Re: Post linearization, pre-Profile
Re: Post linearization, pre-Profile
- Subject: Re: Post linearization, pre-Profile
- From: Roger Breton <email@hidden>
- Date: Tue, 29 Aug 2006 22:29:42 -0400
> When considering Total Ink Limit;
> 1. Is the point of divergence I'm observing an indicator in determining total
> ink limit? Of not is any of this information usable in helping to determine
> TIL?
To me, TIL is a function, first and foremost, of how much ink can be laid
down on a given substrate without flooding or puddling. I triy looking at
TIL in many ways but I'm getting the feeling, now, that it is
implementation-specific. That is, on some RIPs, TIL means one thing while
one some others, TIL comes to mean not quite the same thing. Depends on each
RIP's approaches. But the basic idea always remains the same, to push the
printer/substrate/ink to the limit of what is feasible, technically,
physically and colorimetrically without inducing all kinds of side effects
or unwanted artifacts that will hamper later profiling efforts. The "hooks"
that you see in your Lab plots does not, in my view, directly translates
into a quantitative approach of limiting inks but it's a good starting
point, something to think about to peek into the repro system made up of the
printer, its inks and your choice of paper.
> 2. How much weight should the bottom-out point of the Lab_L curve be given
> when determining TIL.
Ideally, you'd want to go as deep as possible. But there are always trade
offs. Earlier this week, for example, I was calibrating two different
proofing substrates on an EFI RIP. With the first substrate, limiting the
total ink at 215% translated into a black of L=10 and a* and b* very close
to zero (I could not go more than 215% without flooding the paper with ink).
On the second substrate, wanting to push the TIL a little further, at 230%
(this paper could hold more ink), I got the black down to L=8 or 7, good,
but the b* was becoming very negative (go figure!). So I reasoned to hold
back on TIL by leaving at 220% not to induce casts in the darkest neutrals,
figuring that this policy would be best for later profiling.
> Can this same procedure be used for testing linearity and determining channel
> limiting?
Again, depends on who's doing the job: you or the RIP.
> Would Chroma and Hue be good guides?
The ColorBurst people swear by limiting channels by chroma. Anything over
74, for example, on the magenta, they'll consider off limit. So, in their
RIP interface, they don't show screen percentages to the user but chroma
values per channel. The user have to get used to interpreting these values
in order to carry any serious calibration by hand.
> If so, how do you use Chroma and Hue
> curves/values to aid in linearization and channel limiting?
One trick consists of getting a chroma value at least as large as the
process to be proofed. It's not the panacea but it's a well accepted idea.
(that's one of the ColorBurst RIP precept) For photographic reproduction,
this trick is not applicable as the goal of calibration should be to develop
as big as possible a color gamut. Some people like to limit ink right where
ink "hooking" starts. There are pros and cons to this approach, and, in the
end, I think it all boils down to what the RIP manufacturer allows one to
do. GMG, Oris, EFI, Onyx, ProofMaster and ColorBus, to only name these few,
all have their own proprietary methods of going about ink limiting and
calibrating. Hard to say which works best across the board. But in each
case, there are optimal ways of going about deciding on TIL. That's my
experience. Some ideas are generalizable, yes. But not everything.
> Douglas Rhiner
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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