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I purchased an Apple LaserWriter laser printer in June, 1985, which has
kept me much busier than I expected. I had a preliminary <PLOT79>
interface for it working in less than twenty-four hours, and by
rerunning all of the demonstration programs as a test of the FORTRAN 77
version of PLOT, I was able to check that out as well as polish the
interface. Because the LaserWriter supports line intensity, line
styles, and line widths, the device driver takes advantage of all of
these for intensity, pen, and width simulation.
The LaserWriter marks a revolution in the printing industry, in that it
is driven by an on-board controller which translates the powerful
PostScript language sent from the host into operations on the page
bitmap. PostScript was developed by Adobe Systems, Inc., in Palo Alto,
CA, who have expressed a strong desire to make it the industry standard
for controlling printers. PostScript is also used by Mergenthaler for
one of their phototypesetters, and by QMS for a laser printer.
A page described in PostScript is normally independent of output device
resolution, unless it contains bitmap images, but even then, the
PostScript translator is capable of arbitrary 2-D transformations on
all graphical objects, and will scale the image if necessary. For this
reason, the <PLOT79> interface describes pictures in higher resolution
than is necessary for a 300-dot/inch laser printer; if they are output
on a phototypesetter, sharp images will still be obtained.
Although the <PLOT79> device interface to PostScript was reasonably
straightforward, developing a TeX DVI translator for it turned out to
be much more difficult. Apparently, changes have been made to the
PostScript language since the manual was published, and consequently,
none of the manual examples illustrating downloading of fonts worked.
It took some telephone calls to find out that revised manual pages have
been prepared, and with several days of work, I was finally able to get
a working DVI driver (<PLOT79.TEX.DVI>DVIALW.C).
Had these documentation errors not existed, and considerably more
examples been provided for the very complex PostScript font mechanism,
this work should not have taken more than a day. I believe that the
DVI driver is complete and correct, but the Apple LaserWriter still
periodically raises ``VMerror'' messages (the kind the manual says
should not occur) and flushes the rest of the document.
The X-ON/X-OFF flow control bug in PostScript Version 23.0 which is
noted in Apple documentation, and will not be fixed until new ROMs are
prepared, has also been rather troublesome, since its occurrence causes
loss of the entire input stream. On several nights, we lost scores of
files this way. By tedious experimentation, I was able to develop a
procedure implemented in <PLOT79.TEX>LW78.C for driving it reliably,
and apart from the ``VMerror'' failures, we no longer lose files.
LW78.C is very dependent on TOPS-20, mostly because misfeatures of the
PCC implementation made it impossible to use standard Unix library
routines. It can still serve as a model for other sites, and once the
X-ON/X-OFF bug is fixed, it should be possible to remove the frequent
time-out pauses.
The <PLOT79> device interface to PostScript has been designed to
identify the section of the plot file which actually contains plotting
commands, and DVIALW uses this information to allow the TeX
``\special{}'' command to request the insertion of plots into typeset
manuscripts.