[argyllcms] Re: Implementation of setting a black point
- From: Steffen <ssachse@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 29 May 2008 21:09:22 +0200
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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