"Background info about Printing" FAQ
In which forum should I ask general printing related questions?
In this OpenPrinting forum:
General Discussion. This is
the successor of the same named forum previously hosted at
openprinting.org.
What is PostScript?
PostScript is a programming language which specializes in placing
text and images on paper. It is the most accurate method for laying out
text, images and graphics a page.
Does GNU/Linux use PostScript?
Yes. Traditionally, PostScript is created by most applications by default
whenever they are asked to print.
Does MS Windows use PostScript?
Not directly. Windows internally uses a different format for page
descriptions of printable documents. To print to PostScript printers,
Windows uses a special program to convert documents: it is called a
"PostScript printer driver".
Does Apple use PostScript?
Yes. Apple operating systems use PostScript for printing. Apple even
experimented with "Display PostScript" on the Macintosh in 1984 in order
to draw the user interface elements on screen.
Is PostScript "free"?
The PostScript language is described in an open specification created by
Adobe Inc.; it is well defined and free to use by anyone who so desires.
Printers that do not understand (standard) PostScript tend to speak one
of many different "languages", most of them proprietary and not openly
documented at all.
What are the advantages of PostScript?
PostScript is a fullblown programming language. That allows a lot of
flexibility. Features can be implemented to do just about anything before
sending the job off to the actual printing device. These include:
Changing fonts, printing 2,4,8, or 16 pages per sheet; "inverting" the
colors; clipping, scaling, warping, rotating pages; landscaping/portrait
printing; and much more.
What are the disadvantages?
PostScript printers are more expensive than standard consumer printers.
Most consumer printers currently do not support PostScript, and very
likely never will.
What exactly is a PostScript printer?
A PostScript printer can read and understand valid PostScript code send
to it. By "interpreting" this code, it translates the data into raster
images that are put on sheets of paper. PostScript printers ship with
builtin PostScript "interpreters" (a.k.a. "Raster Image Processors").
Can all printers print PostScript?
No. However, PostScript in principle is easily converted to whichever
printjob format you printer consumes. A "printer driver" is doing this
conversion. (In the Unix and the GNU/Linux world we usually call this
conversion "filtering" and the program doing so a "filter".)
Can you name a common printer driver program used on GNU/Linux?
The most famous such program is called "Ghostscript". Ghostscript is
unique in that it embeds in itself many different filters (or printer
drivers) at once, and can convert PS to many different device specific
formats. Hence, Ghostscript can drive many non-PostScript devices even
if most applications only know to produce PostScript for print files.
So how can my non-PostScript printer print PostScript files?
By using an "external" PostScript interpreter, implemented in software,
hosted on your workstation or on a print server. This interpreter can
automatically run specific filters/drivers as needed. CUPS comes with a
builtin PostScript interpreter based on Ghostscript.
Do PostScript printers need filters or drivers?
Yes and No.
"No", not for basic printing. PostScript printers should be able to print
PostScript files directly, without any extra filter or driver. (Caveat:
older "Level 1" devices may not print "Level 3" PostScript files.)
"Yes", if you want to use advanced print job control (printing on both
sides of a sheet; punching the output; make the printer do inline
stapling to create a pamphlet job; use a differently colored paper or a
letterhead sheet for the first page;...), a "PostScript-to-PostScript"
filter needs to add specific job control snippets (PostScript or PJL
code) inside the print file before it goes to the printer.
Do non-PostScript printers need filters or drivers?
Yes, always.
There are various driver families available on GNU/Linux. They all offer
drivers to convert PostScript to whatever various printers need.
- CUPS builtin drivers (not yet mentioned, developed by Easy Software Products), available under a Free Software license.
- Ghostscript (mentioned above, developed by artofcode LLC), available under a Free Software license.
- HPIJS/HP Inkjet (not yet mentioned, developed by HP), available under a Free Software license.
- Gutenprint (not yet mentioned, evolved from Gimp-Print, developed by a small community of people with sophisticated color printing needs), available under a Free Software license.
How do I know if I have a PostScript printer?
If you did not explicitly seek out PostScript printing as a feature, it
is likely you don't have it. (If in doubt, send a small PostScript
testfile (such as /usr/share/cups/data/testprint.ps) to the printer:
if it doesn't print at all, or if it prints only some pages of ASCII, it
is not one.)
How can I identify a suitable printer driver for my model?
Here is a list of models by manufacturer:
printer list on openprinting.org
(or, if you need the exact URL on your printout of this page:
http://www.openprinting.org/printer_list.cgi).
Each model entry on this list also names the driver(s) recommended or
useful for that device.
What does PPD stand for?
PPD is short for PostScript Printer Description. A PPD is just a text
file which exactly describes (in a well-defined format) all supported
features of a printer and contains the exact code that needs to be
injected into the printjobs to invoke each desired feature.
How do PPDs work?
Print software programs can read PPDs, and know how to display the
supported features in the printer driver's user interface shown on
screen. If the user makes specific choices (like duplex, stapling) the
program will know what specific code to inject into the printjob and
how to work with print device. (Note: the exact code to print a
job as "duplex" {i.e. both sides of a sheet} may be different from one
model to the next.)
What features does a PPD list?
A PPD describes all capabilities of a PostScript printer. This includes
available fonts (built into the printer), duplexing capabilities, media
sizes handled, non-printable margins for each media size, available
resolutions, installed and so on.
