                               XFree86 and DPS

                    Juliusz Chroboczek, <jch@xfree86.org>

                              27 February 2001

1.  Introduction

Display Postscript (or DPS for short) is a rendering extension for X11.  DPS
is slightly atypical in that it is based on code mobility, the ability to
send executable code from client to server.  Thus, a DPS client does not
request that a line should be rendered; instead, it uploads code to the
server which, when executed, causes a line to be drawn.

This document does not aim at teaching programming with DPS; it is only a
summary description of the DPS support code included with XFree86.  More
information about DPS, including a DPS bibliography, is available from the
DPS extension site <URL:http://dps.sourceforge.net>.

     Note: Adobe, PostScript and Display PostScript are trademarks of
     Adobe Systems Incorporated which may be registered in certain
     jurisdictions.

As all X11 extensions, DPS consists of client-side and server-side compo-
nents.  The DPS client side consists of a number of libraries and a number of
tools for programming and configuration.  The DPS server side may consist
either of an X server extension, or of a client-side process known as the
``DPS agent.''  In this latter case, the term ``server-side'' is somewhat
misleading.

At the time of writing, only the client side is included with XFree86; the
server side must be obtained separately.  Please see server side (section 3.,
page 1) later in this document for more information.

2.  The DPS client side

The DPS client side consists of four libraries and a number of basic tools
for programming and configuration; these are all included with XFree86.

2.1  Libraries

The libpsres library is a library for management of PostScript resources, on-
disk files representing PostScript data structures such as fonts, font encod-
ings, procsets, etc. It is closely related to the makepsres tool (see client-
side tools (section 2.2, page 1) later in this document).

The basic DPS client library is libdps.  This library contains a number of
functions for connection establishment, resource management, as well as stubs
for all standard PostScript operators.  Normally, all DPS clients should link
with libdps; in addition, libdps may be used for printing by non-DPS clients
(this is done, for example, by Sun's JDK).  This library is documented in
[CLRM] and [CLSX].

The libdpstk library contains a number of additional utilities for managing
DPS contexts, user paths and user objects, and for previewing EPS files.  It
is documented in [DPTX].

The libdpstkXm library contains four Motif widgets.  The DPS Scrolling Widget
is a DPS drawing area that automatically manages issues such as scrolling,
scaling, client-side backing store, incremental redisplay, etc. The Font
Selection Box, and its associated Font Preview, present a convenient and pow-
erful interface for choosing scalable fonts.  Finally, the Color Picker pre-
sents an interface for choosing colours using either of the RGB or HSV
spaces.  The latter three widgets are documented in [DPTX]; some summary
Scrolling Widget documentation is available in the doc subdirectory of the
DPS.tar.gz file, available from <URL:ftp://dps.source-
forge.net/pub/dps/DPS.tar.gz>.

The source code for libdpstkXm is included with XFree86; however, as it
depends on Motif, this library is not built by default.  A GTK-based library
providing some of the functionality of libdpstkXm is available from the
gtkDPS site <URL:http://www.gyve.org/gtkDPS/>.

2.1.1  Libdps and Xt

In X11R5, libdps did not depend on libXt.  In X11R6, however, code was added
to make the Xt main loop dispatch to sundry code on DPS events; with this
addition, all programs that link with libdps need to link with libXt, whether
they use Xt or not.

This state of affairs is unfortunately true of the version of libdps included
with XFree86.  We are currently considering various solutions to this problem
(including the use of weak linker symbols or splitting off the guilty func-
tions into a separate library).

2.2  Client-side tools

In addition to the libraries, the client side of DPS consists of two utili-
ties.

The makepsres utility is used for managing PostScript resources.  Its basic
operation consists in walking recursively a filesystem tree, noting all
resources, and then writing out a ``Unix PostScript Resources,'' file, basi-
cally a directory of all the resources found.  This utility is documented in
the makepsres(1) manual page.

The pswrap utility is a stub generator for PostScript clients.  Roughly
speaking, it takes as its input textual PostScript code, and generates a col-
lection of C functions that transmit that code in pre-tokenised form to the
DPS extension.  Pswrap is documented in [PSWRAP].

2.3  Sample clients

XFree86 contains three sample DPS clients, dpsinfo, dpsexec and texteroids.
They are documented in their respective manual pages.

A number of sample clients that depend on Motif are available in
<URL:ftp://dps.sourceforge.net/pub/ftp/DPS.tar.gz>.  Additional sample
clients can be found as part of GtkDPS (see above).

The GNUstep environment can be compiled to use DPS for all rendering; for
more information, please see the GNUstep site <URL:http://www.gnustep.org>.

3.  The DPS server side

In order to use DPS clients, you need to install a DPS server side, which can
be either a server extension (a ``DPS/X extension''), or a separate process
(referred to, variously, either as a ``DPS/NX agent'' or, rather confusingly,
as ``Client-Side DPS'' (CSDPS).

3.1  Display Ghostscript

Display Ghostscript (note the capitalisation), or DGS, is a client-side
implementation of DPS based on the Ghostscript PostScript interpreter.  DGS
is still in beta at the time of writing; it does, however, provide a very
usable implementation of DPS, although it still has some problems with the
semantics of multiple DPS contexts.

DGS is available from the GNUstep download area
<URL:http://www.gnustep.org/resources/sources.html>.

3.2  The DPS extension

The DPS extension is a much younger project aiming at producing an efficient
server-side implementation of DPS.  The extension is currently in a state
best described as alpha; current versions are known to crash the X server
under some circumstances.

The DPS extension is available from the DPS extension site
<URL:http://dps.sourceforge.net>.

4.  References

Links to electronic versions of all of these, and more, are available from
the DPS extension site <URL:http://dps.sourceforge.net>.

[PLRM2] PostScript language reference manual. Adobe Systems, 2nd ed. Addison-
Wesley, 1990. ISBN 0-201-18127-4.

[PLRM] PostScript language reference. Adobe Systems Incorporated, 3rd ed.
Addison-Wesley, 1999. ISBN 0-201-37922-8.

[INTRO] Display PostScript System. Introduction: Perspective for Software
Developers. 15 April 1993.

[CLRM] Display PostScript System. Client Library Reference Manual. 15 April
1993.

[CLSX] Display PostScript System. Client Library Supplement for X. 15 April
1993.

[DPTX] Display PostScript System. Display PostScript Toolkit for X. 15 April
1993.

[PSWRAP] Display PostScript System. pswrap Reference Manual. 15 April 1993.

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