The X Print Service - The Basics

This document is xc/programs/Xserver/XpConfig/README in recent X.org distributions of the X Window System

Index
   - 1.0 X Print Service Overview

   - 2.0 How the X Print Service Works

   - 3.0 Using the X Print Service
        - 3.1 X Print Server Configuration
        - 3.2 Starting the X Print Service
        - 3.3 Configuring the environment
        - 3.4 General End-User Sequence

1.0 X Print Service Overview

The "X Print Service" technology allows X rendering to devices such as printers and fax. Most of the service is available in the X11 technology stack as Xp, with the remainder in the CDE technology stack as DtPrint. Modifications have also been made to the Motif technology stack to support Xp and DtPrint.

The Xp portion consists of:

Xp Extension for the X-Server (included in the X-Server Xprt)
Xp Extension API for the client side (libXp)
PCL ddx driver that converts core X to native PCL
Postscript ddx driver that converts core X to native Postscript
Raster ddx driver that generates xwd rasters which can be converted to PCL or Postscript rasters

The DtPrint portion consists of:

A collection of print GUIs (libDtPrint)
A Print Dialog Manager that can assist a client in setting printing options (dtpdm, dtpdmd)

From an X clients perspective, it can attach to one of two nearly identical X-Servers, a "Video" X-Server, and a "Print" X-Server which has the additional Xp capability but otherwise looks and behaves the same.

2.0 How the X Print Service Works

The X Print Service expands on the traditional X-Server and Xlib world in four ways.

Most obvious is the use of "print ddx drivers" instead of "video ddx drivers". While a video ddx driver modifies pixels in a video frame buffer, a print ddx driver generates "page description language (PDL)" output such as PCL or Postscript.
Once a print ddx driver generates PDL output, it can be sent to a spooler such as lp(1) or retrieved by the client.
Though not currently done, a single X-Server can support both print and video ddx drivers.
Since printers support "paged" output, unlike video, a portion of the Xp Extension supports APIs to delineate printed output. For example, XpStartPage and XpEndPage tell the X-Server where a physical page starts and ends in an otherwise continuous stream of X rendering primitives. Likewise, XpStartJob and XpEndJob determine when a collection of pages starts and ends. XpEndJob typically causes the generated PDL to be submitted to a spooler, such as lp(1).
Since printers have extensive capabilities, another portion of the Xp Extension supports APIs to manipulate "print contexts".
Once a printer is selected using the Xp Extension API, a print context to represent it can be created. A print context embodies the printer selected - it contains the printer's default capabilities, selectable range of capabilities, printer state, and generated output. Some "attributes" within the print context can be modified by the user, and the X-Server and print ddx driver will react accordingly. For example, the attribute "content-orientation" can be set to "landscape" or "portrait".
Since printers can have "built in" fonts, the Xp Extension in the X-Server works with the print ddx drivers to make available (for printing only) additional fonts on a per print context basis.
When a print context is created and set for a given printer, the X font calls may be able to access additional printer fonts. To do this (typically), the X-Server must have access to "printer metric files" (.pmf) that describe at minimum the metrics of the built in fonts.

3.0 Using the X Print Service

There are three tasks to start the X Print Service: 1) configuring the X Print Server, 2) starting the X Print Service, 3) configuring the user session so that clients can find the running X Print Service.

The tasks are described in detail below.

3.1 X Print Server Configuration

The X Print Server (Xprt) can read a number of configuration files which control its behavior and support for printers. Each vendor platform has a default location for this information. Xprt can also read the environment variable XPCONFIGDIR to locate alternate configuration directories. Common settings include:

export XPCONFIGDIR=/X11/lib/X11/XpConfig/
export XPCONFIGDIR=/proj/x11/xc/programs/Xserver/XpConfig/

Xprt has many built-in defaults, and lacking any configuration files, will immediately try to support all printers visible via lpstat(1).

In order of importance for configuration by a system administrator, the configuration files for a "C" locale are as follows.

   ${XPCONFIGDIR}/C/print/Xprinters

	`Xprinters' is the top most configuration file.  It tells
	Xprt which specific printer names (e.g.  mylaser) should
	be supported, and whether lpstat(1) or other commands
	should be used to automatically supplement the list of
        printers.

   ${XPCONFIGDIR}/C/print/attributes/printer

	The `printer' file maps printer names to model
	configurations (see `model-config' below).  For example,
	"mylaser" could be mapped to a "HPDJ1600C", and all other
	arbitrary printers could be mapped to a default, such as
	"HPLJ4SI".  When depending on lpstat(1) in the Xprinters
	file, setting up defaults in `printer' becomes all the
        more important.

   ${XPCONFIGDIR}/C/print/attributes/document

	The `document' file specifies the initial document values
	for any print jobs.  For example, which paper tray to
	use, what default resolution, etc.

   ${XPCONFIGDIR}/C/print/attributes/job

	The `job' file specifies the initial job values for any
	print jobs.  For example, "notification-profile" can be
	set so that when a print job is successfully sent to a
	printer, e-mail is sent to the user.

