RelCol in Monitor Displays, Was: MS Color Control Applet
RelCol in Monitor Displays, Was: MS Color Control Applet
- Subject: RelCol in Monitor Displays, Was: MS Color Control Applet
- From: Marco Ugolini <email@hidden>
- Date: Mon, 03 Jul 2006 15:01:56 -0700
In a message dated 7/3/06 2:06 PM, Roger Breton wrote:
>> What this means in practice is a reduction in ICC functionality. Using V4
>> profiles
>> and the ICC intents, it is no longer possible to do absolute colorimetic
>> matching on monitors. You're stuck with relative colorimetric.
>>
>> Graeme Gill.
>
> Meaning: we'd still like to retain the choice of "real" rendering intent to
> the screen but, because of ICC v4 bylaws, abscol to the screen is forced to
> "morph" into relcol to the screen, right? Could you say in which section of
> the specs the said restriction could be found, Graeme?
I'm far from claiming to be an expert of the caliber of others in this
forum, and I confess to having a hard time reading things such as the
jargon-heavy ICC v4 specs. So, what Graeme is saying may well be there, for
all I can tell, in the folds of all that tech-talk, and I may just be unable
to interpret it correctly.
But, on its face, this forced-RelCol business for monitor profiles seems to
make no sense. Yes, chromatic adaptation makes a D65 or D50 white point
appear white, after a fitting amount of time is allowed to pass. But I would
think that it's not just the white point to change when printing to
something like newspaper substrates: colors are redefined too -- gamut-,
hue- and saturation-wise -- throughout the whole range.
To make that appear, instead, like something printed on nice white paper --
say, more similar to what may happen with sheetfed coated stock -- offers a
skewed representation of the output, it would seem to me. What would be the
point of soft-proofing, then? To show how great things look, even on
newspaper stock? Doesn't that encourage unrealistic expectations?
But here is a quote from pages 85-86 of "ICC1V42.pdf"
(<http://www.color.org/ICC1V42.pdf>), the document released by the ICC in
2004 for Specification ICC.1:2004-10 (Profile version 4.2.0.0). Honestly,
I'm not sure what to make of it.
"D.7.8 Monitor display
Some special considerations apply to monitor profiles. Since a CRT monitor
is a self-luminous display, the interpretation of tone is somewhat
ambiguous: Should full-drive monitor white be regarded as 100% diffuse
white? In terms of colour appearance, the answer to that question depends on
the state of the observer's adaptation, which is influenced by the viewing
environment. For example, in a brightly-lit office environment, the observer
may adapt to the ambient illumination. In a dim environment, the observer
may adapt to the monitor screen itself. In general, it is very difficult to
predict the observer's actual state of adaptation.
However, for desktop applications the document editor or graphic artist
typically has an expectation that monitor white will be associated with the
blank paper (or other substrate) of the output medium, regardless of his or
her actual state of adaptation. Thus, for practical reasons, it is important
that the monitor profiles be designed to display paper white at full-drive
monitor white (R = G = B = 255 on a typical 24-bit display). Similarly,
there is an expectation that R = G = B = 0 corresponds to "black" and will
be reproduced by the minimum reflectance of the output medium. These user
expectations are based on common practice and convenience and lie outside of
strict colorimetry and colour-appearance considerations.
Furthermore, the monitor profile transforms that are common on many systems
are based on oversimplified mathematical models. Often they take the form of
a linear transformation from XYZ to RGB (a 3 x 3 matrix) followed by a
simple power law in each channel for gamma correction. Such transforms often
fail to model the behavior of the monitor accurately in the shadows, since
they ignore the biases that commonly occur in the CRT and support
electronics. These biases are variable from unit to unit and are also
dependent on the userselectable settings of contrast and brightness.
Fortunately, any departures from colorimetric accuracy that result from
these simple models are relatively minor and are partially masked by
face-plate reflections, often 3 to 5 percent, so that they are generally
tolerated.
These simple monitor profiles will satisfy typical user expectations if
monitor white is mapped to the XYZ values of the PCS white point and monitor
black is mapped to the PCS black point. This means that monitor white maps
to reference medium white and monitor black maps to reference medium black.
When processed through a typical monitor profile transform, therefore,
reference medium white will be displayed at monitor white and reference
medium black will be displayed at monitor black. This provides a practical
mapping between the monitor and the reference medium while permitting the
use of simple monitor transforms to satisfy common user expectations."
--------------
Marco Ugolini
Mill Valley, CA
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