Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.adass.org/adass/proceedings/adass94/goochr.ps Äàòà èçìåíåíèÿ: Tue Jun 13 20:47:06 1995 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:47:53 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: þæíàÿ àòëàíòè÷åñêàÿ àíîìàëèÿ

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.
Space and the Spaceball
R. Gooch
Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping,
N.S.W., 2121, Australia, and Macquarie University
Abstract. The vast quantities of data produced by modern radio tele­
scopes have outstripped conventional visualization techniques available
to astronomers. ATNF staff have developed new visualization techniques
to give astronomers a greater intuitive insight into their data. While vi­
sualization techniques in other areas find some application in astronomy,
problems peculiar to the field require new techniques, such as methods for
identifying three­dimensional regions. This paper presents an overview
of some of the problems of visualization for astronomy and describes ex­
periments with the Spaceball, a three­dimensional pointing device.
1. Volume Rendering
Visualization of three­dimensional data sets has already been researched and im­
plemented in fields such medicine, for visualizing three­dimensional CAT scans.
Great progress has been made through the use of volume rendering tools. These
tools allow a three­dimensional data set to be displayed on a two­dimensional
display (the computer monitor), with controls which the user can rotate and so
obtain different views.
These techniques may also be applied to astronomy. While medical visu­
alization deals with data which truly represent three spatial dimensions, radio
astronomy spectral­line data sets represent two spatial dimensions and one fre­
quency dimension. This does not prevent the data from being displayed as if
they were a three­dimensional object, but the astronomer needs to be aware that
the display is merely a representation of the data.
At the Australia Telescope, we have had considerable success using volume
rendering. While we have experimented with both surface rendering as well as
volumetric rendering tools, we have found the latter to be more successful. The
focus of this technique is to make best use of the astronomers' spatial recognition
functions. By presenting data three dimensionally, the astronomer may identify
structure in the data which is not obvious when using conventional tools. Once
this structure is identified, the astronomer may then proceed to further analysis.
1.1. Limitations of Standard Volume Renderers
Many existing volume­rendering tools are designed for visualization. The ob­
jects of interest are solids and fluids which are purely absorptive. In contrast,
objects observed in radio astronomy contain regions of emission and absorption.
Volume­rendering tools for radio astronomy need to take account of this fact.
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Furthermore, most astronomical data contain noise, which further differentiates
them from medical data. However, even simple shaders are far more effective
in revealing structure than the techniques traditionally used by astronomers.
While these techniques are suitable for revealing two­dimensional structure, the
astronomer missed much of the three­dimensional structure. We find that a ra­
diative transfer (``hot gas'') shader is particularly helpful for volume­rendering
of astronomical data.
Another limitation of standard volume renderers is the lack of analytical
software. While in many cases medical specialists are content with a qualita­
tive assessment and rely solely on visual inspection of images, the astronomer
depends far more on quantitative analysis. Once a feature is identified in a
multi­channel data set, quantitative measurements are required to determine
the physical processes at work. Some of these measurements are simple (such as
identifying the frequency extent of an emission), while others require complex
processing to obtain a meaningful result.
1.2. Identifying Structure
Once the astronomer has visually identified a feature of interest in the data set,
some means of defining that feature is required. Therefore, a way of identifying
points in three­dimensional space is needed. Merely using a two­dimensional
pointing device (e.g., a mouse) in conjunction with a two­dimensional projec­
tion (a ``view'') is insufficient. Some means of moving and displaying a three­
dimensional cursor is required. Once this problem is solved, a visually identified
feature may be related to the analysis software.
2. The Spaceball
The Spaceball is a three­dimensional pointing device, available from Spatial
Systems, Inc. It is a force­sensing device which provides six parameters: three
orthogonal forces and three orthogonal torques. I have coupled the Spaceball to
a volume­rendering tool, allowing the user to rotate the volume in an intuitive
manner. In addition, users can move a three­dimensional cursor through the
volume, allowing them to identify three­dimensional regions of interest. While
the positioning tools available in immersive virtual reality environments (such
as a high­quality data glove) are far more advanced, they are also far more
expensive. The Spaceball and similar devices provide a cost­effective means to
position cursors in three­dimensional space.
When the user pushes the Spaceball in a particular direction, a three­
dimensional cursor is seen to move inside the volume. From experiments, we
have found that depth placement of the cursor is rather difficult. A simple solu­
tion is for the user to set the placement in the X and Y directions, then rotate
the cube by 90 ffi and set the placement in the remaining direction. Clearly, this
is still a somewhat cumbersome interface. An improvement may be obtained by
rendering the cursor as part of the data, rather than overlaying the cursor on
top of the rendered volume. This method is particularly effective when moving
the cursor behind thin opaque regions, as the placement is directly tied to the
data, which is the ultimate goal. For placement relative to larger regions, other
methods must be used to give depth cues to the user.