   ${XPCONFIGDIR}/C/print/models/HPDJ1600C/model-config
   ${XPCONFIGDIR}/C/print/models/HPDJ1600C/fonts/fonts.dir
   ${XPCONFIGDIR}/C/print/models/HPDJ1600C/fonts/9nb00051.pmf
   ${XPCONFIGDIR}/C/print/models/HPDJ1600C/fonts/9nb00093.pmf

	The `model-config' file has attributes that describe the
	printer model's capabilities and default settings.
	Printer model fonts may also be present.  The model-config
        file also identifies the print ddx driver to be used.

	For each printer model supported, a complete hierarchy of
	files should exist.  In most cases, these files do not
	need to be modified.

   ${XPCONFIGDIR}/C/print/ddx-config/raster/pcl
   ${XPCONFIGDIR}/C/print/ddx-config/raster/postscript

	The print ddx drivers can have highly specific
	configuration files to control their behavior.  In most
	cases, these files do not need to be modified.

3.2 Starting the X Print Service

The summary checklist for starting the X Print Service is as follows:

Choose an execution model for the X Print Service. The X Print Service can be run on a per-user session basis, per machine basis, or can be run on a few machines globally available to a number of users.
If print jobs are to be submitted to a spooler (almost always the case), make sure all needed printers are available to the spooler subsystem (most often lp(1)) on the same machine running the X Print Service.
Configure the X Print Server. See ``X Print Server Configuration''.
Depending on #1, start the X Print Server process "Xprt", and then the Print Dialog Manager Daemon process "dtpdmd" at the appropriate times.

The details are described below.

Because the X Print Service is based on X, it can be easily distributed. The most significant factors in which execution model to choose will be driven by:

how many printers will be accessable through the printer subsystem on any given machine. A system administrator may choose to cluster printers on a few given machines, or scatter them across an organization and possibly make extensive use of remote spoolers to make them globally available.
how many machines will need a copy of the X Print Server configuration files. The files have been architected so that one super-set version of them can be maintained and distributed (e.g. via NFS), and a per-machine or per-user version of the `Xprinters' is all that is needed to have the appropriate information in them utilized or ignored.
how many users can demand services from a given X Print Service.

With the above in mind, some obvious execution models include:

Global - in this model, the system administrator is choosing to run the X Print Service on a *few* select machines with appropriate printers configured, and allow clients access to the global resource. This can centralize the administration of printers and configuration files, but may have to be monitored for performance loading.
Startup would likely be done by boot-up scripts.
Per-machine - every machine with potential X Print Service users would run the service. Printer and configuration file administration is decentralized, and usage would be limited to the users on the machine.
Startup would likely be done by boot-up scripts.
Per-user session - every user would run an entire X Print Service for themselves. In the future, the Video X Server normally started may contain Print X Server capability, so this model becomes very natural.
Startup would likely be done at session login or by launching actions or processes manually once the user logs in. Note: the dtpdmd must be started after Xprt.

Starting of the processes is straight forward. In strict order:

   [machineA] % Xprt [-XpFile <Xprinters file>] [:dispNum] &

      Note that Xprt will look for configuration files in either
      a default location or where XPCONFIGDIR points.

      -XpFile specifies an alternate `Xprinters' file, rather
      than the default one or `${XPCONFIGDIR}/C/print/Xprinters'.

   [machineA] % dtpdmd -d machineA[:dispNum] [-l /tmp/dtpdmd.log] &

      The dtpdmd will maintain an X-Selection on the X-Server,
      and will start dtpdm's as required to service requests.

In all but the per-user session model, the machine running the dtpdmd (thus dtpdm's) will need display authorization to the users video display.

3.3 Configuring the environment

Once a X Print Server and dtpdmd have been started -- many of them in some cases -- clients will need to find and use them. There are two mechanisms that allow clients to discover X Print Servers and printers.

"X Print Specifier" - assuming usage of the DtPrint print dialogs, the following notation is understood:
```
          printer_name@machine[:dispNum]
```
For example:
```
          colorlj7@printhub:2
```
In the above example, the X Print Server running at `printhub:2' is assumed to support the printer named `colorlj7'.
"XPSERVERLIST" - assuming usage of the DtPrint print dialogs, the environment variable "XPSERVERLIST" can contain a list of X Print Servers. For example:
```
          XPSERVERLIST="printhub:2 printhub:3 otherdept:0"
```
Then in the dialogs, only a printer name needs to be entered. The dialog will then search the X Print Servers in XPSERVERLIST for a server than supports the printer, and then establish contact.

3.4 General End-User Sequence

From most CDEnext applications, printing is accomplished by bringing down the <File> menu and selecting <Print...>. This will result in the DtPrintSetupBox dialog, which will request the name of a printer, and offer limited capability to configure print options (e.g. number of copies). If the user wishes, they can select <Setup...>, which will start a dtpdm capable of modifying additional print options. Finally, the user should select <Print>.