Re: 1440 or 2880 dpi what exactly does this resolution represent?

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   From: HJswim2@aol.com
   Date: Sun, 28 Apr 2002 10:15:10 EDT

   Close, but no cigar. With Epsons, the higher, first dpi number
   refers to the vertical axis, the lower number is the
   horizontal.

No, the first number refers to the horizontal DPI and the second
refers to the vertical.  So 1440x720 means 1440 dots per inch in the
horizontal direction (parallel to print head travel) and 720 in the
vertical direction (parallel to paper travel).
	       
	       Increases in vertical resolution are achieved primarily
   by speeding up or slowing down the paper transport (also called
   motor-stepping) so you can pack more dots in a given amount of
   vertical space.

It's not strictly speaking the *speed* of paper transport, but rather
the exact distance that the paper is transported between print
passes.  No current Epson printer actually has nozzles spaced closer
than 1/360" apart, and most of them have the color nozzles spaced
1/120" apart.  However, the stepper motor that positions the paper can
be addressed to 1/720" in most cases (1/1440" on the C80 and probably
the C70, and on the 950 and 2100/2200).

(Yes, the C80 can do 1440 DPI vertically.  Printing the CUPS test
pattern, which includes a circle of radial lines spaced 1 degree
apart, is an excellent demonstration of this.  Under a loupe, the
stair stepping is clearly more evident at 1440x720 or 2880x720
vs. 1440x1440 or 2880x1440.  While the 3 pl dots are too large to take
full advantage of 2880x1440 resolution -- that requires 2 pl drops --
having a more balanced resolution is advantageous when printing very
fine detail.)
		   
		   Horizontal resolution is increased by other
   techniques including variable droplet sizing and droplet
   offsetting, which also relates to motor stepping.

Variable drop sizing has nothing really to do with horizontal
resolution (or vertical resolution, for that matter).  It's simply a
way to print more and smaller dots (hence smoother texture) at low
resolution, improving image quality in highlights without requiring
extreme precision in head or paper positioning (which increases cost
and print time).  At 2880x720, 1440x1440, and 2880x1440, the smallest
(3 or 4 pl) drops are sufficient to completely fill the page even with
glossy paper, even though variable drop size mode has to be used to
accomplish that.

With the exception of the Stylus Pro printers, no single drop size
Epson printer can actually place consecutive dots more closely than
1/360" (or 1/720" in single drop size mode) and no variable drop size
Epson printer can do better than 1/720".  However, as you note, the
positioning for the motor allows addressing to 1/1440" or 1/2880" on
all current Epson printers; only a few very old ones are unable to do
1/1440".  So each row actually has to be printed in multiple passes.
The combination of the multiple passes required to fill the page
horizontally and vertically is called "weaving"; it's discussed
extensively in the Gimp-print documentation, and one of our papers on
it is on Alan Womack's web site.

-- 
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 Gimp Print/stp --  http://gimp-print.sourceforge.net

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