[argyllcms] Re: Black point, gamma level

On Mon, Oct 22, 2007 at 11:50:39PM +1000, Graeme Gill wrote:

> Profiles are only useful if applications make use of them. You
> can certainly alter the white point using calibration, but you generally
> sacrifice brightness and (level) resolution to do so if all you've
> got to work with are the Video LUTs.

Brightness loss: Only if I stray from the native white point, right?

Resolution: Yes, I see this, especially with 8 bit LUTs. :)  Do video
cards support 16 bit value LUTs at all these days?  (eg: is this
possibly an X limitation within Linux?)

The main issue right now is that Linux has poor support for profiles. 
Web browsing, GTK, etc., mostly all do not make use of them.  Only a few
applications watch the attribute which "xicc" sets, and typically become
very slow when they do.

> Hmm. I'll take a look at it. It seems to be caused by the target white point
> shifting, and the black point being computed relative to the white point.

I should note that I just tried beta7 (as opposed to beta6), and the
output is now such that 0,0,0 black output from video card LUT is still
0,0,0.  Did something change wrt this in beta7?  The output is now:

0.0000 0.018335 0.011353 0.010989
3.9216e-03 0.023509 0.016135 0.016037
7.8431e-03 0.028616 0.020859 0.021008

It is my understanding that this translates to 8-bit output integers of:

0: 0 0 0
1: 1 0 0
2: 1 1 1

I attached the whole .cal file, as generated with:

        dispcal -K -m -g2.2 -k0.0 -yl -qu -v benq_native_b7

Note that with "-qu", I cannot actually see a visible patch in the first
patch until patch number 9.  I don't think this is because it is too
dark, but because rounding or otherwise is resulting in the output still
being RGB 0,0,0.

> You can't determine the RGB needed to produce a target neutral value
> from the separate channel measurements, unless you assume the device
> is perfectly additive. Making such an assumption would severely limit
> the accuracy of such an approach on real world displays.

Hmm.  Yes, I was assuming that LCDs (especially compared with CRTs) would
be perfectly additive.

Understood.  Thanks!

Simon-
CAL

DESCRIPTOR "Argyll Device Calibration State"
ORIGINATOR "Argyll dispcal"
CREATED "Mon Oct 22 02:24:25 2007"
KEYWORD "DEVICE_CLASS"
DEVICE_CLASS "DISPLAY"
KEYWORD "DEVICE_TYPE"
DEVICE_TYPE "LCD"
KEYWORD "NATIVE_TARGET_WHITE"
NATIVE_TARGET_WHITE ""
KEYWORD "TARGET_WHITE_XYZ"
TARGET_WHITE_XYZ "177.730000 187.860000 207.860000"
KEYWORD "TARGET_GAMMA"
TARGET_GAMMA "2.200000"
KEYWORD "BLACK_POINT_CORRECTION"
BLACK_POINT_CORRECTION "0.000000"
KEYWORD "QUALITY"
QUALITY "ultra high"

