Re: Microsoft's color-management claims
Re: Microsoft's color-management claims
- Subject: Re: Microsoft's color-management claims
- From: Rich Apollo <email@hidden>
- Date: Wed, 21 Sep 2005 10:14:19 -0500
Okay Graeme, you confused the heck out of me. Stupid question time, and
take it easy on me if you answer.
WCS profiles are like just using the Absolute Colorimetric A2B table of
an ICC profile. It's a complete model of the device behaviour, not
just instrument readings (ie. the instrument readings have been
interpolated.) Unlike an ICC profile, it isn't meant to be in a given,
standardized PCS (which includes a standardized viewing condition), but
is meant to be the raw CIE readings for the device plus the actual
viewing
conditions of the device (which are also stored with the WCS profile.)
Are you saying that the D50 Illuminant is not assumed? And that the
measured white is not mapped in relation to D50? How do you store the
viewing conditions of the device?
A WCS profile doesn't contain the equivalent of B2A tables, which
are highly processed (in that they are working in PCS, and entail
pre-packaged gamut mapping and profile inversions).
One of the major flaws of the ICC profile is that it presupposes
that the gamut mapping transform is known at profile creation time.
Given that the source gamut in any device to device transform is
not known at profile creation time, this is an incorrect assumption.
Isn't this gamut mapping why there is a PCS (CIELAB)? Aren't the device
colors mapped to a common PCS?
ICC V4 attempts to work around this flaw, by recommending that
the (perceptual) A2B transform convert the device color into a
standard viewing condition and gamut. Unfortunately no gamut
is yet standardized, and how well this idea will work remains
to be seen.
CIELAB isn't the standard?
The alternate approach (which ArgyllCMS and WCS embody), is
to compute the gamut mapping at linking time, when the source
and destination gamuts are known.
Is this like computing a new profile for each color transform?
Things like viewing condition
transforms are easily dealt with at the same time, as is making
a richer set of intents available. Additional features such
as content dependent gamut mapping (where the source gamut is taken
not just from the source color space, but from the gamut actually
used within the source color space) can also be contemplated.
So the gamut mapping would/could be based on the color composition of
the image? For instance, GCR might occur on a sliding scale depending
on the overall neutrality of an image? Or if the destination color
space was short on red compared to the gamut of the incoming image the
gamut mapping might be adjusted to map reds to a greater degree than
other hues?
Since there is only a single gamut mapping step, possible distortions
due to multiple gamut mappings are avoided.
Multiple gamut mappings meaning each of the rendering intents?
The main penalty, is that linking speed is slower, or there may
be a price with regard to quality to pay, if you want linking to be
fast.
The speed penalty would be extreme, wouldn't it?!
…the forward profile is created by other packages, and
WCS deals with gamut mapping etc. at link time (with the option of
using plug in modules for alternate algorithms for this.)
So the "forward profile" consists of A2-what? Are we talking about
device links of some sort?
One area where they are opening things up a bit, is allowing for higher
dynamic range images. The ICC approach is fastened heavily to the
idea of a print, which can't have a white that is brighter than
the white point. This doesn't reflect the reality of the real world,
which can contain specular values. Digital capture devices (and CGI)
are capable of grabbing this, but ICC color handing can't really
cope.
I don't follow you here. Certainly you can capture specular highlights
in an image, but you cannot reproduce them. I don't understand how you
propose to have a "white that is brighter than the white point".
Rich Apollo
314-344-1144
email@hidden
www.prioritylitho.com
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