>I think, true smart CMMs are still rare. And dumb vs. smart CMM also
>seems to be also a philosophical issue, with followers and opponents on
>both sides. Maybe Marti can tell more ... ?
There is some effort on that subject, and smart CMM would eventually be
taken serioulsly in near future-- as far I can tell, this was one of the topics
in the recent Miami ICC meeting.
But the implications of smart CMM are enormous. In a smart CMM
environment you need not anymore the profile as we know today. Just
the characterization data of both devices. i.e. a sheet containing measured
patches for the scanner and other sheet with measured patches for the printer.
No AToB/BToA tables anymore, since the CMM would compute the
transfer directly. Amazing.
>Except for a few extended features (like BPC, proofing, gamut warning),
>LCMS is basically also a "dumb CMM" engine, which just applies the
>tables contained in the profiles.
Yes.. this is how ICC profiles works today. But please note that is not so
evil at all. Imagine you have an old app using ICC profiles, and want to
continue using this app you are used to. Placing the "inteligence" on the profile
side allows to get latest rocket science gamut remapping on your printer
whithout changing your environment. And this is valuable, too.
Marti.
----- Original Message -----
From: "Gerhard Fuernkranz" <[EMAIL PROTECTED]>
To: "Greg" <[EMAIL PROTECTED]>; "lms mailing list" <[EMAIL PROTECTED]>
Sent: Thursday, February 26, 2004 1:02 AM
Subject: Re: [Lcms-user] Gamut Warning
Greg schrieb:
> Gerhard wrote:
>
>
> It is done by the the BToA tables. The BToA1 table (relative
> colorimetric)
> is supposed to remap PCS values which are out-of-gamut on the device,
> to in-gamut device values.
>
>
> Is this really how it's done, usually? Or, does the CMM typically clip
> out of gamut colours?
> I.e, does the profile typically *actually* contain the explicit
> remapping for all PCS values which
> are out of the device gamut? (this is just a question - I'm not
> suggesting you are incorrect)
Greg,
yes, it is required by the ICC spec, that the BToA tables must be able to
map *all* possible PCS values to valid device values. And for a typical
output
device, the *majority* of all possible PCS values are out of gamut and need
to be remapped, since the PCS is the complete CIELAB space.
Basically the ICC approach is based on a "dumb CMM", which stupidly
applies the tables in the profies which are involved in the transform,
while the
"intelligence" is contained in the profiles (or actually in the profiler
which
creates these profiles).
For speed optimization, the involved transforms are internally often
combined into a single device link (-> see also "-c" option of tifficc).
For matrix/shaper, of course the CMM must do the clipping, since applying
the matrix and shapers may result in negative RGB values, or RGB values
exceeding 255. But I guess, this kind of clipping is usually also done in a
rather simple way, like:
if (R < 0) R = 0;
if (R > 255) R = 255;
...
The "dumb CMM" approach also has some disadvantages. For instance,
the gamut mapping in the perceptual BToA0 table of an output profiles must
already assume a particular source gamut *at profile creation time*, though
the source gamut is not yet known at this time. So the profiler can only
attempt to guess, and try to make a reasonable compromise.
Of course, any "smart CMM" is free to ignore e.g. the perceptual BToA0
tables and to create the PCS->device mapping (incl. gamut remapping) on
the fly at runtime, based on the gamut of the source and destination color
spaces, or it may even use the gamut of e.g. the *image* you want to print,
as source gamut, avoiding unnecessary gamut compression whenever
possible. However, a smart CMM approach may be computationally
very expensive and correspondingly slow.
I think, true smart CMMs are still rare. And dumb vs. smart CMM also
seems to be also a philosophical issue, with followers and opponents on
both sides. Maybe Marti can tell more ... ?
Except for a few extended features (like BPC, proofing, gamut warning),
LCMS is basically also a "dumb CMM" engine, which just applies the
tables contained in the profiles.
Regards,
Gerhard
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