Re: MS Color Control Applet
Re: MS Color Control Applet
- Subject: Re: MS Color Control Applet
- From: Roger Breton <email@hidden>
- Date: Tue, 04 Jul 2006 21:17:11 -0400
Graeme,
Allow me to dissect the MWP tag section and comment along.
> ICC V4.2, Page 33, Section 9.2.25 mediaWhitePointTag:
>
> "Tag signature: wtpt¹ (77747074h)
> Allowed tag type: XYZType
Not much difference here from the older 3.4 specs.
> This tag, which is used for generating ICC-absolute colorimetric intent,
> specifies the XYZ
> tristimulus values of the media white point.
I always felt that the MWP was more designed for printed media than for
self-luminous sources. But I'll admit I see nothing in this last sentence
that "binds" the definition of MWP to a printed sheet or a monitor display
since both can be assigned XYZ tristimulus values. And EyeOnePro or a Spyder
or a DTP94 or a SpectroCam can all return XYZ from a hardcopy and the
faceplate of a CRT or LCD.
> If the media is measured under an
> illumination source
> which has a chromaticity other than D50, the measured values must be adjusted
> to D50 using the
> chromaticAdaptationTag matrix before recording in the tag.
In the previous specs, in the Annex, I remember there was reference to
adapting the colorimetry to D50 but it was left to the profiling package to
decide on which method of adaptation to use, XYZ scaling or some other
methods. In v4 specs, there is no choice. But regardless of the specs
version, adaptation is only be required when measuring self-luminous sources
since all reflectance measurements always use D50. Unless someone is crazy
enough to use D65 and adapts the values to D50 through a CAT!
So this says to me, nothing new, as far as measuring paper is concerned in
v4 specs. But very different, I'd venture to say better, for measuring
displays. Suppose I'm a die-hard for D65 calibration, this means that all
measurements of my screen have to be chromatically adapted to D50 through
Bradford before they find their way in the profile. So, here, I don't yet
see anything suspect or questionnable.
> For reflecting and
> transmitting media,
> the tag values are specified relative to the perfect diffuser (which is
> normalized to a Y value of
> 1,0) for illuminant D50.
Again, here, nothing new. All colorimetry is specified relative to the
perfect reflecting diffuser (I never thought I'd see a "perfect reflecting
diffuser" in my life but BabelColor new white standard comes as close as is
physically possible to this as can be).
> For displays, the values specified must be those of
> D50 normalized such
> that Y = 1,0 (i.e. 0,9642 1,0 0,8249).
Meaning that (absolute) display measurements have to be adapted (CAT02) so
that the XYZ of the white point is encoded as D50 in the profile.
> The MediaWhitePointTag is the information a CMM uses to convert the
> normal relative colorimetric representation used in a matrix or LUT,
> back to absolute colorimetric.
This seems like normal ICC speak for proofing where the information required
to generate AbsCol rendering intent resides in the MWP.
But, how is the new definition of the MWP "absolute" is wreaking havoc for
screen colorimetry? It did not strike me as such in the above.
> Graeme Gill.
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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