Idl Programming

511 Pages·2008·30.44 MB·English

Foreword
David Stern
,rednuoF Research Systems, Incorporated
My goal in writing IDL has been to create a tool for scientists and engi-
neers that could be used both for ad hoc data analysis and as a program-
ming language for building technical applications. IDL is a specialized
fourth-generation language providing a framework that includes func-
tions and components for scientific computing and visualization. It is the
ability to quickly and easily investigate data; to rapidly apply various fits,
filters, and transformations; and to then immediately see the results that
sets IDL users apart from those who run Fortran or C programs. Unlike ,C
Fortran, or even Java, IDL is an interactive tool that can be programmed
or scripted.
I still enjoy using IDL for exploring new data sets and answering new
questions. This is a fun and intellectually challenging trial-and-error
process, somewhat akin to solving a mystery, which usually concludes
with an answer. Once the researcher obtains the answer and learns how
to analyze and visualize a given type of data using ad hoc methods, the
next step is to automate the process, and then to perhaps embed it in a
graphical user interface so that others may benefit from its use.
Many IDL users are comfortable with ad hoc analysis and display but
have not yet mastered the language features or skills necessary to build
more complex programs or applications, in particular those driven by
GUIs. This is not a particularly straightforward process, and this book
does a great job in aiding this transition.
droweroF
Practical LDI Programming begins with a concise presentation of the
basics of IDL, its syntax, and data and control structures. It then explains
how to access data of various types in files, including the complicated
and poorly understood netCDF and HDF formats. Four chapters are
devoted to creating graphics, charts, and displaying images, on the
screen, the printer, and for publication. The final chapter describes how
to build an application with a graphical user interface for a wider range of
users.
Liam Gumley is eminently qualified to write a book on IDL. He has
been a long-time user of IDL and has created a number of impressive
applications used by many scientists. Professionally, he writes software
applications for the Cooperative Institute for Meteorological Satellite
Studies (CIMSS), at the University of Wisconsin. He posts frequently to
the always-active IDL newsgroup, comp.lang.idl-pvwave, and is a
respected voice there.
Liam distills what he feels is important for an IDL programmer to
know, rather than simply presenting a complete reference without editor-
ial comments. In IDL, as in any reasonably complete package, there are
usually 10 ways to perform any task. For any particular problem, one
method will be the best, and one will be the worst, with the remainder
falling somewhere in between. Liam's goal is to impart an understanding
of IDL's operation to the reader, so that he or she can select the best
method for the problem at hand.
The IDL beginner will welcome this book because, while it starts with
the basics, it is nonetheless a comprehensive treatment of the language.
There are a multitude of examples, plus code for designing a GUI-based
application for interactively viewing images. The experienced user will
find the book valuable for its reference sections and discussions on the
proper way to program in IDL, which unfortunately is not always obvious.
On behalf of Research Systems Incorporated and the community of
IDL users and programmers, I would like to thank Liam Gumley for writ-
ing this valuable and long overdue book. Practical LDI Programming
serves as a useful tutorial, source of examples, and reference for those
wishing to learn or to master IDL. It gives me great pleasure to recom-
mend this book to you.
stnemgdelwonkcA
This book arose from a desire to share the lessons I have learned from my
work with IDL. Naturally, I did not learn about IDL all on my own.
Instead, I am fortunate to work in a research environment with experi-
enced and knowledgeable colleagues whose advice helped me to develop
the ideas presented in this book. The genesis of this book was the morn-
ing discussions over coffee with my good friend Paul van Deist, who chal-
lenged and tested my ideas about IDL and programming in general. Paul
Menzel provided me with enough data analysis and visualization chal-
lenges that I had little choice but to hone my skills as an IDL programmer.
And for everyone who stopped by my office to ask a "quick question"
about IDL, this book is for you.
Early in the project it was my good fortune to establish a working rela-
tionship with Research Systems, Inc., the makers of IDL, and I have bene-
fited greatly from their encouragement and support. In particular I must
thank David Stern of RSI for kindly agreeing to write the foreword for this
book. The team at Morgan Kaufmann, led by Denise Penrose, made it
easier than I had ever hoped to bring the book to fruition, and they
deserve credit for turning my manuscript into what I hope will be a pleas-
ing and useful reference for IDL programmers.
