Re: Rendering intents in source profiles?
Re: Rendering intents in source profiles?
- Subject: Re: Rendering intents in source profiles?
- From: Uli Zappe <email@hidden>
- Date: Fri, 25 Jul 2008 08:43:52 +0200
Am 24.07.2008 um 05:05 schrieb Graeme Gill:
Welcome to the path of enlightenment :-)
I already felt more enlightened at times ... ;-)
For instance, take the classic example of a conversion from working
space to printer space. Since the printer space is smaller, gamut
compression or clipping obviously has to be performed. *But* this
should take place in the B2An tag of the printer profile (= target
profile), which "knows" about the limitations of the printer and
can adjust the gamut accordingly.
Oh, how can that work, since the output profile does not "know" what
input profile it is going to be linked to ?
I see your point in that it would be practically unreasonable to
provide for a gamut compression so intense that each and every Lab
color from the PCS could be matched to the target space, even those
that will probably never come from any kind of source profile.
However, impossible as it may be to fulfill it, the task is clear at
least: take care that all colors from the PCS are either clipped (no
problem) or compressed (above problem) so that they fit into the
target space. I can't even formulate the related task for the A2B
direction: map every color to the PCS - yep, this will always work by
definition, so what?
*Maybe* (just maybe) this is a vendor specific strategy: build target
profiles that assume that only a specific subset of Lab colors has to
be matched *AND* take care in your source profiles that only these
colors will be delivered to the PCS (thus already an A2B match)? This
would mean that profiles of such a vendor would be "more compatible"
to themselves than to profiles from other vendors. But this is pure
speculation on my part ...
[ Note that my ArgyllCMS profiler solves this problem by allowing a
source profile or gamut to be specified at output profile creation
time.]
A very compelling idea. :-)
My guess has always been that the the allowance for intent in the
A2B tables is part of a (flawed or at least limited) scheme to make
gamut mapping work in the ICC profile world, by allowing the input
profile to map the input device gamut to a "standard" PCS gamut, and
then have the output profile map from the PCS gamut to the output
device gamut. It's limitation is that it only really works for (what
I would call) saturation intent, since the gamut surfaces will be
mapped to gamut surface (expansion of gamut as well as compression).
OK, so I don't feel alone with my thoughts ... :-)
Up until an addendum to ICCV4 however (PRMG),
Oh, thanks for this hint - I hadn't heard about the PRMG, and this is
extremely interesting in that it basically confirms the above thoughts.
there has been no standard PCS gamut to make this scheme inter-
operable between profiles from different profile creation software.
Exactly ...
Again, thanks a lot, this cleared this up quite a bit, and I'm kind of
relieved that it wasn't only stupidity on my part ;-))
Bye
Uli
________________________________________________________
Uli Zappe, Solmsstraße 5, D-65189 Wiesbaden, Germany
http://www.ritual.org
Fon: +49-700-ULIZAPPE
Fax: +49-700-ZAPPEFAX
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