[argyllcms] Camera profile and non-neutral white patch
- From: Stephen T <stwebvanuatu@xxxxxxxxxxxx>
- To: ArgyllCMS <argyllcms@xxxxxxxxxxxxx>
- Date: Sun, 28 Aug 2011 04:04:50 -0700 (PDT)
Hello,
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?
Stephen.
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