Ghostscript is
an incredibly significant program for Linux printing. Most printing
software under Unix generates PostScript, which is typically a
$100 option on a printer. Ghostscript, however, is free, and
will generate the language of your printer from PostScript. When tied
in with your PDQ printer driver declaration or lpd input filter,
it gives you a virtual PostScript printer and simplifies life
immensely.
Ghostscript is available in two forms. The commercial version of
Ghostscript, called Aladdin Ghostscript, may be used freely for
personal use but may not be distributed by commercial Linux
distributions. It is generally a year or so ahead of the free
Ghostscript; at the moment, for example, it supports many color
inkjets that the older Ghostscripts do not.
The free version of Ghostscript is GNU Ghostscript, and is simply an
aged version of Aladdin ghostscript kindly given to GNU. (Kudos to
Aladdin for this arrangement; more software vendors should support
free software in this way, if they can't handle full-blown GPL
distribution of their code).
Whatever you do with gs, be very
sure to run it with the option for disabling file access
(-dSAFER). PostScript is a fully functional language, and a bad
PostScript program could give you quite a headache.
Speaking of PDF, Adobe's Portable Document Format is actually little
more than organized PostScript in a compressed file. Ghostscript can
handle PDF input just as it does PostScript. So you can be the first
on your block with a PDF-capable printer.
Typically, Ghostscript will be run by whatever magic filter you settle
upon (I recommend apsfilter if your vendor didn't supply anything that
suits you), but for debugging purposes it's often handy to run it
directly.
gs -help will give a brief informative listing of options and
available drivers (note that this list is the list of drivers compiled
in, not the master list of all available drivers).
You might run gs for testing purposes like:
gs options -q -dSAFER -sOutputFile=/dev/lp1 test.ps.
There are a number of things one can do if gs's output is not
satisfactory (actually, you can do anything you darn well please,
since you have the source).
Some of these options, and others described in the Ghostscript User
Guide (the file Use.htm in the Ghostscript distribution;
possibly installed under /usr/doc or /usr/share/doc on your system)
are all excellent candidates for driver options in your PDQ driver
declaration.
Output location and size
The location, size, and aspect ratio of the image on a page is
controlled by the printer-specific driver in ghostscript. If you find
that your pages are coming out scrunched too short, or too long, or
too big by a factor of two, you might want to look in your driver's
source module and adjust whatever parameters jump out at you.
Unfortunately, each driver is different, so I can't really tell you
what to adjust, but most of them are reasonably well commented.
Gamma, dotsizes, etc.
Most non-laser printers suffer from the fact that their dots are
rather large. This results in pictures coming out too dark. If you
experience this problem you should use your own transfer function.
Simply create the following file in the ghostscript lib-dir and add
its name to the gs call just before the actual file. You may need to
tweak the actual values to fit your printer. Lower values result in a
brighter print. Especially if your driver uses a Floyd-Steinberg
algorithm to rasterize colors, lower values ( 0.2 - 0.15 ) are
probably a good choice.
It is also possible to mend printers that have some kind of colour
fault by tweaking these values. If you do that kind of thing, I
recommend using the file colorcir.ps, that comes with ghostscript (in
the examples/ subdir), as a test page.
For many of the newer color inkjet drivers, there are command-line
options, or different upp driver files, which implement gamma and
other changes to adapt the printer to different paper types. You
sould look into this before playing with Postscript to fix things.
Color Printing in Ghostscript
Ghostscript's default color dithering is optimized for low-resolution
devices. It will dither rather coarsely in an attempt to produce
60ppi output (not dpi, ppi - the "apparent" color pixels per inch you
get after dithering). This produces rather poor output on modern
color printers; inkjets with photo paper, in particular, are capable
of mich finer ppi settings.
To adjust this, use the Ghostscript option -dDITHERPPI=x, where x is
the value to use. This may or may not have an effect with all
drivers; many newer drivers implement their own dithering and pay no
attention to this setting. Some drivers can use either the regular
Ghostscript or driver-specific dithering.
This makes for an excellent argument in a PDQ driver declaration, if
it applies.
