[Openicc] Drop size calibration

[Robert Krawitz]

   From: "Hal V. Engel" <hvengel at astound.net>
   Date: Sat, 2 Feb 2008 14:18:07 -0800

   On Friday 01 February 2008 20:31:56 Robert Krawitz wrote:
   >    From: "Hal V. Engel" <hvengel at astound.net>
   >    Date: Fri, 1 Feb 2008 12:25:03 -0800
   >snip:
   >    > >"Hal writes
   >    > >In my earlier testing I also noticed that these curves seemed to
   >    > > change significantly when I switched ink carts.  So if there were a
   >    > > calibration utility it would make sense to recalibrate whenever a
   >    > > cart was changed."
   >    >
   >    > I guess the same would apply for other printers than the R2400.
   >
   >    I do not have any reason to believe that this does not apply to
   >    most if not all ink jet printers.  For some printers, like those
   >    with the nozzles built into the ink carts, this might be even more
   >    of an issue.
   >
   > The 2400 is one of Epson's better non-Stylus Pro printers, to boot.
   > The question is, how much do the curves change.

   I suspect that if the light dark transitions were better tuned that
   the affect of changing ink carts would be less pronounced.  I say
   this because when I was doing these tests the changes I saw when a
   cart was replaced were most noticeable in the shape of the
   anomalies in the light dark transition zones.

You may be correct there.

   > The basic way I would do it in both cases would be to print a
   > linear sweep and try to find a best fit constant that would
   > relate the two curves, or at least the lower part of the curves
   > before dot gain/saturation issues started to come into play
   > (particularly for variable drops).
   >
   > Maybe I really do need a spectrometer...

   Using a measurement device makes these sorts of things objective,
   quantifiable and repeatable.  It also means that these tuning steps
   can be documented.

Correct, although I'm putting together a process that relies more on
the ability to distinguish differences in brightness that form a
pattern than ability to recognize subtle differences.  Our eyes are
better at seeing high frequency detail than recognizing smooth
gradients.  It looks like this process works well for drop sizes, at
least at fairly high resolutions where the drops are large.  But I
suspect it's going to be a lot harder to apply this to tuning light
inks, and that's where objective measurement would help.

(Of course, there's also the question about how much I want to invest
in accuracy at low resolutions.  Trying to get a perfect tuning at 360
DPI seems like a silly exercise.  On the other hand, trying to get a
perfect tuning at 1440x720 DPI has great value, since if it's possible
to get a good match with 5760x1440 output it saves a huge amount of
time.)

   >
   > 1) To calibrate light inks:
   >
   snip
   >    C) Find a best fit constant relating the two curves at each point.
   >       Ideally you should find that the dark cyan curve is consistently
   >       a certain amount darker, at any ink level, than the light cyan
   >       curve.  If you get a good linear relationship in the lower part
   >       of the curve, use that.

   Could this constant be (light ink L*)/(dark ink L*) for
   corresponding patches or perhaps an average of this ratio for a
   number of corresponding patches?  You likely calculate this some
   other way since you currently do not have a measurement device.
   What is your current technique?

The way I currently do it is very crude indeed -- I compare cyan and
magenta sweeps printed with dark ink only with those printed with both
inks, and go for the best eyeball match.  It's not a very good
technique.

What you suggest sounds like the way to go.  I'd want to look at more
than just the average, though; I'd want to look at the shape of the
ratio curve, and in particular target the best match for the
highlights and light midtones.  There's not much sense in worrying
about this in the shadows, where there would be little if any light
ink used.

   snip
   > Would a more detailed writeup on this be of interest to other people?

   The above was a good start.  For this sort of thing it is almost
   impossible to provide too much detail.

Yup.