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[filmscanners] RE: Printer drivers at 720ppi

> From: Berry Ives
> I printed the "Even/Odd" test print 3 times, including 360, 720, and 1440
> dpi settings on the printer driver.  (On all of them, "finest detail" was
> also checked.)  This was on an 1160.
> I don't know what the odd and even means.
> The 1440 "even" came out best, meaning that there was the smoothest gray
> texture of very fine lines.  The "odd" half of that print had the
> appearance of a darker line repeating once every 12 or 13 lines.
> On the 720 dpi, there were darker lines irregularly spaced on
> both the even
> and odd parts of the image, but not identically on both the even and odd
> parts.
> On the 360, the image width shrank to 4 cm, compared to 12.5 cm
> for both the 720 and 1440.  Also the height was extended from 12.5 to 16
> I don't know what any of that means.  All I know is that the 1440
> definitely looks the best.
> Can you shed some light on all this?

First of all, there are two resolutions in question. One is the image
resolution, in pixels per inch, and is determined for instance in Photoshop
by the Image->Image Size dialog. As downloaded, the test image says 720ppi,
which means it should print 1" square. Second, there is the dithering
resolution, in dots per inch, which is set within the driver. This is
completely different and pretty much unrelated. When the driver receives an
image from Photoshop (or any other program), it first converts to 720ppi by
"dumb" sampling (a.k.a., nearest neighbor), and then the dithering algorithm
converts this into chaotically scattered dots at whatever dot resolution
you've selected. The whole issue is the interaction between the 720ppi that
the driver initially resamples to, and the actual resolution specified in
the input file.

"Even" and "odd" refer to whether it's the even or odd pixel rows that are
black. If you print it as-is on a desktop machine, it should correctly
render the fine black and white lines, resulting in gray. If you change the
resolution in Photoshop (or whatever) to exactly 1440ppi (note I'm _not_
talking about the dot pitch in the driver), then the driver will grab, say,
only the even lines, in which case the even side will come out black and the
odd side will come out white. (Or perhaps the other way around, depending
upon the precise position on the page.)

If you see coarse bands of dark and light, then it means that you must have
printed at some other resolution. As I said, the image should print 1"
square at 720ppi, or 1/2" square if you bump it up to 1440ppi. If you're
seeing sizes like 4cm or 12.5cm, then you're obviously blowing the image up
considerably, perhaps as a result of some automatic resizing in the Print
dialog box. The coarse dark and light bands you see are the direct result of
the fact that the resolution you're ending up printing at does not divide
evenly into 720. Let's say you print at 100ppi, which would result in a 7.2"
square image. This means that each pixel in the image must be replicated 7.2
times in each dimension, to get from 100ppi up to 720ppi. Instead, in each
tenth of an inch, it would replicate each line something like the following:

        7 white pixels
        7 black pixels
        7 white pixels
        7 black pixels
        8 white pixels
        7 black pixels
        7 white pixels
        7 black pixels
        7 white pixels
        8 black pixels
     72 pixels total

The extra white line and black line are called aliases in mathematical
terms, or moire in visual terms. They don't occur at all if the resolution
divides evenly into 720.

So that's why it's sometimes desireable to use a superior resampling
algorithm, such as Photoshop's bicubic, to force the image to be an exact
submultiple of 720 before printing. Fortunately, few images contain the sort
of regular, fine detail that causes moire, so in most cases you don't have
to worry about this.

> P.S.  BTW, you have some very nice art on your web site.  All the
> ones I saw
> so far were with the 10DS.  Do you shoot film also?  How large do
> you print from the 10DS?

Thanks for the compliment. I've printed some 10D stuff at 12"x18", with good
results--that's about 180ppi. I have some other photos, including some old
scanned slides and negs, at


but I've pretty much moved on to digicams.


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

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