[argyllcms] Re: Camera profile and non-neutral white patch
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 30 Aug 2011 20:59:46 +0200
Am 28.08.2011 13:04, schrieb Stephen T:
I'm still chipping away at the camera profiling challenge.
The white point (wtpt tag) of a camera profile has important effects on the results. From
Argyll docs: "the white point of the test chart, will be mapped to perfect white
when used with any of the non-absolute colorimetric intents."
Now the white point of my IT8.7/2 target (patch GS00) is slightly bluish: Lab =
92.04 -0.69 -2.10 (RGB (D50, gamma 2.2) = 230 232 235).
Firstly, I have used the colprof parameter -U to scale the profile white point
(wtpt) to L=100. This avoids exposure increasing when mapping the white point
of the test chart to perfect white (non-absolute intent).
But then there remains a slight reddish colour shift. This slight error can
also be seen when processing the test chart - neutral patches are not perfectly
neutral:
reference data
R080505.txt GS00 = 230 232 235
R080505.txt GS06 = 161 161 161
ufraw outputs (WideGamutRGB, custom WB on GS06):
ufraw GS00 = 226 225 226 perceptual 224 225 231 absolute (should be
bluish, error with perceptual)
ufraw GS06 = 160 158 156 perceptual 159 158 160 absolute (should be
neutral, error with perceptual)
Now, can I force the white point to be perfectly neutral? Is their a utility to
edit the wtpt tag in an ICC profile? Is this even a good idea?
AFAIK, the -u option is supposed to assume a perfect diffuse reflector as media white
point (instead of assuming the brightest patch on the calibration chart as
"white").
However, if you want to use the profile for "general purpose shooting" (and not
only for a very particular studio lighting condition), then the shots need to be
white-balanced anyway after applying the profile - and in this case the media white point
in the profile does not really matter.
Regards,
Gerhard
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