Re: fundamental question - monitor profile as working RGB?
Re: fundamental question - monitor profile as working RGB?
- Subject: Re: fundamental question - monitor profile as working RGB?
- From: Graeme Gill <email@hidden>
- Date: Thu, 11 Dec 2003 11:44:49 +1100
Chris Murphy wrote:
Most of the packages are matrix and TRC based. When the TRCs are the
same for each channel, then R=G=B makes gray.
Sounds to me like it should be the other way around. If you insist
on making a profile that has the same TRC for each channel, then
the profile will be more accurate if the display is calibrated to
have perfect gray balance. But why duplicate the same TRC data
for each channel ? It's not a native concept as far as the ICC format
is concerned.
You get to make a matrix profile instead of a table based profile.
This has nothing to do with table vs. matrix. Here's the R=G=B values
for a shaper/matrix version of the profile I used as an example before:
0.000000 0.000000 0.000000 [RGB] -> MatrixFwd -> -0.000000 0.000000 0.000000 [Lab]
0.100000 0.100000 0.100000 [RGB] -> MatrixFwd -> 1.344669 1.622958 -1.997587 [Lab]
0.200000 0.200000 0.200000 [RGB] -> MatrixFwd -> 9.245358 5.703873 -6.391900 [Lab]
0.300000 0.300000 0.300000 [RGB] -> MatrixFwd -> 21.089897 5.434854 -6.445731 [Lab]
0.400000 0.400000 0.400000 [RGB] -> MatrixFwd -> 33.119342 4.630463 -5.936275 [Lab]
0.500000 0.500000 0.500000 [RGB] -> MatrixFwd -> 45.673059 3.595358 -5.125638 [Lab]
0.600000 0.600000 0.600000 [RGB] -> MatrixFwd -> 58.355252 2.632078 -3.958329 [Lab]
0.700000 0.700000 0.700000 [RGB] -> MatrixFwd -> 69.089291 2.375848 -3.582747 [Lab]
0.800000 0.800000 0.800000 [RGB] -> MatrixFwd -> 79.061241 2.729032 -3.337784 [Lab]
0.900000 0.900000 0.900000 [RGB] -> MatrixFwd -> 88.057519 3.193700 -2.347491 [Lab]
1.000000 1.000000 1.000000 [RGB] -> MatrixFwd -> 100.000461 -0.000695 0.000275 [Lab]
But that most products don't do this doesn't mean they're bad and should
be avoided.
It just strikes me as kind of awkward and fragile. It's one thing to
fiddle the analog controls on a CRT (or equivalent digital controls), to
adjust the device gamut and native response to give best possible control
from the frame buffer, and to match a target gamut and white point, but it's
another to try and fake the equivalent adjustments using hardware LUTs in the display card,
particularly if they only have 8 bit entries, and then to rely on that adjustment for the ICC
profile to be accurate. If you're willing to trade color accuracy for small profiles, and
quick profiling, then such compromises are probably fair enough. I sort of thought
that most people in the forum would be interested in high color accuracy though.
In another post Chris Murphy wrote:
That's right. That's essentially what most packages are doing. They calibrate first,
> and then most do a cursory confirmation that the intended white point was hit, and a
> middle neutral to confirm gamma, and maybe they will re-read the primaries if the top
> end of R, G or B were modified in calibration. But that's basically it.
Oh, OK, this sounds all much more primitive than I was imagining, coming from
a print profiling point of view. I know a lot of the approaches to display
profiling are from the "carefully crafted model with a small number of
parameters that works remarkably well on most devices" school, whereas this
approach works fairly poorly in the print world, and the print approach is
"assume as little as possible about the behavior of the device, and take
lots of measurements". This would explain all the activity in tweaking CRT
models to work with LCD devices etc., which seems pointless when using
a generalised approach to device profiling.
[Is it really the case that popular graphics cards still only have 8 bit entries in
their LUts ? Last time I was designing graphics hardware, the RAMDACs were all
appearing with 12 bit LUT entries, and that was about 10 years ago. Surely all
the VGA cards have 12 bits or better for the LUT data ?? Or is the Operating
System software the problem ?)
Graeme Gill.
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