[argyllcms] Re: Gamma wrong for calibration curves?
- From: william.wood@xxxxxxx
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 1 Dec 2006 23:16:01 -0500
> > and the profile applied the gamma is 2.24 again! This seems to
indicate
> > that dispcal is creating a calibration curve with a low gamma and then
> > "profile" is compensating and setting the gamma higher again.
>
> A profile doesn't (in itself) change anything - it's just a
> characterization of the device. Any change you see in applying
> a profile is the combination of two profiles, the source and
> destination profile. So a change in gamma will be the effect of
> the comparative gamma between the two profiles, and the source
> profile gamma may not be a simple power curve (ie. sRGB), and
> will have different black end aim points, the effect of which will
> depend on the linking intent etc.
The source profile in my tests is sRGB - gamma 2.2 (or close) isn't it? So
sRGB converting to Monitor profile (perceptual) should not involve a gamma
change, yet images noticeably darken when I apply the profile conversion.
> The target curve takes into consideration the offset needed in
> the black point, to achieve the black point target chromaticity.
> For a CRT this usually isn't much, and the approximate gamma
> should match the aim curve reasonable well. For an LCD with -k1.0,
> this may be quite an offset, causing a noticeable discrepancy, and
> a loss of contrast. (If you calibrate with -v, the Gamma curve
> offset value is reported, so you could check how much it changes
> between -k1 and -k0)
> You could try calibrating with -k0.0, and see if the approximate
> gamma is now in better alignment with the target one, and your contrast
> ratio is better. I've made the -k0.0 behaviour the default for LCD's
> in future versions, since I suspect this is what will work best as
> a default.
I'm getting the same results with -k0 and -k1; resulting callibration curve
gamma is 2.05, not 2.2. Below I've listed the output from dispcal -E for
.cal files with k=0 and k=1. The gamma offset is the same (~0.08) in both
cases. I don't understand why the gamma offset is low yet the gamma seems
so far off.
Here's the output from dispcal -E on a .cal file with a k of 1:
I:\Util\Argyll\profiles>..\bin\dispwin -d2 t42pk1.cal
I:\Util\Argyll\profiles>..\bin\dispcal -v -d2 -yl -E
Setting up the instrument
Place instrument on test window.
Hit Esc to give up, any other key to continue:
Display type is LCD
Target white = native white
Target brightness = native brightness
Target gamma = 2.200000
Commencing device calibration
patch 6 of 6
Black = XYZ 0.46 0.48 0.48
Red = XYZ 59.73 35.03 6.86
Green = XYZ 48.17 81.70 17.92
Blue = XYZ 22.29 21.10 100.81
White = XYZ 128.15 135.69 123.06
Initial native brightness target = 135.690000 cd/m^2
Target white value is XYZ 128.150000 135.690000 123.060000
Target black point = 0.489440 0.518237 0.470000
Gamma curve offset = 0.079598
patch 100 of 100
Verification results:
Brightness error = 0.030000 cd/m^2
White point error = 0.021150 deltaE
Maximum neutral error (@ 0.095629) = 1.150478 deltaE
Average neutral error = 0.447931 deltaE
The instrument can be removed from the screen.
Here's the output from dispcal -E on a .cal file with a k of 0:
I:\Util\Argyll\profiles>..\bin\dispwin -d2 t42pk0.cal
I:\Util\Argyll\profiles>..\bin\dispcal -v -d2 -yl -E
Setting up the instrument
Place instrument on test window.
Hit Esc to give up, any other key to continue:
Display type is LCD
Target white = native white
Target brightness = native brightness
Target gamma = 2.200000
Commencing device calibration
patch 6 of 6
Black = XYZ 0.32 0.33 0.46
Red = XYZ 59.62 34.87 6.85
Green = XYZ 48.05 81.59 17.93
Blue = XYZ 22.10 20.88 100.79
White = XYZ 128.06 135.58 123.05
Initial native brightness target = 135.580000 cd/m^2
Target white value is XYZ 128.060000 135.580000 123.050000
Target black point = 0.468322 0.495823 0.450000
Gamma curve offset = 0.078043
patch 100 of 100
Verification results:
Brightness error = 0.110000 cd/m^2
White point error = 0.059313 deltaE
Maximum neutral error (@ 0.112249) = 6.966823 deltaE
Average neutral error = 3.416810 deltaE
The instrument can be removed from the screen.
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