I am indebted to the following reviewers who played a large part in the
development of the manuscript: Paul van Deist of the University of Wis-
consin-Madison, Nick Bower of Curtin University of Technology, Martin
Schultz of the Max-Planck-Institut fuer Meteorologic, Molly Houck of
vii
stnemgdelwonkcA
RSI, and Randall Frank from Lawrence Livermore National Laboratory.
Their suggestions dramatically improved the content, style, and organi-
zation of the book, and any errors that remain are mine alone.
I have also learned a great deal from the contributors to the Usenet
newsgroup c0mp.lang, i dl-pvwave, whose willingness to discuss the
intricacies of IDL often provided the stimulus to ii~vestigate a topic fur-
ther. Some results of exchanges on the newsgroup are to be found in this
book.
Finally, I owe the greatest debt to my wife, Janine. If not for her
patience and understanding, I would never have completed this book.
Introduction
T his book provides a solid foundation in the fundamentals of programming in Interac-
tive Data Language (IDL), an array-oriented data analysis and visualiza-
tion environment developed and marketed by Research Systems, Incor-
porated (RSI) of Boulder, Colorado. IDL is available for Windows, MacOS,
UNIX (including Linux), and SMV platforms.
The first version of IDL for VAX/VMS was released in 1981 by ISR
founder David Stern. Since then, IDL has emerged from its roots in the
astronomical and space sciences to become a widely used tool in
research, educational, and commercial settings in areas as diverse as the
earth sciences, medical physics, and engineering test and analysis.
Examples of real-world tasks where IDL is used include analyzing and
visualizing images from the Hubble Space Telescope; rendering three-
dimensional volumes of the human body from magnetic resonance
imaging; and monitoring and controlling the plasma field of a nuclear
fusion tokamak.
The long-term success of IDL stems from its flexible modes of opera-
tion, which include rapid interactive analysis and visualization; a power-
ful programming environment; and fully developed cross-platform end-
user applications that range from an advanced remote sensing image
processing suite (ENVI) to a comprehensive terrain and river network
analysis package (RiverTools). In addition, IDL offers a broad range of
fundamental data types, making it possible to read almost any input data
retpahC 1 Introduction
format. The availability of IDL on all major UNIX platforms (including
Linux), Windows, and MacOS facilitates data analysis in multiplatform
environments and ensures that applications developed on one platform
can be successfully ported to new platforms when hardware changes are
necessary. Finally, the worldwide community of IDL users contributes to
and maintains a collection of freely available Internet-based libraries,
which at the time of writing included over 5,000 separate routines.
1.1
ABOUT THIS BOOK
This book begins by explaining the fundamentals of IDL and then moves
on to explain howt he fundamentals may be used to develop data analysis
and visualization applications. The first chapter begins with a discussion
of the philosophy behind the book; explains where the supporting mate-
rials for the book may be found; and summarizes the conventions used
for IDL code examples. The chapter continues with instructions on how
to start an IDL session and how to use the built-in online help. The chap-
ter concludes with an outline of the remaining chapters in the book.
Who Should Read This Book?
Readers of this book will include anyone who wishes to use IDL effec-
tively to analyze and visualize data. Newcomers to IDL will find a thor-
ough description of the basics of the IDL syntax, with special attention
paid to the effective use of arrays, as well as the fundamentals of visual-
ization in IDL. Readers with a moderate level of prior exposure to IDL
(perhaps only in interactive mode) will learn the mechanics of writing
understandable and efficient IDL programs and receive practical advice
such as how to create publication-quality PostScript output. Advanced
users of IDL will appreciate the many hard-won nuggets of advice about
IDL peculiarities, such as a detailed description of the differences
between 8-bit and 24-bit graphics modes and the implications for IDL
programming. Readers at all levels will appreciate the realization of IDL
programming concepts in example programs that can be used immedi-
ately for real-world tasks. Prior exposure to a procedural language such
as C or Fortran will be helpful, but not essential, for readers of this book.
1.1 About This Book
Why Write This Book?
