[argyllcms] Re: vcgt meaning with regard to the ICC profile
- From: Etienne Dechamps <etienne@xxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 25 Apr 2013 10:19:09 +0100
On 04/25/2013 12:22 AM, Graeme Gill wrote:
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 ?
Well, it could have been designed the following way: if your workflow
fully supports ICC, then ignore vcgt (you don't need it); if it doesn't,
then just load the vcgt LUTs. One could make the argument that since
vcgt is non-standard, you cannot rely on users loading it, and so the
rest of the ICC profile should not be made dependent of a non-standard
extension.
Right now if a system supports ICC but not vcgt (which is a completely
valid situation since vcgt is not part of the ICC standard), then it
will not get the correct result because the rest of the profile
describes the display's behaviour assuming the vcgt is applied.
On a related note, I guess ICC-aware PC software could have some special
logic that basically looks at the currently loaded hardware LUTs, and
then makes processing decisions based on whether the vcgt from the
profile is loaded or not, so that the result is always correct. I am not
aware of any software that does that, though.
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.
Yes, that's what I meant.
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.
I'm new to ArgyLL, so pardon me if I'm going to ask a dumb question -
how can I specify a white point other than D65 when generating an ICC
profile with vcgt tables? colprof doesn't seem to have an option for that.
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
Yes, actually I meant the primary chromaticities. I realize that I'm a
bit sloppy with the terminology - sorry about that, I'm quite new to
color concepts.
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.
Understood.
Thank you for your very useful answer.
--
Etienne Dechamps
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