[argyllcms] Re: Wrong gamma / color rendition worse than before calibration

  • From: Florian Höch <lists+argyllcms@xxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 07 Sep 2011 16:09:57 +0200

Hi,

if the profiles are any indication, then the color gamut of that screen is (a lot) smaller than sRGB (not uncommon for TN panels, especially in Laptops/Notebooks), leading to clipping. Same for black. Many Laptop/Notebook displays have a pretty poor black level (some monitor profilers cheat by scaling the black of the profile to absolute zero. Argyll does not do this). I wouldn't pay too much attention to those gamma test images, as TN panels are very viewing angle sensitive, and combined with the usually pretty 'bendy' calibration curves needed to make such a display behave, there isn't much useful information to discern from those images. In summary, I'm afraid your problems are not fixable via different calibration/profiling settings, as they seem like hardware limitations of the screen.

Am 07.09.2011 13:18, schrieb Thomas Bartosik:
Hi list!

I am somehow at a loss as to how to solve my gamma problems..

1) What I want to do:
Have a calibrated display for a) web work and primarily b) print work sent to a 
printing studio. They accept ISO coated CMYK PDFs.

2) What I have done:
I got a spyder2 for cheap, i know it's not the best, but I'd like you to 
comment on the issue and tell me whether the colorimeter's quality might be the 
problem.

I used the colorimeter to calibrate/profile really bad and cheap displays (e.g. 
one notebook display that did not differentiate between 235,235,253 white till 
255,255,255 white, everything was white)
The calibrated/profiled results of these displays are very pleasing and now 
show nuances that were completely unseen before.

Now my problem is my own display in a Thinkpad T61p. This is (an expensive) LG 
1920x1200 display that has really good reviews in most articles I have read so 
far. In an uncalibrated/unprofiled state, colors look really good but a bit 
bluish and the brightness distibution is also very good (subjectively, to my 
eyes). i.e. I can differentiate 252,252,252 white from full white and also 
3,3,3 black from full black. This has not been the case with nearly all other 
uncalibrated/unprofiled displays I have seen so far.

I did several dispcal runs, and they more or less ended up in the same color 
rendition.
(I tried with -t 6504 and -gs and without both)

Now the problem I face is this:
All dark nuances in pictures are too dark. And I think I can really say that in 
an absolute manner, it's not only a subjective thing.
The same goes for bright tones.

If I look at
http://www.visibone.com/color/chart_847.gif
in a color managed app like gqview or gimp with the .icc profile loaded and 
applied, I cannot differentiate between FF66FF, FF33FF and FF00FF while I can 
do so unmanaged or with just dispwin loading the profile and the app not 
loading it. As far as I have read till now this huge difference between using a 
color managed desktop (i.e. loading the profile with dispwin or xcalib) and 
then applying the same profile again with a color managed app should not be 
that dramatic.
I suspect something's wrong with the measurements taken or the profile created.
Can anyone give advice as to in which direction I should look to solve this 
problem?

As an interesting sidenote, I found a profile for a similar display ( A T61 LG 
panel, resolution could be different, I don't know more about this profile), 
and apart from its D50 whitepoint the effect of not seeing different magenta 
nuances is quite the same (at least FF66FF and FF33FF are indistiguishable). 
This profile was taken with a gretag macbeth device, I guess on windows.
I found an additional profile that's also for an LG in a Thinkpad, with the 
same effect on magenta.
This makes me believe my profiling data and measurement values seem to be 
probable at least. But why don't I see those magenta nuances?(the same is also 
valid for some green and blue areas...)

All of the profiles and my cal and icc file are at
http://rockbrew.com/argyll
(The T61p-full-brightness... is mine)

Do you see differences in these colors?
If I do the monitor test on
http://www.lagom.nl/lcd-test/
after having applied a profile, I can barely make out the lowest stripes in the 
contrast test. The gamma test seems more accurate than without being 
calibrated, but the area of blending in is a bit broader. The bars all blend it 
at around 2.2 which I never managed to do with just the nvidia controls, but 
the blue gamma is somewhere around 2.6, which puzzles me.
NB: This is when I test it in opera, i.e. the .icc profile is loaded by dispwin 
but not the browser!
If I download the contrast and gamma .png and look at them at zoom 1:1 in 
gqview with the profile loaded, I get these gamma readings:
         48%     |     25%      |     10%
  R   G   B   W  |   G     W    |   G    W
2.3 1.6 2.8 2.3 |  2.2   2.3   |  2.2   2.3

The 1.6 at green is pretty extreme, and the 2.8 at blue as well (tho I cannot 
really say it is 2.8 for sure as 2.5 till 3.0 is nearly the same level to my 
eye)

Without any calibration, the gamma and contrast test are perfectly OK, but 
could be a bit better (this is what I wanted the calibration/profiling to do 
for me!)

If I tune the values in nvidia-settings for the lagom lcd test and dump the 
values with dispwin -s I get nvidia.cal (also at the webspace above). This is 
the (subjectively) best setting I have till now...

Maybe I have to give different -g or even -G values? I do not understand -G at 
all and I think -g should best be set to the display's native gamma as it will 
result in the least banding artifacts and does not really matter as the color 
managed app should compensate for it anyway...



I hope there's some advice you can give as I am somewhat clueless..

