On Jun 1, 2005, at 5:20 AM, Marttila Jouni wrote:
Chris!
I have been following this conversation and I must admit that I
have had the
same misperception about gamut mapping. So I went to test this
straight away
and I must say that you are right. So this mean that ICC-profile
software
manufacturers assume some kind of wide colour space to start with.
And this
also means that if one is using small RGB space and perceptual
rendering one
never fully utilise the whole CMYK colour space.
Correct. This is why I've not been a proponent of perceptual
rendering for most images. It does a fine job with respect to
preserving detail, but it totally clobbers the image with respect to
saturation. I'd say some 90% of the images I come across render much
better with Relative Colorimetric + Black Point Compensation. Now
that BPC is document, I consider its use a basic requirement for
conversions, other than for proofing or spot color matching. The
exception images are the ones that are highly saturated and have fine
detail - those typically render better using perceptual or sometimes
saturation rendering.
Supposedly there is some possibility for dynamic gamut compression
due to changes in ICC v4. But it seems to me for this to happen, we
need new CMM's that are not only v4 compliant, but also are
specifically written with dynamic gamut compression in mind. It's not
like the ICC profile is smart enough to do this on its own. While
there are ICC v4 compliant CMMs (a handful anyway), the only one I'm
aware of that appears to be doing something that hasn't been done
before is from ColorIQ (
www.coloriq.com). I have not yet had an
opportunity to test this new CMM however, so I really don't know much
about it at this point. But I am absolutely in favor of smarter CMMs.
Now in a PDF/X-3 or PDF/X-4 context, smart CMM's turn into a
challenge because what if I use a different smart CMM than my
printer? Oops. We have different rendering, and my expectations are
not met. There is an argument to be made for allowing pre-rendering
using a smart CMM, without committing to a specific output space, to
solve this particular dilemma. That is, you'd use the smart CMM to
prerender your images based on a close approximation of the gamut of
the destination device, and then build a device-independent PDF/X-3
document. Then the PDF/X-3 goes to the printer where "normal"
conversions and gamut mapping practices apply to convert from device-
independent space to device-dependent space.
Is there somewhere info what kind of starting colour
spaces different
software (e.g. Gretag ProfileMaker, PrintOpen) have? It must be
quite large
- otherwise colours that would be outside this assumed colour space
would be
clipped?
It's a secret :) The guts of profiles use proprietary computations
which is in large part what profile companies use to claim copyright
for the profile. I personally find this argument specious because
there are equally complex algorithms found in Photoshop, and if you
use multiple layer effects arguably you are creating a composite
algorithm much more complex than used to build an ICC profile. Yet
Adobe makes no claim to copyright the image you've created using
their algorithms and software.
In any event, it's unlikely this information is documented for public
consumption.
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)