Re: ICC v4
Re: ICC v4
- Subject: Re: ICC v4
- From: Chris Murphy <email@hidden>
- Date: Mon, 21 Nov 2005 09:07:32 -0700
On Nov 17, 2005, at 6:35 PM, Roger Breton wrote:
I have a feeling that all these gamuts talk all boil down to
changing the
colorimetric encoding, just to halt the wholesale waste of PCS
space. Isn't
it? The Destination gamut has no clue about the Source gamut: that
seems to
me like one of the golden rule of the ICC founders. Wasn't that a
limitation
then as it still is a limitation today? Or are you saying that V4
"ICC PCS
reference gamut" has the potential of changing all that?
It will change that in the sense that it places some burden of gamut
mapping on the source profile to a Reference Medium. From there, that
reference output medium space is what all profiling packages use to
predicate their gamut mapping for the perceptual rendering intent.
It's not as good as what a smart CMM could accomplish, which would be
knowledge of what portion of the gamut an image actually occupies and
predicate gamut compression dynamically to the destination space.
One of the limits of ICC V2 is that the destination profile
doesn't usually know the source colorspace.
And I'll bet WCS has a cure for this ill? But ICC V4 too then?
ICC v4 has a workaround, which is the ICC PCS Reference Medium. The
key point of perceptual rendering is foreknowledge that you can't get
from A to B. You can't exactly reproduce the image because too much
is out of gamut. That would be a scene-referred image state. The
image state needs to be changed before gamut mapping to a specific
output device. The source profile is thus responsible, in a
perceptual rendering context, to change the image state from scene-
referred to reference-output-referred. From there, the destination
profile changes image state (using gamut dynamic range compression,
but not as much as in an ICC v2 context) to actual-output-referred.
Colorimetric rendering holds that the image state doesn't need to be
changed. For example from press to inkjet printer. This implies
actual-output-referred to actual-output-referred.
One key thing we could see in future implementations is the tagging
of the image with its current image state and from there we'd be able
to automate the selection of rendering intents for most applications.
More metadata!
As for what WCS will do I can only guess based on current publicly
available data. The notion that there are pluggable gamut mapping and
color appearance models implies at least the potential for dynamic
gamut mapping on an image specific basis. This is possible in the ICC
world too but necessitates smarter CMM's that can do this analysis
rather than simply defer the mapping to a prebuilt table in the
source and destination profiles.
How would you make this process "more explicit"? Or how does V4
helps make
this process more expicit, I should say?
Well it doesn't yet because I don't think the ICC PCS Perceptual
Medium is fully defined yet.
I wish I could have made it to Scottsdale this year. I'm sure you
guys had
many interesting conversations.
It hasn't happened yet. It's in two weeks.
Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
-------------------------------------------------------------
Co-author "Real World Color Management, 2nd Edition"
Published by PeachPit Press (ISBN 0-321-26722-2)
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| >Re: ICC v4 (From: Roger Breton <email@hidden>) |