Thanks in advance!

th

Here's a rundown of the calibration I've done.

blackknight ~ # dispcal -yl -v -R
XRandR 1.2 is faulty - falling back to older extensions
Setting up the instrument
Instrument Type:   ColorVision Spyder2
Serial Number:     00633553
Hardware version:  0x0307
Place instrument on test window.
Hit Esc or Q to give up, any other key to continue:
patch 3 of 3
Measuring VideoLUT table entry precision.
patch 6 of 6
patch 6 of 6
patch 9 of 9
patch 9 of 9
Uncalibrated response:
Black level = 0.40 cd/m^2
White level = 119.19 cd/m^2
Aprox. gamma = 2.29
Contrast ratio = 300:1
White chromaticity coordinates 0.3269, 0.3578
White    Correlated Color Temperature = 5726K, DE 2K to locus = 12.5
White Correlated Daylight Temperature = 5726K, DE 2K to locus =  9.6
White        Visual Color Temperature = 5366K, DE 2K to locus = 12.2
White     Visual Daylight Temperature = 5475K, DE 2K to locus =  9.3
Effective LUT entry depth seems to be 10 bits
The instrument can be removed from the screen.


blackknight ~ # dispcal -m -yl -t 6504 -gs -v -O "Thinkpad T61p 1920x1200 LG full 
brightness Argyll-1.3.1 sRGB D65 Spyder2" -o 
/usr/share/color/icc/T61p-full-brightness-argyll-1.3.1-sRGB-D65XRandR 1.2 is faulty - 
falling back to older extensions
Setting up the instrument
Instrument Type:   ColorVision Spyder2
Serial Number:     00633553
Hardware version:  0x0307
Place instrument on test window.
Hit Esc or Q to give up, any other key to continue:
Display type is LCD
Target white = 6504.000000 degrees kelvin Daylight spectrum
Target white brightness = native brightness
Target black brightness = native brightness
Target gamma = sRGB curve
Commencing device calibration
patch 6 of 6
Black = XYZ   0.35   0.35   0.40
Red   = XYZ  44.06  26.35   3.27
Green = XYZ  33.71  59.85  10.98
Blue  = XYZ  17.84  17.99  78.99
White = XYZ  95.06 103.78  92.64
patch 128 of 128
Initial native brightness target = 103.784294 cd/m^2
Had to scale brightness from 103.784294 to 81.568913 to fit within gamut,
corresponding to RGB 0.899985 0.837122 1.000000
Target white value is XYZ 77.528580 81.568913 88.828359
Adjusted target black XYZ 0.34 0.35 0.40, Lab 3.88 0.45 -0.30
Target black after min adjust: XYZ 0.342 0.351 0.399, Lab 3.883 0.447 -0.304
Gamma curve input offset = 0.000000, output offset = 0.004299, power = 0.000000
Total Iteration 3, Final Samples = 64 Final Repeat threshold = 0.600000
Creating initial calibration curves...
Doing iteration 1 with 16 sample points and repeat threshold of 1.200000 DE
patch 16 of 16
Brightness error = -0.462568 cd/m^2 (is 81.106344, should be 81.568913)
White point error = 0.625266 deltaE
Maximum neutral error (@ 0.895486) = 1.116106 deltaE
Average neutral error = 0.685531 deltaE
Number of measurements taken = 26
Computing update to calibration curves...
Doing iteration 2 with 32 sample points and repeat threshold of 0.848528 DE
patch 32 of 32
Brightness error = -0.322559 cd/m^2 (is 81.246354, should be 81.568913)
White point error = 0.646947 deltaE
Maximum neutral error (@ 0.248961) = 0.841565 deltaE
Average neutral error = 0.534482 deltaE
Number of measurements taken = 49
Computing update to calibration curves...
Doing iteration 3 with 64 sample points and repeat threshold of 0.600000 DE
patch 64 of 64
Brightness error = -0.351730 cd/m^2 (is 81.217183, should be 81.568913)
White point error = 0.201203 deltaE
Maximum neutral error (@ 0.100647) = 0.920412 deltaE
Average neutral error = 0.409086 deltaE
Failed to meet target 0.600000 delta E, got worst case 0.559542
Number of measurements taken = 159
The instrument can be removed from the screen.
Written calibration file 
'/usr/share/color/icc/T61p-full-brightness-argyll-1.3.1-sRGB-D65.cal'
Luminance XYZ = 0.000000 79.468347 0.000000
White point XYZ = 0.951621 1.000000 1.047750
Black point XYZ = 0.004093 0.004148 0.004938
Created fast shaper/matrix profile 
'/usr/share/color/icc/T61p-full-brightness-argyll-1.3.1-sRGB-D65.icc'

blackknight ~ # dispcal -yl -v -r
XRandR 1.2 is faulty - falling back to older extensions
Setting up the instrument
Instrument Type:   ColorVision Spyder2
Serial Number:     00633553
Hardware version:  0x0307
Place instrument on test window.
Hit Esc or Q to give up, any other key to continue:
patch 3 of 3
Current calibration response:
Black level = 0.41 cd/m^2
White level = 80.85 cd/m^2
Aprox. gamma = 2.20
Contrast ratio = 196:1
White chromaticity coordinates 0.3122, 0.3277
White    Correlated Color Temperature = 6541K, DE 2K to locus =  4.0
White Correlated Daylight Temperature = 6542K, DE 2K to locus =  0.7
White        Visual Color Temperature = 6395K, DE 2K to locus =  3.9
White     Visual Daylight Temperature = 6566K, DE 2K to locus =  0.7
The instrument can be removed from the screen.

--
Florian Höch


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