I wrote this book to help IDL users at all levels advance to the point where
writing understandable and efficient IDL programs is second nature. Part
of the attraction of IDL is that you can accomplish a great deal with only a
few commands in interactive mode. A side effect, however, is that a sizable
fraction of IDL users never progress beyond the point of entering individ-
ual commands and are never able to fully exploit the capabilities of lDL as
a programming language. In my experience, the transition from casual
command-line user to effective IDL programmer is often long and frus-
trating, if the user makes the transition at all. All the necessary informa-
tion can be found somewhere in the manuals that accompany IDL, but
with several thousand pages spread over five or more volumes, it is often
difficult to know exactly where to look when you have a question. I felt
strongly that there was a place for a book written by an experienced IDL
programmer that gathered together essential information about IDL pro-
gramming, assembled it in a logical sequence, and presented it with prac-
tical and illustrative example programs that solve real-world problems.
What about Objects?
A new object data type and an Object Graphics system were introduced
in IDL 5.0 in 1996. These new features help to broaden the scope of the
language and allow complex graphical displays to be constructed, partic-
ularly in the area of three-dimensional visualization. However, effective
object-oriented programming in IDL requires a rather different approach
when compared to the more commonly used procedural programming
paradigm that has existed in IDL since it was first released. In this book, I
have chosen to limit the discussion strictly to procedural programming,
since I believe a larger audience for this topic exists in the community of
IDL users. Likewise, this book only describes the Direct Graphics system
in IDL and does not explore the realm of the Object Graphics system. At
the time of writing, the Direct Graphics system offered much simpler (i.e.,
easier to program) ways to create visualizations in IDL. To do justice to
objects and the Object Graphics system in IDL, I believe a separate book
is required that describes how object-oriented principles are applied in
IDL; hopefully such a volume will emerge in the not-too-distant future. In
retpahC 1 Introduction
the meantime, the chapters of this book that describe the fundamental
syntax of the language should prove useful to those writing object-
oriented programs in IDL.
Supporting Materials
The source code for the example procedures and functions shown in this
book are available online at the author's website:
http'//www.guml ey.com
It is recommended that you download the programs to a directory on
your system, and then add the directory to the LDI path by one of the fol-
lowing methods:
(cid:12)9 Ifyou are running the LDI Development Environment, select File, Pref-
erences, Path, and add the directory to the default search path.
(cid:12)9 On UNIX platforms you may add the directory to the IDL path envi-
ronment variable .HTI AP_LD For example, if you stored the programs in
a directory named/h0me/bi 11 g/i dl, the syntaxw ould be as follows:
% setenv HTAP_LDI ${IDL_PATH}./home/billg/idl (Cshell)
$ export IDL_PATH=${IDL_PATH}'/home/bilIg/idl (Kornshell)
Typographic Conventions
Throughout this book the following typographic conventions are used.
Commands that are intended to be entered at the LDI command line will
appear as follows:
IDL> tvscl, dist(256)
Procedures and functions (i.e., programs) will appear as follows:
ORP OLLEH
print, 'Hello world'
DNE
The program text may be entered in the LDI Development Environment
editor or in any other text editor. The name of the text file should be the
2.1 About IDL
same as the procedure or function name (e.g., he l l o. pro), and lowercase
file names are recommended.
Special Characters
IDL recognizes a number of special characters within IDL programs or
when commands are entered at the command line. The most important
special characters are described below.
The semicolon character designates the beginning of a comment. Any
text following a semicolon on a given line is ignored. For example:
ORP OLLEH
A (cid:12)9 simple program
print, 'Hello world' (cid:12)9 This line prints a message
DNE
If the last character on a line is a dollar sign, the current command is
continued on the next line. For example:
IDL> print, 'The quick brown fox ', $
IDL> 'jumped over the lazy dog'
If the first character on the line is a dollar sign, any text that follows is
executed as an operating system command. For example, to list the con-
tents of the current directory on a UNIX platform:
IDL> $1s-I
1.2
ABOUT IDL
One of the strengths of IDL is its support for a variety of hardware and
operating system combinations. Whether you are accustomed to a Win-
dows, MacOS, or UNIX (including Linux) environment, a version oflDL is
probably available for your system. The platforms supported by IDL 5.4
are shown in Table 1.1. For an up-to-date list of supported platforms and
operating system versions, see the RSI website (http://www. -hcraeser
systems, com).