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To assist the user, a wire frame is displayed, with color­coded lines pro­
jecting from the cursor (a simple three­dimensional crosshair) to the three or­
thogonal corner planes. By upgrading to a stereo display, we expect to provide
the necessary depth cues to enable the cursor to be placed in three­dimensional
space.
3. Applications of a Three­Dimensional Cursor
3.1. Extracting Quantitative Information
To address the problem of extracting quantitative information, the user must
be able to define and extract sub­regions of the data set and process these with
a wide variety of algorithms that provide measures of the physical processes in
the observed astronomical object. Work is in progress to allow subsets of data
to be passed seamlessly to the analysis tools (such as AIPS++).
3.2. Viewing Small­Scale Structure
To expose small­scale structure the astronomer needs to isolate a region of in­
terest. One method is to integrate a ``slicer'' tool which allows the astronomer
to view the three orthogonal slices which intersect at a specified position. While
slicer tools have been available in some astronomical analysis packages for a
few years, specifying the slices was done using three separate linear controls
(knobs or sliders). To my knowledge, this is the first time an integrated input
device such as the Spaceball has been used to control a slicer tool for analyzing
astronomical data sets. Coupling this slicer tool with a volume­rendering tool
allows the astronomer to specify a point in three­dimensional space relative to
the overall structure while at the same time displaying small­scale structure.
3.3. Understanding Data
Most radio astronomy is not solely a matter of collecting, viewing and inter­
preting data. Many theoretical models exist which attempt to explain observed
phenomena. Once an astronomer has identified structure in the data and pro­
ceeded to perform quantitative analysis, the data need to be compared with
existing models. Various astronomical data analysis packages provide tools to
do this. However, they are limited to one­ or two­dimensional data sets.
A greater challenge will be to provide model­fitting tools for three­dimen­
sional data sets using algorithms tuned to such data. New model­fitting algo­
rithms need to be developed to take full advantage of emerging visualization
technologies.
4. Results
The feedback we have obtained from astronomers indicates that they can extract
more science from their data using the visualization tools we have developed than
was previously possible. By taking advantage of the spatial recognition and in­
tegration powers of the brain (powers which are tuned for three­dimensional
moving objects), features that would not appear using conventional techniques

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become readily apparent using volume­rendering tools. Objects that would ap­
pear as a number of faint, disjoint fuzzy patches in individual images appear as a
clearly defined object in three dimensions. In a number of instances astronomers
have found previously unknown features in their data when they used the new
visualization tools.
Using the Spaceball to identify regions of interest is proving an effective
means to extract quantitative information and focus on small­scale structure,
especially when coupled with a slicer tool.
5. Future Work
Further into the future is the possibility of experimenting with fully immersive
virtual reality environments. Virtual reality offers the potential to present far
more information to the user's brain than current video display technology. With
the capability to present more information will come the challenge to structure
that information in a cohesive, meaningful way. For example, the current tools
we are developing allow the user to view the data from the outside, using the
controls to enhance and suppress regions of the data. Using virtual reality
techniques, astronomers could walk through the data to regions of interest; this
would give them more selectivity in viewing data.
6. Summary
Visualization of three­dimensional data sets for radio astronomy will continue to
develop over the years. The field is currently in its infancy. While more­general
visualization of three­dimensional data sets is at a slightly more advanced stage
(but by no means mature), the problems unique to radio astronomy are chal­
lenging. Experiments with stereo displays, Spaceballs, analysis and modelling
software may open up new vistas for the astronomer, and provide interesting
and perhaps unexpected challenges for those developing visualization systems.
Acknowledgments. I thank Ray Norris and Tom Oosterloo for ideas and
contributions to the visualization project.