SLxpa binds the XPA library to the S-Lang language. It provides an importable module and associated .sl script which make XPA callable directly from S-Lang scripts. The S-Lang interpreter is part of the widely-used, open-source library of the same name, and provides a scripting environment well-suited for scientific and engineering tasks, due to the powerful, compact, fast, and robust multidimensional numerical capabilities that are native to the language.
XPA is a set of libraries (ANSI C, Tcl/Tk) and commandline tools, written at the Smithsonian Astrophysical Observatory, which provide a clean and simple IPC (inter-process communication) mechanism that is also surprisingly powerful. It can be used to build loosely-coupled client/server programs that interact either on a single machine or across local and wide-area networks; we have also found it useful for automated regression testing, especially for GUIs that are traditionally difficult to test in this manner. The XPA library provides much of the functionality of more well-known mechanisms like CORBA, Java RMI, and the now-defunct ToolTalk, but is much smaller and, we feel, can be easier to learn, embed, deploy, and utilize.
SAOds9 image display and analysis tool is perhaps the most
well-known XPA command server. Also written at the Smithsonian
Astrophysical Observatory, SAOds9 is one of the single most widely
used programs in observational astronomy.
CIAO, a software system
created to assist in the analysis of data
generated by the Chandra X-Ray Telescope, provides a number of programs
Prism) that function both as XPA clients
with DS9 and each other) and servers (by providing XPA access points of
their own). Other software employing XPA includes
POW (a FITS file viewer and plotting tool, respectively,
available within both the
HEASOFT astronomy software
suites), as well as the
SAS package created for the
XMM-Newton X-Ray Telescope. A set of Perl bindings to XPA also
exists in CPAN.
S-Lang bindings for XPA were originally developed at the Smithsonian
Astrophysical Observatory in 2002, as part of
CIAO, an analysis
software package developed in part to support the Chandra X-Ray
Telescope. The SLxpa package, created at the MIT Center for Space
Research between 2003 and 2004, adds significant enhancements to the
original bindings, in order to:
Moreover, SLxpa extends their scope of potential use to a considerably wider audience, in that the package may be obtained independently of CIAO, allowing developers to avoid downloading hundreds of megabytes of astronomical software and data just to obtain a 100 Kb module.
As noted in the respective function descriptions, some of the SLxpa
enhancements described above change the semantics of the XPA bindings
as originally developed within CIAO. However, SLxpa may still be
used as a drop-in replacement for those bindings, simply by declaring
_CIAO3_XPA_COMPAT_ in the
Global namespace prior
to loading the package, e.g. as
The variable need not be initialized to any particular value.
public variable _CIAO3_XPA_COMPAT_;
SLxpa ships with a configure script generated by autoconf, and in most cases can be built by issuing the following standard commands within a UNIX-like (e.g. Linux or Cygwin) environment:
The third step is optional, but recommended. Version 2.1.4 or later of XPA is recommended. Versions of SLxpa prior to 0.5 were compatible with S-Lang 1, but SLxpa versions 0.5 and later require S-Lang 2.1 or later (e.g. to support the use of named qualifers in function calls). The
./configure [options] make make install
./configure --helpcommand will display all available configure options.
To use the
XPA module in a S-Lang script it is first necessary
to make the functions in the package known to the interpreter, via
or, if the application embedding the interpreter supports the
() = evalfile ("xpa");
may be used. You may also load the package into a namespace, such as
This would place the XPA symbols into the
() = evalfile ("xpa", "xpans");
xpansnamespace (creating it, if necessary). Once the package has been loaded functions and variables defined within may be used in the usual way, e.g.
Finally, recall that the S-Lang
require ("xpa"); . . () = xpaset("ds9","quit");
autoloadfunction may be used to avoid explicitly
require-ing the package prior to using the functions defined within.
The remainder of this document assumes the reader is familiar with the XPA suite of command line tools and/or the XPA application programming interface (api). For more information consult the XPA website.
To make the most of the examples given below it is encouraged that you run them and experiment. In support of that you should first:
examplesdirectory of the SLxpa distribution
The examples reference programs mentioned in the introduction with which
you may not be familiar. Do not fret, as you need not actually run an
example within the given application for it to still serve most of its
didactic purpose, because what's being shown are not specific features of
the application, per se, but rather how XPA function calls may be made
through the S-Lang interpreter. The XPA functions may be called in
the exact same manner from any application which embeds the S-Lang
interpreter and supports module importation. In fact, most of the
examples can be cut and pasted into a S-Lang script and then invoked
from within the S-Lang shell (
slsh, distributed with the S-Lang
library, and assumed to be installed on your system).
As described in chapter DS9_Package, SLxpa also bundles a script which streamlines interactions with the SAOds9 image display and analysis tool.
By augmenting the numerical strength and modular extensibility of the S-Lang platform with the imaging capabilities of SAOds9, the DS9 script can make a powerful addition to your image analysis toolset.