Re: Questions on fundamentals

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I speak from the perspective of someone who's actually written (or
rather, very extensively modified and adapted) drivers for a lot of
Epson inkjets.  Basically, all of the Epson inkjets (and, for that
matter, most if not all of other the low-end, high quality inkjet
printers) are completely dumb raster devices.  You give the printer
input dots and it lays 'em down.  The programming's actually a bit
more complicated than that, because the dots have to be presented in a
rather funky order (at least on Epson printers), but the rendering is
done precisely for each and every dot.

   Date: Wed, 14 Jun 2000 06:31:13 +0100
   From: "Michael D. Garrett" <garrettmd@chartertn.net>

   Regarding the fundamental problem of representing many tonal variations on
   4color printer with basically two states (on/off) per color:

A lot of modern inkjets can represent considerably more than two
states, with variable dot sizes and light magenta and light cyan inks,
but the general principle remains -- the number of levels needed to
represent the tonal range far exceeds the number of states per dot
position.

   1) Do halftone concepts apply to inkjet printing (without an interceding
   RIP)?

The Windows driver doesn't appear to (visual inspection), and the
driver I work on (a free software/open source one for the Gimp and
Ghostscript) doesn't use halftoning.  A driver could certainly do that
if it wanted to, although with a maximum resolution of 1440x720 dpi,
the screen would be quite coarse.  What happens instead is a dither is
used to achieve an appropriate dot frequency while spacing the dots as
evenly as practicable to achieve a smooth appearance.

I guess this depends upon what you mean by "halftone concepts" --
certainly tonal variation is accomplished by varying the amount of ink
deposited on the paper, but it's not done by varying the size of the
drops (there may be a limited number of choices of dot sizes, which
dramatically improves quality, but there's not the smooth variation
that halftoning does).

   2) if halftone dots are not generated from the pixel image, how is tonal
   variation simulated?

   a) how is a 300 pixel/inch image transformed into a 720 dpi print?

Usually by "sampling" the image at 720 dpi and dithering each point.
This, of course, means that each image pixel can be sampled more or
less than once.  An individual driver could do what it pleases; it
could interpolate before dithering, for example.

   b) if I send a photo scan to an inkjet printer with default print settings,
   does it basically dither the pixels.
   c) Do I have control over the dithering algorithm?

That depends upon the driver, or you could write your own to use any
dithering algorithm you please.  The gimp-print driver does offer an
explicit choice of dithering methods, partly because it's still under
development.

   3) I've read that inkjet printers 'require' RGB input. That they have
   conversion tables optimized for the printer.

And sometimes for the paper, and the particular inks, and the phase of
the moon :-)

   a) Where is that conversion done, in the driver or in the printer?

Driver.  As I stated above, the printer has no intelligence.  We're
looking at allowing CMYK input to the gimp-print driver, but it won't
be that useful right away because there really isn't much if any
software under Linux/UNIX that generates CMYK separations.  But Ian
Young (also on the list) has talked about a Photoshop plugin based on
the gimp-print software that *would* use CMYK input.

-- 
Robert Krawitz <rlk@alum.mit.edu>      http://www.tiac.net/users/rlk/

Tall Clubs International  --  http://www.tall.org/ or 1-888-IM-TALL-2
Member of the League for Programming Freedom -- mail lpf@uunet.uu.net
Project lead for The Gimp Print --  http://gimp-print.sourceforge.net

"Linux doesn't dictate how I work, I dictate how Linux works."
--Eric Crampton
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