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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
Applying Public Access Programming Techniques To
SAOimage
E. Mandel
Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA
02138
D. Tody
National Optical Astronomy Observatories, P. O. Box 26732, Tucson,
AZ 85719
Abstract. This paper describes our application of the X Public Access
(XPA) interface to the new version of SAOimage. XPA allows an Xt
program to define named public access points through which data and
commands can be exchanged with external programs. It makes possi­
ble the external control of the program's main functions, including image
display, image zoom and pan, color map manipulation, cursor/region def­
inition, and frame selection. It also supports ``externalization'' of internal
algorithms such as file access and scaling. Finally, we describe how XPA
is used to support user­configurable ``quick­look'' analysis of image data
and bi­directional communication with other processes.
1. Introduction
Astronomical software development needs to adopt more ``open systems'' con­
cepts and designs. Users increasingly want to extend existing software and to
combine tasks from different systems in order to create their own heterogeneous
research environments. Our software designs must reflect this increasing need
for flexibility and extensibility without sacrificing functionality and without vi­
olating budget constraints.
The SAOimage display program has been very popular with astronomers
over the past decade. In the age of ``open systems'', it needs updating to support
extensible features. These features include providing an easy way to display
arbitrary data file formats, control image display from an external process, and
integrate analysis routines into the image display.
In order to update SAOimage efficiently and cost­effectively, SAO and
NOAO will collaborate to develop a high­quality image display program, basing
our work on the ximtool program developed by D. Tody for IRAF. Ximtool
uses the NOAO widget server and ``gterm'' image widget to support customized
graphical user interfaces (GUIs), image and line graphics, multiple frame buffers,
user­defined color maps, region markers, etc. It provides a high­level image dis­
play programming interface as well as low­level access to gterm­image widget
functions. Our aim is to layer open­ended functionality on top of ximtool in
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order to develop SAOtng (SAOimage: The Next Generation). In this way, SAO
and NOAO can share development of image display, while satisfying our indi­
vidual needs.
2. The X Public Access Mechanism
To add open systems features to SAOtng , we will utilize the X Public Access
mechanism (XPA), described in Mandel & Swick (1994). XPA allows an Xt pro­
gram to define named points of public access, through which data and commands
can be communicated to/from external programs:
SAOtng
command line
program
XPA names are used to define public access
The xpaget program receives data and info
The xpaset program sends
The OpenXPA subroutine allows standard I/O
routines to exchange data and info with SAOtng
from SAOtng
data and commands to SAOtng
points for SAOtng commands and data
Figure 1. We have used XPA to extend the ximtool image server into
an open­ended SAOtng display service that can cooperate with other
processes and programs.
3. Controlling SAOtng
The standard SAOtng graphical user interface is a simple menu­based GUI that
parallels the original SAOimage menu/command paradigm. It provides menus
for image and frame selection, scaling, color maps, zoom/pan, regions, etc. All
of these menu functions also are defined to be XPA public access points. Thus,
SAOtng can be controlled directly through its menu­based GUI or externally
through the XPA mechanism (see Figure 2).
For example, the xpaset program can be used to change the SAOtng scaling
algorithm, and the xpaget program can be used to retrieve the current scale:
csh? echo ''scale log'' --- xpaset SAOtng
csh? xpaget SAOtng scale
log

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In fact, the SAOtng GUI uses XPA to send commands to itself. This implemen­
tation ensures that support for external control of SAOtng does not lag behind
the internally­supported user interface: the GUI initiates commands as if it were
just another external process!
4. External Support For File Access
A FITS file can be sent directly to SAOtng for display using XPA:
cat foo.fits --- xpaset SAOtng
Other file formats are supported by writing two file access programs: a header
access program to generate a FITS header of the full image, and a data access
program to generate a FITS image of a specified data section (given the center,
dimensions, and block factor). These access programs are defined at start­up
time in a user­configurable ASCII file. To load an image, the programs are
run externally (using the system() subroutine) to create FITS­format data for a
specified data section, which are passed back to SAOtng for display using XPA.
When a new file name is sent to SAOtng, the appropriate header access program
is run to gather the overall image dimensions. Next, the data access program
is run to extract the desired image section, which then is scaled to 8 bits and
displayed. Finally, the data access program again extracts the data section and
stores it as raw data for later use (e.g., re­scaling the image). Note that adding
new formats does not require re­compilation of SAOtng.
5. Integration of Analysis Routines
Each file type can have user­defined analysis commands associated with it. These
analysis commands are defined at start­up time by means of ASCII descriptions.
The analysis commands associated with the currently displayed image are avail­
able for execution via an analysis menu. When selected from this menu, an
analysis command is macro­expanded and then executed externally. Results
can be displayed in a separate window or even can be sent back to SAOtng, i.e.,
a command can create an image and then send it to SAOtng for display:
$data --- smooth ... --- xpaset SAOtng ''frame new''
In the example above, the ``$data'' macro is expanded into an XPA command
that retrieves the FITS representation of the displayed image. The FITS image
generated by the smooth program is piped into a new frame buffer of SAOtng.
6. Cooperation With Other Processes
XPA allows the re­use of GUI programs so that new systems can be built from
existing high­level components. For example, the SAOtng ``Load Image'' menu
option pops up the XDir program, a directory and file browser that also offers
XPA services (see Figure 2). The XDir program then can be used to browse
through directory trees using template filters. Double clicking on an image file

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Figure 2. SAOtng has a simple menu­based GUI. It can be com­
manded externally (lower right) and can communicate with programs
such as XDir (upper right).
causes XDir to send an XPA command to SAOtng to load the new image. Thus,
SAOtng can make use of sophisticated XDir capabilities without linking them
explicitly: GUI programs can be re­used in the same manner as subroutines!
7. Conclusion
SAOtng exemplifies a new type of astronomical analysis program that is exten­
sible and that cooperates with other processes. It takes us closer to an era in
which our heterogeneous analysis systems can work as an integrated whole.
Acknowledgments. This work was supported under NASA contracts to
the IRAF Technical Working Group (NAGW--1921), the AXAF High Resolution
Camera (NAS8--38248), and the AXAF Science Center (NAS8--39073).
References
Mandel, E., & Swick, R. 1994, PASP, 96, 198