KEYWORD "RGB_I"
NUMBER_OF_FIELDS 4
BEGIN_DATA_FORMAT
RGB_I RGB_R RGB_G RGB_B 
END_DATA_FORMAT

NUMBER_OF_SETS 256
BEGIN_DATA
0.0000 0.018335 0.011353 0.010989 
3.9216e-03 0.023509 0.016135 0.016037 
7.8431e-03 0.028616 0.020859 0.021008 
0.011765 0.033657 0.025529 0.025902 
0.015686 0.038634 0.030145 0.030722 
0.019608 0.043547 0.034707 0.035470 
0.023529 0.048399 0.039217 0.040146 
0.027451 0.053189 0.043676 0.044753 
0.031373 0.057911 0.048082 0.049288 
0.035294 0.062559 0.052435 0.053743 
0.039216 0.067127 0.056731 0.058110 
0.043137 0.071609 0.060963 0.062382 
0.047059 0.075998 0.065126 0.066558 
0.050980 0.080295 0.069217 0.070638 
0.054902 0.084500 0.073234 0.074627 
0.058824 0.088620 0.077181 0.078530 
0.062745 0.092662 0.081062 0.082356 
0.066667 0.096633 0.084882 0.086109 
0.070588 0.10054 0.088647 0.089800 
0.074510 0.10439 0.092362 0.093435 
0.078431 0.10819 0.096034 0.097022 
0.082353 0.11195 0.099665 0.10057 
0.086275 0.11566 0.10326 0.10408 
0.090196 0.11934 0.10683 0.10756 
0.094118 0.12300 0.11038 0.11102 
0.098039 0.12663 0.11391 0.11447 
0.10196 0.13024 0.11742 0.11790 
0.10588 0.13384 0.12092 0.12132 
0.10980 0.13743 0.12442 0.12473 
0.11373 0.14101 0.12792 0.12815 
0.11765 0.14458 0.13142 0.13156 
0.12157 0.14815 0.13493 0.13496 
0.12549 0.15173 0.13843 0.13836 
0.12941 0.15530 0.14192 0.14176 
0.13333 0.15887 0.14541 0.14515 
0.13725 0.16246 0.14889 0.14854 
0.14118 0.16605 0.15238 0.15193 
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0.14902 0.17326 0.15934 0.15872 
0.15294 0.17687 0.16283 0.16213 
0.15686 0.18048 0.16631 0.16554 
0.16078 0.18409 0.16979 0.16895 
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0.17255 0.19497 0.18022 0.17924 
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0.26667 0.28243 0.26416 0.26400 
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0.27451 0.28967 0.27127 0.27115 
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0.52157 0.52434 0.50346 0.50156 
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0.98431 0.98536 0.98563 0.98460 
0.98824 0.98902 0.98925 0.98847 
0.99216 0.99268 0.99286 0.99233 
0.99608 0.99634 0.99644 0.99617 
1.0000 1.0000 1.0000 1.0000 
END_DATA
CAL

DESCRIPTOR "Argyll Calibration options and model"
ORIGINATOR "Argyll dispcal"
CREATED "Mon Oct 22 02:24:25 200"

KEYWORD "R_P"
KEYWORD "G_P"
KEYWORD "B_P"
NUMBER_OF_FIELDS 3
BEGIN_DATA_FORMAT
R_P G_P B_P 
END_DATA_FORMAT

NUMBER_OF_SETS 32
BEGIN_DATA
1.2935e-03 1.0595e-03 2.6921e-03 
0.98508 1.0011 0.99090 
1.9717 1.7176 1.6774 
0.23808 0.24069 0.24319 
0.20029 0.21122 0.22817 
0.10062 0.10997 0.12530 
0.13803 0.14578 0.13892 
0.073937 0.066531 0.078886 
0.083974 0.074218 0.079305 
0.095012 0.080651 0.039439 
0.049038 0.023802 0.027801 
0.063340 0.048454 0.080184 
0.073513 0.078130 0.059165 
0.034052 0.032320 0.035064 
0.016057 -0.015569 0.052980 
-0.015938 0.055733 0.042782 
0.023479 0.064277 0.035548 
0.062193 0.078489 0.035571 
0.083516 0.034310 -2.8974e-03 
0.10962 4.7840e-03 -8.6567e-03 
0.022616 0.028641 -0.011806 
7.8459e-03 -0.031027 -5.2081e-03 
-0.021340 4.2090e-03 0.057432 
-0.073483 -6.9834e-03 0.040526 
-0.049960 -0.014240 0.021157 
-0.011578 0.039493 0.040706 
0.012976 0.058999 0.032757 
0.015815 0.088667 0.023319 
0.067505 0.094733 0.039018 
0.12209 0.11402 0.091758 
0.20028 0.094886 0.16139 
0.27724 0.066283 0.24233 
END_DATA

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