Thanks for the note.
An imagesetter would need a 16 x 16 matrix for 256 tones. If the imagesetter is set to a native resolution of 1800 dpi, then the file would have to be 112.5 ppi = 1800/16. If it is set to 2450, it would be 153.125 ppi or lpi = 2450/16.
This is assuming they aren't using some algorythm with a variable dot size.
This is why I was confused with Jon's note regarding a comparison of the different screens:
" The basic difference between
the finer one I use and Dan's/Elizabeth's is that the pattern with
theirs is visible to the naked eye. Mine just looks like a dark grey
translucent screen. I'd guestimate theirs to be somewhere on the order
of 150-200 dpi compared to the 1800-2540 dpi screens I've been using. "
Jon, If you have Peter Elsey's current email address, could you send it to me off list?
Precision Digital Negatives - The System
PDNPrint Forum at Yahoo Groups
In a message dated 5/6/07 12:03:30 AM, firstname.lastname@example.org writes:
IF they're using it properly, i.e. creating a stochastic screen function
and then just telling the imagesetter to output a box so many points
high by so many points wide, filled 80%, then the resolution is device
dependent, and is determined by the RIP, accounting for the matrix size
and rotation. For a 256 gray scale at 0 degrees (typically used), it's a
4x4 matrix, though the rotation, if used affects the matrix size.
There's a good explanation of Postscript halftones in the Postscript
reference books, I think the blue book, though it's been years (~20)
since I've written raw Postscript, so I may be mistaken. The reality is
likely that they're doing it from within a graphics program, and unless
they're intimately familiar with Postscript (and I've only met 3 or 4
people that care enough to be) the likelihood is the graphics software
is outputting a 600 ppi image (Illustrator, in particular is pretty bad
about this, since version 7, as I recall), as opposed to the code to
create a stochastic screen function at the raw imaging unit resolution.
(yes, at one time I did that sort of stuff, and had 2 Lino 330s of my own)
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