Re: A New Topic!
Re: A New Topic!
- Subject: Re: A New Topic!
- From: Terence Wyse <email@hidden>
- Date: Wed, 6 Feb 2008 16:45:46 -0500
Hi Mark,
What you've described, at least conceptually, is very close to what a
GMG RIP already does with it's "MX3" profiles and via its Calibration
Wizard routine. The only thing NOT included with a GMG MX3 calibration
is how the K channel interacts with combinations of CMY. CMY is
iterated via L*a*b* while the K is iterated/linearized via L* only.
But it works better than any straight linearization process I've ever
seen in terms of keeping an inkjet printer within a given tolerance.
Keeping it within .5-.75 dE average and 2-3 dE peaks is pretty routine
these days.
And what does this have to do with ColorSync exactly? (just kidding)
;-)
Terry
On Feb 6, 2008, at 3:22 PM, Mark Rice wrote:
I am a fan of color management, but I have one major problem in
dealing with
in the context of most inkjet printers: most inkjet RIPS are not
consistent
or repeatable in calibration procedure. That flaw means that color
management is larger doomed on these RIPS for exacting users. My
experience
came from the photographic digital imaging field, where extreme
precision is
a must. All photographic photo printers and film recorders use
"iterative
linearization" to obtain precisely controlled grayscales that match
target
densities very precisely. I have been begging RIP makers Onyx and
SAI to
produce such a RIP, but they don't really see the need for it. My
problem is
that if one achieves a linearization that is close, but not dead on,
one can
re-linearize, but it is just as likely to make the linearization
worse as
better. And, of course, if the linearization changes, the ICC profiles
become nearly useless.
[>]
Here is my proposal for an Iterative Linearization RIP:
"Procedures for an Iterative Linearization RIP for Inkjet
Copyright Mark Rice 2007
This is a theory derived from working with photo equipment such as
the Kodak
LVT, Durst Lambda, and Oce Lightjet.
1. Print test chart similar to existing Ink Limit chart.
2. Measure each value densitometrically until D-Max is achieved in
each
color. Or, D-Max may be limited arbitrarily to assure that a similar
value
can be achieved under all conditions of humidity, temperature, etc.
Comparable to process deviation in photo equipment.
3. Build a series of aim points from D-min to D-max based on some
form of
theory - photo devices usually use L-Star RGB or "equal brightness"
RGB for
good viewing discrimination. Aim points should be in a text or XML
file for
editability and should be high precision values-up to four decimal
points of
density. D-Min and D-Max may need to be edited in particular for
repeatability, and that often entails adjusting nearby points for
evenness.
4. Discuss what cross color ink limitations should be.
5. Print the 4 or 6 or N-color grayscale, all colors combined to
include
crosstalk characteristics. Present software doesn't seem to adjust for
crosstalk, except for ICC profiling.
6. Read the resulting gray scale into appropriate software. Compare
Difference of read values with aim points. Use an algorithm to move
toward
aim points. LVT and Lambda use a different algorithm for initial
reading
than subsequent readings.
7. Perform step 6 again, but with a new grayscale incorporating the
adjustments from step seven. A new algorithm that steps more "lightly"
toward the aim points is needed to avoid overshooting and "chasing its
tail".
8. Repeat until a minimum established variance from Difference is
achieved.
(It can't aim for dead on or it will keep trying forever.)
9. Prepare the ICC profile.
This procedure will produce highly repeatable results that also will
match
one machine and RIP to another with great accuracy. Presently I have
this
problem: I may get close to a good linearization, and attempt to
improve it,
but it may get worse instead of better. There is no possibility of
improving
on an old linearization. If a filter is used, all the value of an ICC
profile is lost.
It also potentially allows one manufacturer's RIP profiles to be
converted
to another manufacturer's profiles, a big problem in the industry at
this
time.
Mark Rice Wednesday, February 06, 2008"
Comments, please!
Mark Rice
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