Re: Custom ICC Profiles/Conversion Rending Intents
Re: Custom ICC Profiles/Conversion Rending Intents
- Subject: Re: Custom ICC Profiles/Conversion Rending Intents
- From: Chris Murphy <email@hidden>
- Date: Sun, 19 Jun 2005 22:57:45 -0600
On Jun 19, 2005, at 10:31 PM, Graeme Gill wrote:
. It's unlikely that an increased ink limit
won't change the gamut in some way, either increasing it
(albeit only slightly), or making it ill-behaived ("non-monotonicity",
"aliasing", or "folding").
Agreed. At least with well behaved output, like printing presses, the
benefits of heavy GCR are clear when it comes to gamut. Of course the
gotcha is the requirement for rigorous process control for the black
channel.
If increasing an ink limit increases the gamut
in a well behaved way, then in principle there is nothing
wrong with using the extra gamut (color wise), and it's really up
to human judgement whether the ink limit is a reasonable one.
Absolutely. For inkjets, there is an ideal, that's certainly output
condition specific. There is a wider range for presses, as it's more
"mechanical" nature lends to a certain meandering of its behavior. So
black generation is as much image dependent as it is output condition
dependent.
What sort of criteria would one use to automatically compute
a reasonable ink limit, based on the device response (and ignoring
other factors like ink running off the page etc.)?
This is a valid question and raises all kinds of additional questions
as well. However I was speaking more from a generic sense. Clearly
400% heavy GCR with the vast majority of devices out there, is a
contradiction of sorts. You really don't have such a high ink limit
and a high black generation. This is not made obvious in the UI of
profile building applications. That it cannot be entirely automated
is something I don't spend a whole lot of time lamenting. Most people
would like a starting point to determine an general purpose black
generation and ink limit.
What you say may be generally true, although only at or near the gamut
surface. The GCR choice will have an effect in other areas of the
gamut,
and won't be affected by the ink limit. I'd really ask "so what ?"
though.
GCR is a secondary preference, that shouldn't affect the gamut size
or color reproduction.
Ahh but it very clearly does with the way it's generally implemented
today, as a relative setting, rather than an absolute setting
explicitly tied to ink limit. High GCR does clearly improve shadow
gamut quite a bit (while potentially obliterating shadow detail,
depending on the output device's propensity to plug in shadows).
Wow, that's pretty silly.
It's not silly, it's the nature of the maths.
The silly part, and ultimately my complaint, has nothing to do with
math. It has to do with the whole concept of an ink limit. What we
have here is a legal loophole. Legal mathematically, but not a good
idea in practice.
At 50% interpolation between these nodes, we would
get a raw CLUT CMYK of 24.5 56.5 74.5 74.5 %, and
output CMYK of 49.5 75.2 86.3 86.3 %, giving a total
ink of 297%, exceeding the limit.
By 17%. Depending on the printing process, this could be a big
problem. Most likely it's an annoyance, or "not ideal" but that 17%
could actually cause problems in drying, off set, unnecessary ink
consumption, etc. I don't like the idea that an "ink limit" is
considered a suggestion, even if it can be explained mathematically.
I'd certainly like to see a total ink limit tag in profiles, as
this is essential in defining the gamut of the device.
Yes. I'd like to see a black generation tag. It means some work on
the part of the ICC to separate the ambiguous from unambiguous
varieties of black generation (I think completely unambiguity isn't
going to happen), but it would be very useful metadata to have for
repurposing separations.
Chris Murphy
Color Remedies (TM)
www.colorremedies.com/realworldcolor
---------------------------------------------------------
Co-author "Real World Color Management, 2nd Ed"
Published by PeachPit Press (ISBN 0-321-26722-2)
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