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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.
An IRAF­Independent Interface for Spatial­Region
Descriptors
D. Schmidt
Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA
02138
Abstract. Regions for spatial masking are a valued part of PROS. To
move toward open analysis software, we plan to make the PROS regions
subsystem available from processes outside of IRAF. We also plan to
make the region descriptor parsing system available for purposes beyond
the creation of masks, such as an interface to an image display program,
accepting a region descriptor and generating commands to display the
region on an image.
Last year, we put in place support for arbitrary actions using region
descriptors. We discuss this architecture and its implications for open­
ing up the use of regions. We then discuss the challenge of giving the
subsystem an IRAF­independent interface.
1. Introduction
Limiting attention to photons from specified source and background areas is a
necessary part of astrophysical X­ray analysis. The IRAF/PROS X­ray analysis
system provides the required masking capability through its regions subsystem,
described by Mandel et al. (1993). The interface to the PROS regions subsystem
is a language for constructing ASCII region descriptors. The language is based
on a set of simple geometric shapes and a scheme for specifying complex shapes
as combinations of the simple shapes. The regions subsystem supports a variety
of coordinate systems, both celestial and pixel­based.
Anticipating two kinds of new requirements, we reconsidered the design of
the PROS region descriptor processing subsystem. One class of new require­
ments would be for products other than spatial masks from region descriptors.
The other new requirement would be to allow new tasks, within or outside of
IRAF, to use the subsystem to generate masks or other products.
2. Functions of a Region Descriptor
The primary function of a PROS region descriptor is to create an IRAF PLIO
pixel mask to select photons for analysis. Certain secondary functions were
already present in PROS and SAOimage:
ffl to display the regions on an image;
ffl to document the regions used in an analysis;
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ffl to extract specific regions and their parameters for special processing, such
as validity checks;
ffl to transform coordinates into a different system (e.g., to move data be­
tween SAOimage and PROS, when using two different pixel­based sys­
tems).
The presence of these secondary functions in PROS, however, did not satisfy
our goals of functional extensibility and open interfacing.
3. The Problems We Faced
Each of the secondary functions of region descriptor processing was implemented
ad hoc, each in its own program with its own lexical analyzer and parser. Each
parser defined the region descriptor language somewhat differently. Each parser
had side effects, making it difficult to add or change functionality. There was no
provision for defining new functions or novel combinations of existing functions,
and the system was available only within IRAF.
4. Our Solution
We defined a set of requirements for a new implementation of the regions sub­
system: (1) there should be a single language definition, and a single parser, (2)
there should be a single entry to the descriptor­processing subsystem, (3) selec­
tion of functions and return of resultants should be through a simple, extensible
control structure, (4) functions should be independent, so that any combination
would be selectable in a single call to the parser, and (5) the interfacing of func­
tions to the general language processing scheme should be modular, so that we
could define recipes for adding and changing functionality.
We determined that the underlying metaphor of a software CPU, executing
instructions compiled by the parser, remained a sound conceptual framework for
the syntactic and semantic analysis of region descriptors. In this framework, we
treat a descriptor like a program for a CPU that we emulate in software:
ffl Lexical analysis provides tokens to a compiler.
ffl Compilation produces an object program of software CPU instructions.
ffl Execution of the object program creates a mask or other resultant.
The implementation, however, needed a new layer of design to provide a general
method of plugging in new functions; the program interface needed a uniform
access protocol, so that we might open the subsystem to general tasks.
We retained xyacc as the language definition language. Using xyacc sim­
plified generating a correct and reliable parser. The integral processing, with
xyacc, from lexical analysis through compilation challenged flexibility that we
wanted in using the lexical analysis and object program. We defined a uniform
and extensible system of control flags to achieve the desired flexibility in using
information at any stage of the linguistic analysis.

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compilation
Interpretation /
driver
Parser
Object
list
program
Virtual
CPU
analysis
Lexical
Expanded
descriptor
Execution
Single­region
notes .pl mask
Lexical
tokens
Application 1 Application 2 Application 3
<< Resultants
Requests >>
Expanded descriptor
Object list
Single­region notes
.pl mask
Options (parsing control structure):
Figure 1. The PROS regions subsystem program interface.
Figure 1 shows schematically how various applications use a uniform pro­
tocol to interact with the subsystem via a versatile parsing control structure,
and how the underlying subsystem is structured to enable functions to access,
as needed, any of the three stages of the linguistic processing.
5. Using the New Design
This design prepared us for extending the regions subsystem's functionality and
for making the subsystem available for use by programs in a more open envi­
ronment. To make it easy for programmers to begin to exploit the new design,
we created ``recipes'' for two ways of extending region descriptor functionality.
Outlines of those procedures follow.
First, a recipe for processing a region descriptor: (1) call rg open parser(),
to create a parsing control structure, (2) call one or more of the request setup
routines, to create request/resultant structures and link them into the pars­
ing control structure, (3) call the parser, rg parse(), (4) retrieve the resultants
through a set of macros provided, and (5) call rg close parser(), to release any
request/resultant structures and the parsing control structure.
Second, a recipe for adding a new function: (1) define a request/resultant
structure for the new option, and standard­format macros to access its fields, (2)
create a slot for the new option in the parsing control structure, (3) create option
setup and release routines, using standard templates, (4) integrate the option

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into the system of ``request set query'' routines and control flags, according to
the processing required for the option, and (5) add the new functionality at one
of four locations in the system, according to what form of the descriptor it uses.
Full utilization of the new capabilities remains for future applications.
6. Conclusion
The redesign and re­implementation of the program interface to the IRAF/PROS
regions subsystem has provided PROS programmers with a cleaner environment
for developing new regions applications. It has been used successfully for more
than a year. We anticipate greater benefit in the future, when applications out­
side of IRAF may make use of the new request/resultant protocol for much freer
access to the functions of region descriptor processing.
Acknowledgments. This work was supported under NASA contract to the
ROSAT Science Data Center (NAS5--30934).
References
Mandel, E., Roll, J., Schmidt, D., VanHilst, M., & Burg, R. 1993, in Astronomi­
cal Data Analysis Software and Systems II, ASP Conf. Ser., Vol. 52, eds.
R. J. Hanisch, R. J. V. Brissenden, & J. Barnes (San Francisco, ASP), p.
430