From: Michael A. Litscher <litscherm@co...> - 2008-01-08 04:03:38
(I am resending this, as it didn't appear to get into the mailing list
the first time.)
Thank you for all of your help thus far. Seeing what you were
attempting to do with scaling the XYZ numbers in the spreadsheet was
very enlightening. From what I learned, I used the XYZ numbers from the
RGB(0, 0, 0) to RGB(255, 255, 255) measurements to offset and scale all
of the XYZ numbers from XYZ(0, 0, 0) to XYZ(monitor's white point),
which made quite a difference, because once that was done and converted
to Lab, and then transformed to RGB using lcms and the monitor's
profile, the difference between the displayed RGB and the calculated RGB
(from actual measurements) was impressively small.
I would prefer to use the XYZ data for the RGB(0, 0, 0) and RGB(255,
255, 255) measurements taken during the profile creation (i.e. that seen
in the IT8-formatted area between BEGIN_DATA / END_DATA sections inside
the .icc file) to do the scaling so I do not need to re-read the black
and white swatches each time I want to measure a color on the monitor in
order to properly scale it. By using the RGB(0, 0, 0) and RGB(255, 255,
255) measurements taken during the profile creation, I will also be able
to see if the brightness of the monitor has drifted or was changed since
the profile was created. Can I be assured that this data is embedded in
monitor profiles? If so, which lcms functions can be used to retrieve
this data? The lcms IT8 functions do not appear written to parse IT8
data out from the middle of an ICC profile.
I have also been attempting to use lcms to get the 'lumi' data from the
profile, but I have been unable to determine which, if any, of lcms's
exported functions will return the data for the icSigLuminanceTag tag.
But even then, I understand that all you are guaranteed to get is the
raw and unscaled Y value, with no guarantee of getting the X and the Z
values, which may in fact be set to zero. I attempted to scale the X,
Y, and Z values using only what Kai-Uwe Behrmann told me the profile's
luminance value was (116), which resulted in adequately scaled Y values,
but not so adequately scaled X and Z values, resulting in some
So, how can I retrieve the absolute measurement data for the black and
white swatches from the profile? If I can not do that, how do I get the
absolute luminance data from the profile?
Thank you again for all of your time,
Michael A. Litscher
CTO, ColorMetrix Technologies, LLC
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