[argyllcms] Re: vcgt meaning with regard to the ICC profile
- From: Graeme Gill <graeme@xxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 25 Apr 2013 09:22:09 +1000
Etienne Dechamps wrote:
> What I don't know, however, is whether the rest of the ICC profile contents
> are a characterization
> of the device *before* or *after* the vcgt LUTs have been applied.
The ICC profile should be the display character with the vcgt in place -
there's no
point otherwise. See <http://www.argyllcms.com/doc/calvschar.html>:
"Note that a characterisation (profile) will only be valid for a device if it
is in the same state
of calibration as it was when it was characterised."
> In other words, when using
> ICC-compliant software (e.g. Photoshop, Gimp), should the vcgt be loaded, or
> will it result in
> double correction (and thus bad results)? Is there a consistent behavior
> between vendors in this
> regard?
Yes, it should be loaded. What other purpose would it have if it is included in
the profile ?
> As far as I know, applying the vcgt usually results in the following:
> - No change in gamut, because it's not possible to alter gamut using RGB
> 1d-LUTs, you need a
> 3d-LUT or a matrix for that;
Not technically true - it can reduce/change the absolute 3D gamut volume by
moving the white point.
You can't shift the primaries chromaticities with 1D curves though.
> Am I correct? If I am, is there any rationale behind the seemingly arbitrary
> choice of D65 besides
> the fact that it is the standard sRGB white point?
Many people use something other than D65, so that their display is a better
visual match
to print under a D50 WP lamp.
> And a final question: is calibrating the white point without adjusting the
> primaries (e.g. when
> using non-ICC-aware software on a system with the vcgt loaded) such a good
> idea? Could it result in
> a worse perceived result because the white point is moved away from the
> center of the gamut? I've
> read something about chromatic adaptation but I'll admit I'm way over my head
> here…
I'm not sure what you mean by "without adjusting the primaries" - changing the
white point
using calibration always adjusts the primary intensities. The gamut width is
basically
determined by the primary chromaticities, and useful systems have primaries
pretty close
to the spectrum locus boundary. If you look at a chromaticity plot of the
daylight locus
(preferably a u'v' one, not an xy one!), the difference between D50 and D65 is
not very
significant relative to the gamut boundary, and the change in relative
colorimetric gamut
shape is small.
Graeme Gill.
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