Re: Tell me again

["Thomas Michanek" <thomas.michanek@xxxxxxxxx>]

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From: "Doug" <forwardtcm@xxxxxxxxxxxx>
> What's the best DPI setting to use when inserting graphics, when the
> document is meant for printing to PDF and from there to paper?  144?

If it doesn't matter what the (bitmap) image looks like on-screen
in FrameMaker or in Acrobat, then the only thing you need to
consider is the resolution of the output device (the printer).
A bitmap image consists of pixels in different colors, and the
printer needs to use a certain number of "printer dots" to
represent each pixel (a "square" of X times X printer dots).
For a black-and-white laser printer, the pattern of printer dots
in the "square" is used to give the impression of different levels
of gray, representing colors. This process is called "halftoning"
and the square is called a "halftone cell".

For the image to be printed without any distortions, the printer's
dot resolution should be an even multiple of the image's pixel
resolution. So, for a 600 dpi printer, you should use import DPI
values of 300, 200, 150, 120, 100 or 75.

If you use 100 dpi for a 600 dpi printer, each pixel in the image
will be represented by 6 x 6 printer dots (600 / 100). So for
each pixel, the printer can make a pattern of 36 printer dots
to represent the particular color or grayscale of that pixel.

If you use 150 dpi, 16 printer dots will be used (4 x 4) for each
pixel. The image will then become smaller, but at the same time
a smaller number of colors can be represented and the image may
appear "darker" or less detailed. There's a trade-off between
size and grayscale/color resolution.

If you have a color printer, things become a little more complex,
but the basic principle of "even multiple" is still true.
However, for modern printers with more than 600 dpi resolution,
the need for "even multiple" is less important. For an old 300
dpi printer, you'll definitely will see a difference.

This was probably more than you wanted to know :-)

____________________________________________
Thomas Michanek, FrameMaker/UNIX/MIF expert
Technical Communicator, Uppsala, Sweden
mailto:Thomas.Michanek@xxxxxxxxx
http://go.to/framers/
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