Am 28.01.2011 22:51, schrieb Gerhard Fuernkranz: > Using a profile created from Elena's ofps2.ti3 data I did take a closer look > at a short line crossing one of the discontinuities (a straight line in the > source sRGB space, which is then converted with perceptual intent to CMYK). I was furthermore curious about the "kh" and "kz" locus in this small region (see attached plot). While the "kz" locus is indeed smooth, the "kx" locus is discontinuous at two points. So any other black locus derived from the "kx" locus is supposed to be discontinuous too. But I'm curious where this "jump" is coming from. Is this just the result of inverting an almost flat, but still noisy/wavy region of the A2B table [i.e. a region which would be monotonically increasing or decreasing, if there weren't some ripples which break the monotonicity]? I had also rather expected that (for the same given L*a*b* color) ramping the relative k locus from 0 to 1 (=> -kl) would also result in a smooth ramp of the K channel from the lowest possible K level (-> kz) to the max. possible K level (-> kx). But this seems not to be the case either, but there is also a steep jump from K=0.38 to K=0.83: $ xicclu -fif -l300 -L100 -kz ofps2.icc 35.7038 10.9054 7.4473 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.599189 1.000000 0.797272 0.229041 [CMYK] Lim 2.625501 (clip) $ xicclu -fif -l300 -L100 -kx ofps2.icc 35.7038 10.9054 7.4473 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.248006 1.000000 0.601538 0.836248 [CMYK] Lim 2.685793 $ xicclu -fif -l300 -L100 -kl ofps2.icc 35.7038 10.9054 7.4473 0 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.599189 1.000000 0.797272 0.229041 [CMYK] Lim 2.625501 (clip) 35.7038 10.9054 7.4473 0.05 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.588819 0.994304 0.791400 0.259401 [CMYK] Lim 2.633924 35.7038 10.9054 7.4473 0.1 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.584088 0.992378 0.789968 0.289761 [CMYK] Lim 2.656195 35.7038 10.9054 7.4473 0.15 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.577425 0.987182 0.780403 0.320122 [CMYK] Lim 2.665132 35.7038 10.9054 7.4473 0.2 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.565605 0.988832 0.774003 0.350482 [CMYK] Lim 2.678923 35.7038 10.9054 7.4473 0.25 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.554522 0.990714 0.768928 0.380842 [CMYK] Lim 2.695007 35.7038 10.9054 7.4473 0.3 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.251803 1.000000 0.603638 0.834065 [CMYK] Lim 2.689506 35.7038 10.9054 7.4473 0.35 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.251803 1.000000 0.603638 0.834065 [CMYK] Lim 2.689506 35.7038 10.9054 7.4473 0.4 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.251803 1.000000 0.603638 0.834065 [CMYK] Lim 2.689506 35.7038 10.9054 7.4473 0.45 35.703800 10.905400 7.447300 [Lab] -> Lut -> 0.251803 1.000000 0.603638 0.834065 [CMYK] Lim 2.689506 Should I conclude that the given L*a*b* color can be represented with CMYK combinations with K levels of 0.23...0.38 and 0.83, but not with any K levels in between? If so, then this would be quite strange as well. Furthermore I noticed that "xicclu -fif -kz" and "xicclu -fif -kx" reported different 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 preserving colorimetry takes precedence over the black generation rule, so if an ambiguous CMYK representation of a given color is not possible, then kx and kz should simply return the same CMYK numbers, shouldn't they?). Regards, Gerhard
Attachment:
kx_vs_kh_locus.png
Description: PNG image
