20041117-001
Common
PDLs, Print Streams & Technologies
This
document attempts to explain popular PDLs, (Page Description Languages),
or more simply “Printer Languages” and the different technologies available
that you may run into in your setup. A PDL is a protocol used to control how
text and images are formatted into a printable image, which is then printed by
the printer. In most cases, the printer uses the PDL to render the printable
image. The printable image is made up of rasters, or scanned lines of
spots or dots and the process is
called RIP, or Raster Image Processing. The PDL usually outlines
a set of print controls to format the data and control the print job, which are
then combined into a print stream, being the PDL commands and the data, sent
together to the printer.
However, the vocabulary-term line
between PDLs and print streams is becoming more and more blurry. PDLs such as
Postscript and PCL are now commonly used in the same arena as print streams
such as IPDS, AFPDS, Metacode and LCDS. Complicating it further, many people
refer to their PDL or print stream by the overall architecture name and
vice versa. IBM’s AFP is commonly
referred to as a PDL, however it is really an entire system that can use
multiple PDL and print stream formats within. Xerox’s Metacode is referred to as a print architecture as well
as a PDL/print stream. Finally, to make it even more confusing,
long-established document and image formats such as PDF and TIFF
are now also being referred to as print streams, because they can be sent to
intelligent printers in much the same way as a traditional PDL/print stream.
The following is a list of PDLs, Print Streams and Data Formats (for lack of a
better term), which you may encounter in your printing adventures.
·
Interpress – Developed by Xerox in the early
1980’s, Interpress was a PDL for use with the Xerox printer line. It was not
widely adopted and failed to flourish. Interpress was the precursor to Postscript.
·
Postscript – Two developers of Interpress
left Xerox to form Adobe, where they developed Postscript. Driven by
early Apple printing and typesetting environments in the 1980’s, Postscript has
grown by leaps and bounds over the last twenty years and is the leading format for
high-end printers from today, regardless of the computer environment. It is
actually a programming language which formats text and images into a printable
bitmap image. The computer sends the Postscript program to the printer where it
is interpreted and the images are generated. Postscript is widely supported on
Apple computers, UNIX workstations, and Desktop PCs, and inside most high-end
printers as well as a growing number of mid-range printers. It is generally
considered by the majority to offer better graphic output than it’s end-user
counterpart, PCL, found in low-end desktop to mid-range office printers which
typically do not support Postscript. Recent developments in Postscript (Level)
3 have allowed Postscript programs (documents) to also make use of variable
data, executing inside the printer itself. It continues to flourish in the
professional printing industry, partly because of the wide acceptance of it’s
cousin, PDF.
·
PCL – Hewlett
Packard was developing PCL, or Printer Control Language for their
printers about the same time Postscript was being developed. Like Postscript,
it is actually a programming language, interpreted in the printer. PCL is most
widely used in low to midrange laser and inkjet printers. It’s popularity
exploded with the continued growth of the HP LaserJet line from the mid 1980’s,
which has set the standard for other desktop printers up to the last few years.
Most all desktop printers either support or use PCL as their native PDL, though
many have begun to include support for Postscript Levels 2 and 3 as well, due to
the stagnation in PCL development at HP, and the need for higher quality bitmap
processing and variable data capability. PCL still executes faster than
Postscript, though the size of the PCL program generated is roughly three to
four times as large as it’s Postscript counterpart of the same data.
·
AFP – Advanced Function Presentation, developed
by IBM for use with their high-speed printers on mainframe systems, is the
architecture for how the print process is controlled from the initial print
command to the output. While technically not a PDL, AFP controls the print
resources, programs and output sent to the printers. Within AFP, either IPDS or
AFPDS are used. It is important to determine which one when working with
AFP-processed data:
o
IPDS – Intelligent Print Data Stream is a
bi-directional data stream used by AFP to drive printers. It can use channel,
TCP/IP or SNA protocols to send print data to the printers, and manage the
print process, from printer resources to security to error recovery.
