Gerhard Fuernkranz wrote:
After taking a look at the old and the new specs, I'm getting more and
more the impression that V4 hasn't really introduced a significant
change, but only extensions. Couldn't it be that the *intended*
semantics have not realy changed, but that they have been misinterpreted
previously by CMM and profile makers, simply because the early specs
weren't clear enough?
I don't disagree with your analysis. The old spec. was ambiguous about
how to fit emissive displays into the idea of icc-absolute being
an illuminant relative standard. The ambiguity was in whether
to regard the white point of the emissive display as the illuminant
or the media color. There was also the anomaly of recommending
"wrong von-Kries" for the white point transform between D50 and
the media white point.
What's happened is that V4 has resolved the ambiguity, but in the *wrong way*.
It's resolved in a way that:
* Breaks expectations about what Absolute Colorimetric is meant to do,
by making it's application to emissive displays different to other profiles
(emissive profiles are now the odd ones out for no understandable reason).
* Failed to fix the use of "wrong von-Kries" transforms in computing white
point adaptation for non-emissive displays.
* necessitated the introduction of a "real absolute" intent in CMM's that
want to give users back the functionality they had before.
* introduced confusion and incompatibility with existing, widely
available profiles (srgb for example).
If the ambiguity had been resolved the other way (declaring an emissive
display is assumed to have an illuminant of D50, and a media color of
it's white point), and that Bradford should be used instead of "wrong von-Kries"
in transforming the profile data to relative colorimetric (possibly supplemented
by the 'chad' tag to make the transform maths explicit), then none of
these problems would exist.
The overall effect has been to make "absolute colorimetric" the same
as relative, just for emissive displays, necessitating the introduction
of another absolute intent to achieve things like spot color matching:
Effect V2 V4 Refl. V4 Emiss.
------ -- -------- ---------
Absolute Absolute Real Abs. Real Abs.
ICC Abs.
Relative Relative Relative ICC Abs.
Relative
Perceptual Perceptual Perceptual Perceptual
Saturation Saturation Saturation Saturation
Actually there is a perfectly good reason to introduce another
absolute intent into the ICC profile, but it has to do with
reflective media. Something similar to the 'chad' tag could
be used to indicate the transform back to the media under a
different intended ("Other") illuminant. With V4 that would
leave us with:
Effect V2 V4 Refl. V4 Emiss.
------ -- -------- ---------
Other Abs. - Real Abs. Real Abs.
D50 Abs. Absolute ICC Abs. Real Abs.
Relative Relative Relative ICC Abs.
Relative
Perceptual Perceptual Perceptual Perceptual
Saturation Saturation Saturation Saturation
still leaving emissive displays the odd one out.
Graeme Gill.
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