QTR-Quadtone RIP

Phil

Looking over your posts on this topic it's not clear to me what you are
changing on each iteration. I don't know of any mechanism in QTR for one
linearization to refine the results of a previous linearization. Iteration
is useful, but as Terrence Lowe points out it should occur before
linearization. To elaborate somewhat on Terence's suggestion, here's what I
do:

After the curve is setup on the Ink Setup tab and I am happy with the limits
etc on that tab, I print a step wedge, measure the values and plot them in
Excel. I compare this non linearized curve with the so called ideal curve
for the dmax I am achieving with the ink/paper I'm using. If there is a
"big" discrepancy between the measure curve and ideal, I'll adjust the
Highlight, Shadow and Gamma parameters on the Gray Curve tab and print
another step wedge, measure and plot it. You may also have to adjust the
same parameters on the Toner tabs if appropriate.

I don't have any suggestion as to how to judge "big". All I can suggest is
that if you're not happy with the results of linearizing your curve, you
could try making your "big" smaller. Also, the Gray Curve parameters are
sometimes useful in eliminating flat spots or small reversals that make
linearization impossible.

I may adjust these parameters, print, measure and plot the results several
times before I feel the curve is close enough to get QTR to linearize it
satisfactorily. After doing several curves the number of iterations required
drops off. Of course once you've done one curve for an ink/paper combo you
can usually use the same values for other curves for that paper - at least
as a starting point.

As an indication of how much adjustment can be required, the default and
final values for these parameters for a curve I created for Premier Fine Art
are:
Parameter       Default         Final		
---------       -------         -----
Highlight         10              6
Shadow            10              4
Gamma              1             1.6

The Gamma adjusts the mid-tones and the other parameters adjust the obvious
parts of the curve. In general, "L"arger values "L"ighten the curve. As
noted in the User Guide, the Gamma adjustments are dramatic and should be
small compared to the other 2 parameters. Once you reach acceptable values
for these parameters, you can use the measured values from the last
iteration as the linearization values to create your final curve. If you
need to adjust these parameters again, you must clear the linearization
array and print and measure a step wedge so you can enter the new
linearization values (and regenerate a new curve).

I recall that adjusting these parameters was suggested in a post by Roy
because the linearization algorithm is not able to do an effective job on
curves that require a large adjustment. I don't recall him stating how close
you need to be to get a good linearization.

I created the curveval2.xls spreadsheet (available in the files section) as
a tool to aid in visualizing the curve. I use Xkey, a key-stroke grabber
from X-Rite, to place the values from my 810 directly into the spreadsheet.
I find I can make a quite good (in my humble opinion) curve using the 21
step wedge. The 51 step wedge is more work and, because of the smaller step
is more prone to reversals.

Hope this helps. Good luck.

Tom Moore

> -----Original Message-----
> From: QuadtoneRIP@yahoogroups.com [mailto:QuadtoneRIP@yahoogroups.com] On
> Behalf Of Philip Schwartz
> Sent: Wednesday, December 20, 2006 7:19 PM
> To: QuadtoneRIP@yahoogroups.com
> Subject: [QuadtoneRIP] Re:can't properly linearize ink profile in QTR
> 2.4.3.10
> 
> Hi Jeff. I am familiar with Keith's image, but I didn't actually try
> it. My reasoning: I need to create curves that will produce
> (reasonably) identical prints on different papers. If I can't reliably
> linearize my profiles this will be an exercise in futility. I may get
> good prints, but none will match. I did also try the 51-step wedge,
> but I kept getting QTR errors complaining that the linearization array
> sequence was invalid because it was not constantly increasing so curve
> generation failed. To be honest, I got this message most of the time
> using 21 values, but did manage to generate a couple of curves. It
> seems like QTR doesn't like multiple steps with the same value, or
> steps that are too close together, or, horrors, a value that actually
> decreases. All of these things can happen in my world, and I don't
> know how to fool QTR without invalidating my measurement.
> I also tried to load a Photoshop CS acv I use all the time, but got a
> QTR error about invalid values. Sorry, I'm not in front of that
> computer at the moment but I have captured all these errors on my home
>  PC.
> --- In QuadtoneRIP@yahoogroups.com, "Jeff Randall" <jrandall@...> wrote:
> >
> > Phil: Have you printed Keith Coopers test image
> > <http://www.northlight-
> > images.co.uk/article_pages/black_and_white_test.html>
> > and looked at the circular gradient and printed the offsetgrad8.jpg
> > file located in this group's Files section to see if you can
> > actually see the non-linearities you are measuring?
> >
> > Another thing to try is a 51 step calibration approach using the 51-
> > step file in the OneEye folder of the QTRip folder.  I had problems
> > getting a linear profile using the 21-step approach, but the 51-step
> > approach worked very well for me.  YMMV.
> >
> > BTW, I stretched the 51step image to print landscape -- bigger
> > patches for my clumsy hands.
> >
> >
> > --- In QuadtoneRIP@yahoogroups.com, "Philip Schwartz" <pschwart@>
> > wrote:
> > >
> > > Thanks Terence. I am a new to QTR but have lots of experience
> > printing
> > > fine art B&W. QTR has lots of controls, but they are not well
> > > documented so results are often not predictable, making the
> > profiling
> > > process incredibly tedious. I am spending far too much time
> > profiling,
> > > and not enough time creating and printing images :( Hoping this
> > will
> > > change ...
> > >
> > > --- In QuadtoneRIP@yahoogroups.com, Terence Lowe <televe@> wrote:
> > > >
> > > > I, too struggled with the QTR linearization process.  My approach
> > > was to
> > > > try to get the best density measurements by modifying the ink
> > setup and
> > > > grey curve tabs, but with the linearization array clear.  This
> > took
> > > many
> > > > iterations.  Eventually, when I thought that the measured
> > density curve
> > > > was close to the ideal density curve (I plotted both in Excel
> > for
> > > > comparison purposes), I linearized, printed the step tablet
> > again and
> > > > checked the new, linearized density measurements to confirm (or
> > > > otherwise) that my final curve was good enough.
> > > >
> > > > I made the assumption that it was incorrect to re-linearize the
> > > > linearized curve during the many iterations and so only
> > linearized once
> > > > as the final step in curve creation..  I hope this was the
> > correct
> > > > assumption.
> > > >
> > > > Cheers,
> > > >
> > > > Terence Lowe.
> > > >
> > >
> >
> 
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