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MathGroup Archive 1991

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  • To: mathgroup at yoda.ncsa.uiuc.edu
  • From: shawn
  • Date: Tue, 19 Feb 91 09:42:05 -0600

I would like to make a few remarks.

  * The notes that I sent out are quite old, I hope they are still useful.

  * There is some confusion, I think, between encapsulated PS files and
conforming PS files.  The EPSF format was invented on the Mac.  It simply
placed PS source in the data fork, and a PICT preview image in the
resource fork of the file.  Since then, EPSF has come to mean attaching
a preview image to PS source so that an enclosing program can show an
approximation of the image on screen easily without interpreting the PS.
Adobe has suggested a standard for including a preview image as PS 
comments near the beginning of the file.
However, EPSF has become to more loosely mean "PostScript".  That is,
when an enclosing programs like say page layout programs say that they
support EPSF, they mean that you can designate a box on the page in
which some PS gets rendered.  If a preview image is available in the file
you will see on your screen an approximation of printed image.  If the
preview image is missing you get an empty or gray box, but you still get
a nice image on paper. 

  * The perhaps more important issue is conforming PS.  A PS file is 
said to be conforming if it makes suitable declarations to an enclosing
program about its contents so that the enclosing program can do something
reasonable without interpreting the PS itself.  (The declarations are made
as PS comments, which of course a PS interpreter ignores.) These comments
allow the creator of the PostScript to include such information as the
bounding box inside which ink will be laid down (which presumably the
creating program or person knows); when the PS for the current page ends,
and a new page begins, and other useful things.  This allows the enclosing
program to say scale and translate the coordinate system before sending
the included PS off to the printer; or print pages in reverse order; or
print thumbnail pages, &c.

* What psfix emits is minimally *conforming*  postscript, not encapsulated
PostScript.  After all, psfix is just a shell script.  To include a preview
image, it would have to render the postscript and grab the resulting
bitmap -- a tough job for a shell script.  (Although it could ask for help
from sunps, or x11ps, &c.--it doesn't.)  What it does is include a *few*
conforming comments for enclosing programs and a prolog containing
definitions that make the PS from Display[] understandable by a 
standard PS interpreter.  The bounding box comment is all that a
page layout program needs; preview comments would be nice a nice extra,
but are not provided by psfix.

* The enclosing program is supposed to redefine showpage in the scope of
the included PS to do nothing or something reasonable.  Whether a file is
conforming PS or encapsulated PS has nothing to do with showpage.

--Shawn Sheridan


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