Can I use any vendor PPD that describes my PostScript model?
Usually yes. Two points to take care off:
- Using one that is labelled "for Apple operating systems" may not work on GNU/Linux!
- Many vendors still do not label their PPDs for Linux suitability; however those suitable for "MS Windows NT" usually work just fine for us (GNU/Linux with CUPS).
Where do I get a PPD file?
PPD files are available from many places:
- openprinting.org
- Manufacturer websites
- Driver disks
Most current Linux distributions ship a big selection of PPDs already on
their installation media too.
Can I use any PPD file for my printer?
The PPD should be made for your printer model, or for one similar to
yours.
I've heard that CUPS does use PPDs also for non-PostScript devices. Is that true?
Yes. Since CUPS acts as a PostScript interpreter for all supported
non-PostScript models, it is natural to use PPDs (and their well-defined
file format) to control all aspects of a printing device. CUPS extended
the original PPD specification created by Adobe in that it adds
information about which specific filter to use for the target device that
converts PostScript to printer-specific raster or otherwise data. (The
filter information is contained in the PPD in a way that does not disturb
other PPD consuming programs.)
The PPD is missing a feature. Can I edit it by hand?
In principle, yes. But if you are not a PPD expert, we do not advice you
to do so. Programs are very sensitive to the structure and exact syntax
of the PPD file; making manual changes can cause reading the file to fail.
If you want to edit it anyways (quite a few people have done so
successfully), make a backup first. Know where to look for the PPD file format specification.
This name alone sounds scary; what is it?
Ghostscript is a software that interprets the PostScript (instruction)
files. Ghostscript is able to translate these instructions (which mostly
describe how the visual appearance of pages should show images, text and
graphics) into other "printer languages".
What does it do?
Ghostscript converts PostScript files into raster "pictures" or commands
that your non-PostScript printer can understand. The PostScript language
is described in an open specification created by Adobe Inc.; it is well
defined and free to use by anyone who desires. Printers that do not
understand (standard) PostScript tend to speak many different
"languages", many of them proprietary and not openly documented at all.
How many different languages are there?
Many. Virtually each one of the major vendors has developed his own
printing language. See for example [ this list]. Some manufacturers
support not only their own language, but also that of one or more of the
competitors.
What are some of the other printer languages that vendors have invented?
For a nearly exhaustive list, look at the website of Undocumented Printing.
The most commonly known ones are:
- AFPDS (IBM)
- PCL (HP)
- HP/GL (HP)
- ESC/P (Epson)
- RPCS (Ricoh)
This is very complicated!
If you want to understand every aspect of printing and print file
conversions, yes. In practice, most current Linux distributions nowadays
do a good job to support printing for many different models, and
automatically configure the printing software with little user input.
Many people never need to know more than the name of their printer to
setup their printing during install time.
However, there are still more than enough cases, where this process does
not work as desired (especially with printing hardware that is not
supported for Linux by their vendors). We are working hard to make
improve that user experience even more.
Direct printing vs. Spooling
What is direct printing?
Sending print files directly to the printer, without buffering or
filtering.
What are the disadvantages of direct printing?
Most printers usually can take and process only one print job at a time.
In multi-user environments with shared printers, allowing every user to
access each printer directly very frequently leads to "clashes" (the
same is true even for a single user printing multiple jobs in short
intervals).
In the best case, users have to wait for a previous job to be completed
by the printer. Until this is done, his current "directly printing" job
is waiting and blocking his other intended actions even to the point of
freezing his current user interface.
What is spooling?
Spooling is said to mean "Simultaneous Peripheral Operation On Line".
Spooling lets all users and applications hand print jobs very quickly to
a special program (or the operating system). This program first buffers
the jobfile (usually on a special directory on the harddisk) and then
handles all communication with the printer. It patiently waits and
"queues the job" until the printer is free again, should it be busy with
another task. After handing a printjob to the "spooler", the user
interface is ready very quickly again for the user's next task.
Are there different spooling software applications?
A few years ago it was different -- but nowadays most GNU/Linux
distributions have been standardizing on the
Common Unix Printing System which comes with a
builtin spooler. However, should you want to use a different spooling
software -- (luckily) openprinting.org has documented the most important
ones of them.
How does printing work with a spooler? Do I need to know any specifics?
No. Not unless your job is to troubleshoot a printing problem.
Generally, the spooler uses various helper programs to accomplish his
jobs (filters, backends). The sequence is this:
- Your program passes a print job (in most cases a PostScript file) to the spooler (in most cases to CUPS) once you have selected the printer (in case you have a choice of several).
- The spooler determines if the job needs any filtering. If so, it runs the filters on the job as required. (If the file was not PostScript, the first filter invoked may be one that converts the input into PostScript in the first place.)
- Finally, the spooler contacts the print device and sends the file off (either PostScript "as-is", or PostScript stuffed with specific job control codes according to your preverences, or even converted to a different-from-PostScript printer format) as soon as is ready to be received.
Sophisticated installations may involve user authentications, print
quota, job prioritizations and more. All this is handled by the spooler
and does not need any specific user attention once your printers are
correctly installed.
Whom should I contact for suggestions with this FAQ?
If you are sure about any correction or addition you may want to make,
feel free to edit this Wiki directly. If you want to discuss it first,
write to Kurt Pfeifle (kpfeifle AT danka DOT de).
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