Ultimately, IPDS will be used at the IBM printer.
o
AFPDS – Advanced Function Printing Data
Stream is a page presentation format used by IBM software, and it
is pre-IPDS meaning a temporary format used by the mainframe’s Print Service
Facility (PSF) before conversion to IPDS. PSF is similar to a printer
driver, in that it collects and manages the resources needed, converts AFPDS
into IPDS for a specific printer, delivers the print stream and handles error
recovery. AFPDS contains all the necessary information to render the pages, and
is commonly used as an archive format for mainframe print data. AFPDS can also
be structured as AFP Conversion and Indexing Facility (ACIF) data, where
the AFPDS document data and all resources are concatenated into one file. AFPDS
is also sometimes referred to as MO:DCA-P by IBM (how’s that naming
convention for ya?), which stands for Mixed Object:Document Content
Architecture-Presentation.
·
PPDS – Personal Printer Data Stream from IBM
·
Metacode – Metacode
is a PDL used by many Xerox Centralized Electronic Printers (CEP),
high-speed mainframe printers. It is a lightly-documented PDL which uses a
binary stream, proprietary to Xerox. Often, Metacode is viewed as a competitor
to AFP in high-speed mainframe or digital press environments.
o
DJDE – Dynamic Job Descriptor Entries are used
inside Metacode streams to manage the print job. DJDE is mentioned because it
is commonly referred to in the print arena as a data format, when really it is
a directive inside a print format.
·
LCDS – Line Conditioned Data Stream by Xerox
is a PDL/stream comprised of ASCII data. It is considered a high-level
formatting language.
o
DJDE – LCDS may
also contain DJDE entries inside the stream.
·
IJPDS – Ink Jet Printer Data Stream is a
proprietary PDL/stream for Scitex printers.
·
LIDIL –
Lightweight Imaging Device Interface Language is a newer PDL developed by HP recently for their
low-end desktop inkjet printers. At this stage, it is still very proprietary to
HP and is not seen at all in the professional printing industry, however only
time will tell how this language will be used in the future. For now, it is
relegated to the user’s desktop.
·
PDF – Portable Document Format is a file
format from Adobe which has become increasingly popular in the digital world
throughout the 1990’s and continues to this day. It is most commonly associated
with the Adobe Acrobat program, however other viewers like Ghostscript, and
xPDF are able to display PDF documents. The PDF format retains the document
specifications including size and quality, across most any environment due to
the way it handles the text and images. A PDF file is very similar in layout
and language to Postscript. It differs from Postscript in that it is a Display List only, meaning like
Postscript programs, PDFs contain the objects to assemble the final printed
image, however unlike Postscript there are no programming constructs for loops,
image caching, imposition, paper handling, duplexing, etc. A PDF file is not
executed, it is displayed. However, it is becoming the lingua franca of the
digital pre-press workflow in the printing industry. With the PDF format,
customers can now send their documents to the printing vendor exactly as they
wish to have them printed, including all objects and copy, already typeset. Some
PDF viewers such as Acrobat now even allow for minor editing and adjusting of
the PDF document without changing its primary formatting. For this reason, some
printers are now capable accepting a PDF file as input, and RIPping it right
inside the printer. However, for most typical printers, the PDF file is still
RIPped to a Postscript file in the pre-press workflow before being sent to the
printer.
·
TIFF – Tagged Image File Format is a
flexible raster file format developed by Aldus (purchased by Adobe) and
Microsoft in the 1980’s, and still in use today. It has long been considered an
image file format, which it is, but also has application in the document
industry for its long standing and wide acceptance. Many printers accept TIFF
files as input as well, after all, the text and image objects have already been
RIPped into a bitmap already for the print engine. A TIFF file has no ability
to contain programming constructs such as document imposition, paper handling,
duplexing, loops or caching.
Many software packages are available to allow the dynamic
capture and conversion of most of these formats. In fact, some packages are
also capable of extracting data from these print streams and reassembling it
later, which has application in things such as mail sortation for postage discounts,
logging and digital archiving. The first step when working with or around a
complex print environment is to properly identify the data format being used,
and your intended destination format. From there you can decide if capture or
conversion is necessary, and begin planning your workflow.
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