[argyllcms] Re: Implementation of setting a black point



Graeme Gill schrieb:

I was just wondering if the intent was to trick the calibration
to have a "dead" zone from zero, and I'm not sure it's really setup
to work that way.
It should work that way. "Black" should begin a little bit earlier so that some headroom is left. 
In what way do you mean it's not a truly neutral gradient ?
Does the gradient not have a consistent white point, or is it
consistent but not the right white point ?
I think the gradient doesn't have a consistent white point. There are "stripes" in it. I don't think banding would be the right term, as the gradient is very smooth, but it shows color casts at certain r=g=b values. 
Have you confirmed this with instrument readings ? If so,
which software did you use to do the instrument readings ?

I can postulate 3 possible explanations:

  dispcal is not making the gradient conform to the target curve.

  dispcal is not setting an appropriate target curve.
I just calibrated and profiled again with argyll and took some readings with spotread combined with the DTP-94 and I got the following results: (I calibrated to -b 120 -gs -yl -qh -k0 -w 0.3127,0.3290 and used photoshop to create patches) 

L = #, a=0,b=0 - Whitepoint in [K] (XYZ)
----------------------------------
0    -      7270 (XYZ: 0.13 0.12 0.17)
10    -    5960 (XYZ: 1.76 1.86 1.84)
20    -    6230 (XYZ: 3.69 3.89 4.05)
30    -    6280 (XYZ: 7.24 7.5 8.1)
40    -    6480 (XYZ: 12.74 13.45 14.53)
50    -    6420 (XYZ: 20.77 21.88 23.48)
60    -    6400 (XYZ: 31.74 33.42 35.79)
70    -    6400 (XYZ: 45.38 47.55 51.36)
80    -    6440 (XYZ: 63.45 66.74 72.03)
90    -    6420 (XYZ: 85.07 89.61 96.2)
100 - 6460 (XYZ: 111.32 117.23 126.76) 
dispcal -yl -r reports the whitepoint to be:
White chromaticity coordinates 0.3141, 0.3300
White Correlated Color Temperature = 6422K, DE to locus =  4.4
White Correlated Daylight Temperature = 6422K, DE to locus =  0.3
White Visual Color Temperature = 6271K, DE to locus =  4.2
White Visual Daylight Temperature = 6432K, DE to locus =  0.3


  There is a difference between the manufacturers colorimeter calibration
that Argyll uses, and the instrument calibration used by iColor, leading to 
  a different white point.

Graeme Gill.
I get similar results to the ones with iColor when I use basICColor Display 4. I hope this wasn't too much of unrelevant information, but thanks for 
taking an interest anyway!

Steffen

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