuous at two 
So any other black locus derived from the "kx" locus is supposed to be 
too. But I'm curious where this "jump" is coming from. Is this just the result 
inverting an almost flat, but still noisy/wavy region of the A2B table [i.e. a 
which would be monotonically increasing or decreasing, if there weren't some 
which break the monotonicity]?

The way I would explain it (assuming it's not a bug), is that there are several
possible solutions for at that point, and so the maximum K solution "jumps"
as a folded edge comes into range. A way of investigating in more detail whether
this is the case, is to go into xicc/xlut.c about line 940 in the 
function, and enable the "#ifdef NEVER" code. This will then report the possible
solution K locus segments for each lookup.

Should I conclude that the given L*a*b* color can be represented with CMYK 
with K levels of 0.23...0.38 and 0.83, but not with any K levels in between? If 
then this would be quite strange as well.

I've certainly seem bifurcated black regions in some profiles.

Furthermore I noticed that "xicclu -fif -kz" and "xicclu -fif -kx" reported 
subsets of my region as "clipped". Actually I had expected that both would 
report the
_same_ sets of colors as "clipped".  Either a given color can be represented 
with at
least one CMYK combination, or it cannot (and my understanding is that 
colorimetry takes precedence over the black generation rule, so if an ambiguous 
representation of a given color is not possible, then kx and kz should simply 
the same CMYK numbers, shouldn't they?).

Hmm. yes, they should be the same.


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