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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
The NCSA Horizon Image Data Browser: a Java Package
Supporting Scientific Images
R. L. Plante, 1 W. Xie,
J. Plutchak, 2 R. E. M. McGrath and X. Lu
National Center for Supercomputing Applications (NCSA), University
of Illinois, Urbana, IL 61820, Email: rplante@ncsa.uiuc.edu
Abstract. The NCSA Horizon Image Data Browser is a Java package
which includes ready­to­use applets and applications as well as reusable
classes for building new applications for browsing scientific images locally
or over the network. We present the current status of the development
of this package focusing on three general features: 1) flexible support for
metadata through a schema­independent design, 2) the implementation of
our metadata model to support world coordinate systems, and 3) support
for use of Horizon applications within the NCSA Habanero Collaborative
environment.
1. Introduction
The NCSA Horizon Image Data Browser 3 is a Java package for browsing scien­
tific images. It is a collection of ready­to­use applets and applications as well as a
toolkit of reusable classes that can be mixed and matched to create new applica­
tions. The basic goal of the package is to provide tools that can give a ``first­cut''
look at image datasets read from the network or local disk and act as a smooth
pipeline from data repositories to specialized native software. For many users,
particularly non­scientists, it can serve as a cheap, platform­independent tool
for visualizing real scientific data. The design of the package is independent of
data format allowing for the support of multiple formats. Initially, the package
will come with support for FITS, HDF, GIF, and JPEG. The package provides
such features as zooming, animation, pixel value and coordinate position display,
spreadsheet display, and color fiddling. (For a full discussion of the goals of the
package see Plante et al. 1997.)
In this paper, we will focus on three specific design features the Horizon
package: the support for metadata, world coordinate systems, and collaboration.
1 Astronomy Department, UIUC
2 Atmospheric Science Department, UIUC
3 http://imagelib.ncsa.uiuc.edu/Horizon
99

100 Plante, Xie, Plutchak, McGrath and Lu
2. Metadata
Metadata---data about data---are an important part of scientific data. They are
the ancillary information that accompanies some primary data set, allowing it
to be interpreted intelligently by scientists and the software they use. It plays a
critical role when data must be transported between di#erent software systems
or formats. In the Horizon package, metadata are a set of name­value pairs where
the name is of type java.lang.String and the value, java.lang.Object.
An important assumption that always exists when handling metadata is
the agreement between the creators of metadata and its users on the mapping
of a metadatum's name to its value type (e.g., Double, String, etc.) and its
conceptual meaning. A set of agreed mappings of names to types and meanings
is often referred to as a schema. Di#erent formats and di#erent scientific fields
often use di#erent schemata to represent metadata.
At the center of Horizon's support for metadata is the Metadata class
(Plante 1997). Its design is schema­independent and takes into account that
there may be multiple schema in use within a single application. Furthermore,
metadata can be hierarchical, so the Metadata class allows direct access to any
individual metadatum in the hierarchy, or whole sections of the hierarchy. Hori­
zon also supports array data as metadata. The following code illustrates both
the array and hierarchical features:
Metadata mdata;
...
// retrieve all metadata for coordinate axis 0
Metadata axmdata = (Metadata) mdata.getMetadatum("CoordinateSystem.Axes[0]");
// retrieve a specific coordinate axis sub­metadatum directly
String name = (String) mdata.getMetadatum("CoordinateSystem.Axes[0].name");
Other features of the Metadata class include support for default values that
can be overridden but not erased; this feature is used a way of giving read­
only access to metadata that are to be shared with many objects. The class
also supports on­demand loading of values; if the process of loading metadata
is expensive (e.g., they are read from the network), one would prefer that they
only be read if the user explicitly requests them.
The Horizon package defines a ``slim'' metadata schema which it uses to do
basic visualization. Part of the job of the format­dependent reader is to convert
selected native metadata into the ``horizon'' schema. Higher­level Horizon classes
use this schema to interact with the image data in a format­independent way,
including extracting chunks of data and displaying positions in the data's world
coordinate system.
3. World Coordinate Systems
A scientist usually wants to know where a piece of data exists in physical or
conceptual space--its World Coordinate System--rather than its location in some
data array. In the Horizon design, data can exist in a data space of arbitrary
number of dimensions which maps to a world coordinate space of the same or

