Re: Monitor calibration & profiling in a multiuser OSXenvironment.
Re: Monitor calibration & profiling in a multiuser OSXenvironment.
- Subject: Re: Monitor calibration & profiling in a multiuser OSXenvironment.
- From: Graeme Gill <email@hidden>
- Date: Tue, 02 Sep 2003 13:55:10 +1000
Roger Breton wrote:
>
Someone contributed some worked out example to calculate that last step but
>
I have not had the chance to revisit those calculations since the last part
>
of summer.
>
>
You must have all that math figured out in Argyl?
To a degree. The Absolute <-> Relative transform is poorly defined
in the ICC (V3.4) spec. In the context of printed media, the assumption
is that the media is measured under a D50 illuminant, and the
media white point difference to D50 w.p. reflects the non-perfect-white
nature of the substrate. The examples in the ICC spec. then
use a "wrong" Von Kries transform to make the (usually) slight
adjustment from the media white (abs) to D50 (rel). A device (like a CRT) that
has a fundamentally different white point, isn't directly covered
by the spec., although there is some reference in the appendices to
using an appropriate white point transform to convert to equivalent
D50 measurement conditions.
Given the confusion, I've taken the point of view that the transform
to D50 white point is primarily for chromatic adaptation purposes
(in spite of the fact that an observers actual adapted white probably
isn't the same as the white point of the colorspace), and used a
Bradford Transform for converting between the absolute white point
and the D50 white point of the PCS. This is then completely compatible
with sRGB (which has a large white point shift), and consistent with
generally accepted practice for a "good" chromatic adaptation, while
introducing only a small error compared to using a "wrong" Von Kries
for print media measured under D50. The V4 chromatic adaptation tag,
is (as I understand it) an attempt to address the fact that in the confusion,
different systems are using different, unspecified, absolute<->relative
adaptation conversions.
The V4 chromatic adaptation tag raises some interesting practical
problems though - what if the transform doesn't actually transform
D50 to/from the media white point ? Which do you take as authoritative ? :-)
Graeme Gill.
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