Printing info (and apologies)

[P3D Peter F Davis <pd@xxxxxxxxxxxxx>]

I apologize to the group for again bringing up the subject of printing
and halftoning.  I hope this will be an isolated post, and not start a
long discussion with little relevance to 3D.

I also apologize to John Ohrt, Gregory Wageman and others with whom I
argued about scanning resolution vs. halftone screen density.

The current industry wisdom is indeed to scan at twice the resolution
of the halftone screen density.  So, if you're scanning for a 75lpi
screen, scan at 150ppi.  Likewise, for 150lpi, use 300ppi.  I think
this must be related to the fact that halftone screens are rotated, so
there isn't an even correspondence between image pixels and halftone
dot placement.  By using the resolution doubling, each halftone dot is
the average of 4 image pixels.

I had argued that a 1-to-1 correspondence was appropriate, but I was
ignoring the fact that halftone screens are rotated.  In the typical
case of a black screen rotated 45 degrees, the halftone cell overlaps
Sqrt(2)=1.414... times the width and height of the cell if it were not
rotated.  I believe this is why 1.5 to 2 times the screen density is
generally recommended.

As for determining the screen density, this is usually set in
software, but should probably be somewhere between 1/8 and 1/16 of the
physical resolution of the output device for photographic output.  For
a 600dpi printer, 1/8 is 75 lpi, which yields 65 gray levels
... barely adequate.  1/16 would be about 37 lpi, giving 256 gray
levels.  Of course, the lower the screen density, the coarser the
dots, so it's a trade-off.

Again, apologies for taking up so much of your time.

-pd

--------
                                Peter Davis
                        http://world.std.com/~pd

            "If you can't behave yourself, who can you behave?"


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