[Openicc] Drop size calibration

[Hal V. Engel]

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.

>
>    > And until then your spreadsheet is a really
>    > nice guide for gutenprint curves adjustments.
>
>    I put the spreadsheet together to help me visualize what was going
>    on and to see how my adjustments affected things.  It does not give
>    you any guidance on what those curves should actually look like
>    (beyond of course being smooth) or exactly what adjustments need to
>    be made to correct specific issues in the curves.
>
> One thing to consider would be to compare curves with 4-color inks
> with 6 or 7 color inks.  You can do that by setting the ink type to 4
> Color Standard rather than the default.  Then we could try to make the
> two sets of curves match.  That would take care of the light inks.  To
> handle drop sizes, compare the curves with single drop sizes with
> those using multiple drop sizes (the CVS repository currently contains
> code that allows you to adjust all of this).

I will install CVS.

>
> 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.

snip
>    In addition I don't know how much variation there is from printer
>    to printer.  That is some of the problems I was seeing in the
>    curves of my printer may not show up in your R2400 or the one
>    Robert has.  Perhaps you could run a set of test measurements using
>    the same settings I did and drop those into the spreadsheet.  If
>    the curves look like mine then this is likely not something
>    specific to my printer.
>
> In this particular case I'm willing to all but guarantee that you'll
> get very similar results.

This is a higher end printer than most consummer grade ink jets so I would 
expect the sample to sample variation to be smaller than lower cost printers.

>
> So here's a very rough cut at a workflow (with a bunch of steps left
> out for now; if this sounds interesting I'll flesh it out) I'd suggest
> for calibrating drop sizes and light inks.  It's basically what I try
> to do by eyeball, but with measurements it could be done a lot more
> easily.
>
> 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?

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.

Hal