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[filmscanners] RE: Printer drivers at 720ppi



> From: robg@wordweb.com
>
> I was on the Epson list for a long time and this is the first time I've
> heard someone make this statement about the internal behaviour of
> the Epson
> driver.  Can I ask where this knowledge comes from?  In the past I found
> a formula written by Epson themselves which gave the optimal ppi
> for photos
> as 240ppi.

It's easy to prove for yourself. Take this test image:

        http://www.pbase.com/image/22493977

which consists of alternating 1-pixel black and white lines, at 720ppi, and
print it out. It will look gray. Print it at 1440, and one half will look
white, because it skips the black lines, and the other half will look black,
because it skips the white lines. Print it at some odd size, like 730, and
you'll see nasty alias bars.

> I'm also wondering how the figure of 720 ppi you state above is unaffected
> by the model of the printer.  The old Epson 3000 was only "720 dpi", and
> most of the Epson printers are "720 dpi" in one direction.  But some of
> the recent printers are "2880 dpi" in one direction.  Does the
> magic number
> still apply to the most recent printers?  It sounds surprising
> that it would
> apply to every Epson ever made.

I've seen an official Epson document that says that its desktop printer
drivers resample to 720ppi, and the wide format printers resample to 360ppi.
(I don't recall the link, though.) Then, the dithering algorithm generates
the actual dots from these pixels at whatever dot pitch the driver is set
for.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@ix.netcom.com

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