The NCSA Horizon Image Data Browser 101
Browse FITS
image header
Track World
Coordinates
Subsampled View Zoomed View
Select Zoom
Figure 1. The ADILBrowser Applet: a viewer for browsing images
in the NCSA Astronomy Digital Image Library (ADIL).
di#erent number of dimensions. The transformation between data voxels and
coordinate positions can be non­linear and even non­reversible.
The two Horizon world coordinate classes used most by the application pro­
grammer are CoordinateSystem and CoordPos (Plante 1997). A Coordinate­
System object can be obtained for a particular dataset through its format­
independent interface. This object has full knowledge of the dataset's coordinate
system and its mapping to the data voxels. Thus, the programmer can give the
CoordinateSystem a data voxel and get back a CoordPos object, an encapsu­
lation of the corresponding coordinate position. The reverse transformation is
also supported. One special feature of the CoordPos object is that it knows how
to print itself; e.g., it knows to print right ascension as HH:MM:SS.S, galactic
longitude in decimal degrees, and velocity with the appropriate metric units.
These default formats can be easily overridden.
The transformation engine within the CoordinateSystem class is the Coord­
Transform class, used for converting between two coordinate systems. Horizon
comes with a number of specialized CoordTransforms that do things like spheri­
cal projections, switching between celestial and galactic coordinates, etc. (Some
of these have been implemented using FITSWCS, a Java port of the WCS li­
brary by Calabretta, see Greisen & Calabretta 1995.) Multiple CoordTransform
objects can be strung together to produce complex transformations. Thus, ev­
ery CoordinateSystem maintains an internal stack of CoordTransform objects
through which it can pass data voxels or coordinate positions. Users of the
CoordinateSystem can attach additional CoordTransform objects to the sys­
tem at any time for on­the­fly switching of coordinate systems.
The combination of Metadata and CoordTransforms is Horizon's recipe for
a smart CoordinateSystem. When a CoordinateSystem is constructed, it uses
Metadata gotten from the dataset to configure the needed CoordTransform ob­
jects. Included in the Metadata are AxisPosFormatter objects (one for each

102 Plante, Xie, Plutchak, McGrath and Lu
axis) that control how coordinate positions are printed out. If the user attaches
an additional CoordTransform object, say to switch from celestial to galactic
coordinates, the Metadata are consulted to determine which axes the transfor­
mation should be applied to. The Metadata are then transformed as well to
reflect the new coordinates system; thus, for example, the format can automat­
ically be switched from DD:MM:SS.S to decimal degrees.
4. Collaboration
Collaboration is supported within Horizon via the NCSA Habanero package, 4
a collaborative environment written entirely in Java. Habanero allows a group
of participants located on di#erent machines to interact with a single tool as
a group. (See Crutcher et al. 1998 in this volume for more details on the use
of Habanero with astronomical applications.) The Horizon classes will come
with special hooks that allow them to be run within Habanero environment in
a collaborative way.
Figure 1 from Crutcher et al. (1998, this volume) shows an example of
running a Horizon applet, the ADILBrowser (upper right corner; see also Figure
1 of this article) within a Habanero session. Participants use the applet to
browse images located in the NCSA Astronomy Digital Image Library. 5 The
applet tracks world coordinates and allows zooming into subregions of the image.
The remaining windows are part of the Habanero environment which include a
session manager, a chat window, and a white board.
Acknowledgments. The Horizon development team thanks Jef Poskanzer,
Mark Calabretta, and Tom McGlynn for contributed code. The Horizon Image
Data Browser package is supported in part by Project Horizon, a cooperative
agreement with NASA, and by the NASA Applied Information Systems Research
Program (96­OSS­10).
References
Crutcher, R. M., Plante, R., & Rajlich, P. 1998, this volume.
Greisen and Calabretta 1995, Representations of Celestial Coordinates in FITS 6 .
Plante, R. 1997, Supporting Metadata and Coordinate Systems for Scientific
Data in the Horizon Java Package 7 , white paper.
Plante, R., Goscha, G., Crutcher, R., Plutchak, J., McGrath, R., Lu, X., &
Folk, M. 1997, in ASP Conf. Ser., Vol. 125, Astronomical Data Analysis
Software and Systems VI, ed. Gareth Hunt & H. E. Payne (San Francisco:
ASP), 341.
4 http:://www.ncsa.uiuc.edu/SDG/Software/Habanero
5 http://imagelib.ncsa.uiuc.edu/imagelib
6 ftp://fits.cv.nrao.edu/fits/documents/wcs/wcs.all.ps.Z
7 http://imagelib.ncsa.uiuc.edu/Horizon/docs/articles/CoordsAndMetadata