[Openicc] Drop size calibration

[Robert Krawitz]

   Date: Mon, 28 Jan 2008 23:31:45 +0100
   From: Gerhard Fuernkranz <nospam456 at gmx.de>

   Hal V. Engel wrote:

   > I suspect that drop size calibration has a lot in common with
   > calibrating the light/dark ink transitions in CcMmYKk type
   > printers.

   Btw: This statement, which brings drop sizes and light/dark
   transitions into relation, reminds me on another approach I have
   seen somewhere in the literature. This approach does the dark/light
   splitting in the dithering engine: It treats the e.g. 16 dot-on-dot
   overprint combinations of the three dot sizes of "C" and the three
   dot sizes of "c" as if they were 15 drop sizes of a single virtual
   cyan channel. Then these 15 virtual drop sizes are sorted by
   density, and dithering is done between the resulting 16 virtual
   levels. A requirement for this approach is of course that the hues
   of the dark and light inks must match sufficiently well, i.e. the
   dark inks should be basically the same colorant as the light ones,
   but at a higher concentration. I have no idea though, how well this
   approach works in practice...

Gimp-Print 4.2 did this, but we abandoned it in Gutenprint 5.0.  It
worked quite well for single drop sizes where the required base
density was high (older printers), but it doesn't work very well (at
least the way I implemented it) with variable drop sizes.  It's less
useful with really tiny drop sizes, where even the dark drops aren't
particularly visible.  The problem is ink loading: a smaller virtual
drop size may actually deposit more ink, and that resulted in either
having to set the base density low or getting washed out dark colors
(or worse!).

What we did in practice was use only one virtual drop size at any
given point (i. e. no overprinting), and we'd eliminate some of the
virtual sizes (e. g. large drop size light ink, and/or small drop size
dark ink depending upon the relative drop sizes and darkness).

-- 
Robert Krawitz                                     <rlk at alum.mit.edu>

Tall Clubs International  --  http://www.tall.org/ or 1-888-IM-TALL-2
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