[argyllcms] Re: perceptual black too light

Gerhard,
        using a more robust search for the black point (20 trials with
random starting points) on your data set results in the following:

XYZArray:
  No. elements = 1
    0:  0.018478, 0.018829, 0.015137    [Lab 14.860508, 0.784551, 0.455625]

xicc/xicclu -ip -fb -pl CHP410-1200.icm
0.000000 0.000000 0.000000 [Lab] -> Lut -> 0.999641 0.101813 0.132937 1.000000 
[CMYK]

Note that Lab 0 0 0 returns the closest point from the perceptual table,
not the black point. Profile chooses the black point to be the lowest
L point within the gamut with the same direction as K only. (This
seemed a reasonable thing to me, since K only is often used as a reference
black hue within CMYK systems.) There may be points in the gamut with a
lower L, but they may have a not so neutral color.

xicclu -ia -ff -pl CHP410-1200.icm
0.999641 0.101813 0.132937 1.000000 [CMYK] -> Lut -> 13.500272 0.608876 
-3.316293 [Lab]

/src/argyll/test/gfuer2:../../xicc/xicclu -ia -fif -pl -l250 -kh CHP410-1200.icm
15.000000 0.800000 0.400000 [Lab] -> Lut -> 0.222745 0.177431 0.194287 0.998518 [CMYK] Lim 1.592982
14.000000 0.800000 0.400000 [Lab] -> Lut -> 0.292572 0.228957 0.231581 1.000000 [CMYK] Lim 1.753109 (clip)
13.000000 0.800000 0.400000 [Lab] -> Lut -> 0.292572 0.229783 0.141170 1.000000 [CMYK] Lim 1.663525 (clip)


Which is consistent with what profile found. Does this seem a better result ?

[Thanks for drawing this problem to my attention.]

Graeme Gill.



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