[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 profiles.

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