[Openicc] Drop size calibration

[Lars Tore Gustavsen]

On 2/2/08, Robert Krawitz > wrote:

>
> 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...

If so, there is a new rebate going on in US if you live there.
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Relatives in US picked up mine up last summer with an similar  rebate
from this ebay seller:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=150211241322
In the ends it was around $500. In Norway a colorimeter from xrite are
more than that.




> Hal writes
>    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.
>

I can always print a sample, but since I don't have the Ilford paper
it will takes a while.

> 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:
>
>    A) Use the Ordered dither algorithm at a sufficiently high
>       resolution to guarantee use of only one drop size.  For printers
>       with 4 pl or bigger drops, use 2880x1440; for 2 pl or bigger,
>       use 2880x2880 if available, or otherwise 5760x1440; for 1 pl or
>       bigger, use 5760x2880.  Don't use a resolution much higher than
>       this; you'll run into saturation problems.
>
>    B) Using the paper of your choice, print a sweep of dark cyan using
>       an appropriate number of patches, and also print a sweep of
>       light cyan using the same number of patches, using Raw color
>       correction.  This can be done with the test pattern generator
>       (this allows you to access the individual ink channels); I'll
>       need to provide instructions.  Then let dry as appropriate and
>       measure.
>
>    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.
>
>    D) Set the Light Cyan Value to the constant, and Light Cyan
>       Transition to 1, and compare a sweep printed with the "normal"
>       cyan (mix of light and dark) against the dark cyan only curve.
>       If you see a dip in the curve centered near the constant, reduce
>       Light Cyan Transition appropriately.  If you see a significant
>       difference overall in the lower part of the curve, that means
>       that your constant is wrong.  If the lower part of the curve is
>       too light, your constant is too high (not enough light ink is
>       being used).  If the lower part of the curve is too dark, your
>       constant is too low (too much light ink is being used).
>
>       For example: if you determine in step C that the constant
>       relating light and dark cyan is 0.25, and you see a dip in the
>       composite curve relative to the dark-only centered around 0.25,
>       look to see where the dip starts.  If the dip starts around 0.2,
>       you want to reduce the light cyan transition to 0.8 (this means
>       that dark cyan ink starts to be used when light cyan has reached
>       0.8).
>
>       If the lower part of the curve is too dark overall, then your
>       constant of 0.25 (meaning that light ink is 0.25 the value of
>       dark ink, so 4x as much light ink is used to produce the same
>       darkness) is too low, and you'll need to remeasure.  Similarly,
>       if it's too light overall, your constant of 0.25 is too high,
>       and not enough dark ink is being used.
>
>       Now repeat the measurement.  If the two curves match, you're
>       done.  If not, make adjustments as described above and repeat.
>
>       The Light Cyan Transition controls where light ink starts
>       transitioning to dark ink.  Generally, setting it to a lower
>       value will produce a smoother and more "accurate" curve, but
>       will yield grainier results, while setting it to a higher value
>       will produce smoother texture but an increased dip in the
>       midtones.  You may find that after you've adjusted per-channel
>       ink limits that you can make the value of this control larger.
>
>    E) If you have three level inks (black/light black/light light
>       black) or quadtone, the basic procedure is the same, except that
>       you should do it pairwise with adjacent inks starting from the
>       darkest inks (black/light black first).  First calibrate black
>       to light black.  When you've achieved good calibration, then
>       calibrate light black to light light black.  When you've
>       achieved good calibration there, combine all three.  You'll need
>       to look more carefully at errors in the curves...
>
> 2) To calibrate drop sizes:
>
>    This needs to be done separately at each resolution you're
>    interested in.  I calibrate drop sizes using black.
>
>    The basic process is similar to light ink adjustment, except that
>    the upper part of the curve (when the ink is approaching
>    saturation) will not yield a linear relationship.  Again, you want
>    to find a relationship that yields the smoothest composite curve.
>    I usually use Ordered dithering for this, but you may want to use
>    Ordered New, which will probably yield better transitions due to
>    the three level dither.
>
>    Again, the test pattern generator provides a way to print sweeps of
>    the three drop sizes separately.


Thanks for a very nice writeup. Some examples with the testpattern
tool will certainly  help. And this kind of documentation should be
include in the users' manual. I am also sure I have more questions. I
just have to read the instruction 10 more times :-). Of course more
information is also welcome.

Lars Tore Gustavsen