[argyllcms] Re: perceptual black too light
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sat, 18 Feb 2006 21:35:03 +0100
Graeme Gill wrote:
I'm not interpreting the ICCV4 spec change to be a shift from assumed
media adaptation to illuminant adaptation.
It also took me a while to digest this, but the ICC spec explicitly sais
"Note that the scaling between media-relative and ICC-absolute
colorimetric values is done under the PCS
illumination source. Also note that the observer is assumed to be
adapted to the perfect diffusers not to
the media white."
The chromatic adaptation from the illuminant (or in other words, from
the adapted WP) to the PCS illuminant (D50) is described by the chad
tag, while the wrong von Kries transformation between PCS white and the
media white point (after adaptation to PCS white) is rather meant to be
a media-relative encoding - just to align src and dst media white
points, which are not yet necessarily aligned after the chromatic
adaptation from src to dst illuminant.
I could imagine the following relaxed interpretration of the spec:
If we believe, that for a particular device the observer is not really
adapted to the actual illuminant used by the device, but to a different
WP, then maybe we are free to assume a different adapted WP, but then I
think we must treat the assumed adapted WP as if it were the
"illuminant", and as a consequence, the chad tag should describe the
chromatic adaptation from the assumed adapted WP to the PCS illuminant D50.
That's in fact also what happens for display profiles. Since the
observer is basically assumed to be adapted to monitor white, monitor
white becomes the "illuminant", and thus media white (== monitor white)
and "illuminant" become equal (-> wtpt tag becomes D50).
Applied to a reflective print, if the viewing environment is set up in a
way, that the oberver adapts to paper white, this would IMO imply that
the chad tag would need to describe a chromatic adaptation from media
white to D50, and the wtpt tag would be D50, like for a monitor profile
(but that's a hypothitical consideration, since the basic assumption is
obvioulsy, that the observer of a reflective print is rather adapted to
a perfect diffusor, and not to media white).
On the contrary, the continued use of the media white as the point of
reference for "Relative Colorimetric" indicates to me the opposite.
The transform from measured (reflectance) absolute values to relative
colorimetric is still being done using the "wrong Von Kries" for no
apparent technical reason, but the media is still assumed to be
adapted white.The shift from using the emissive white point as the
media white rather to the illumination, has made side by side hard
copy to display proofing and spot color reproduction harder to
achieve, and injected uncertainty into the interpretation of display
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