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% Revision 1.1  2002/06/26 19:31:53  slevy
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\begin{document}
\title{Interactive 3-D visualization for particle systems with Partiview}
 \author{Stuart Levy}
\affil{National Center for Supercomputing Applications, 
        University of Illinois Urbana-Champaign,
	605 E. Springfield,
	Champaign, IL 61820}

\begin{abstract}
This paper describes partiview, a software tool for 
interactive graphical display of collections of particles
in 3-space, and its application in studying the results
of N-body collisional stellar dynamics calculations from Starlab.
\end{abstract}

% ---

\section{Partiview}
Partiview is an interactive graphical software tool, focused on 
flexible display of particles in 3-space.

Input data to partiview is provided as a (possibly time-varying)
collection of particles, each with a 3-D position and an arbitrary number
of other floating-point attributes ("fields"), and a configuration script
specifying which fields to map into visible properties,
including color and luminosity.  For example, if a field were named Tlog,
{\verb"  color Tlog 3.2 4.5
"}
assigns colors by using the Tlog field as an index into a
user-supplied color table via a linear mapping
that associates 3.2 and 4.5 with the colormap's endpoints.
Text-based commands can change these selections interactively,
switching between coloring galaxies by e.g. morphological type,
local density, or group membership on the fly.

From each particle's luminosity and distance from the current
viewpoint, partiview draws a dot whose screen brightness and size
suggest its computed apparent brightness.  With dots up to a few
pixels across, apparent brightness may usefully range by several
hundredfold, and larger ranges can be suggested by adding textured
polygons -- "haloes" -- whose size varies similarly.  The result
is good enough to give plausible naked-eye starfields given a table
of stellar luminosities, colors and 3-D positions as in Figure~1,
drawn using Hipparcos data.

This sort of viewpoint-dependent apparent brightness
is a feature that few other scientific visualization
packages don't seem to offer, even though it's
inexpensive to compute and can be useful.
(Where not useful, as when making orthographic
plots of 3-D scenes, it can be switched off in partiview.)

Some simple database-like operations are provided.
For example, one can display only the subset of particles where
some (single) field has values in a given range or set,
or look only at particles lying within a given rectangular subvolume,
Also one can print a histogram of values of a field,
over all particles or the selected subset.

\begin{minipage}{3.5in}
	\centerline{\hbox{
	\psfig{figure=starfield.ps,height=2.7in}
	}}
	\centerline{Figure 1. Partiview showing star field from Hipparcos data with 
		Sun at upper left, marked by 0.1 pc crosshair.}
\end{minipage}\    \
\begin{minipage}{3.5in}
	\centerline{\hbox{
	\psfig{figure=galaxies.ps,height=2.7in}
	}}
	\centerline{Figure 2.  Virtual Director version.}
\end{minipage}


% \begin{minipage}{3.5in}
%   \begin{figure}
%    \psfig{figure=starfield.ps,height=2.7in}
%    \caption{Star field from Hipparcos data.  The Sun is
%     at upper left, and the crosshair has radius 0.1 pc.}
%   \end{figure}
% \end{minipage}\    \   
% \begin{minipage}{3.5in}
%   \begin{figure}
%    \psfig{figure=galaxies.ps,height=2.7in}
%    \caption{Galaxies.}
%   \end{figure}
% \end{minipage}


\section{Desktop and dome}
The same graphical and data-handling code is embedded
in multiple guises for different computing environments.
Both accept the same data and configuration files, and most of the same
text-based interactive commands.  Figure~1 illustrates the
desk- (or lap-)top version, mouse and keyboard driven
with conventional buttons and sliders for common controls,
available for Unix-like systems and for Windows.
Figure~2 shows the virtual-reality version,
built using the Virtual Director virtual-choreography framework
and the CAVE library; it can run on Silicon Graphics computers
with multiple graphics pipes.

Though the latter was originally written for the
CAVE virtual reality room at NCSA, it is used elsewhere too.
The Hayden Planetarium at the American Museum of Natural History
in New York built a Silicon Graphics-driven display for their
planetarium dome; this turned out sufficiently
CAVE-like that the same software runs in the Hayden dome and is
regularly used there.


\section{N-body dynamics: examining Starlab traces}

Stellar dynamics simulations done in Starlab [ref?]
produce "traces", recording various information about each star
as a function of time: physical properties such as mass,
luminosity and temperature; position and three time derivatives;
and hierarchical descriptions (binary trees) of interacting groups.

Partiview, coupled with the Starlab libraries to read and interpolate
traces, is adapted to display these properties as the cluster evolves.
Figure~3 shows a tiny test case, comprising 16 stars.  Bound or
strongly-interacting systems are circled[[, and colored according to the
number of members of the group]].  Circle sizes are related
to separation or semimajor axis, but can be constrained to a range
of screen sizes to ensure that even tightly-bound systems are always
visibly marked.   Groups' binary trees, with stars as leaves and
center-of-mass points as nonleaf nodes, are shown as stick figures.
Tick marks perpendicular to the tree edges, drawn in the screen plane,
serve a dual purpose: their positions show the center-of-mass location,
and their lengths are proportional to the instantaneous true separation
of the nodes on that edge.  
Thus in the triple system [[in magenta]], although in this view the
lower-right pair of stars seem very close together, this must be a 
projection effect; as shown by the tick marks, their true separation
is nearly as large as that from their center-of-mass to the upper-left star.

Figure~4 shows a larger cluster evolving in a tidal field
[[with stars colored by mass]].  "Trails" show the recent motion
history of each star, so long trails show high speed, and curved ones
high acceleration.  Note the tidally escaping stars at left and right.

\begin{figure}
\psfig{figure=primbin16.ps,height=2.5in}
\caption{Interacting groups of stars from Starlab, with hierarchy shown.
Tick marks on each segment (a) mark center-of-mass position and (b)
have length proportional to true, not projected, separation of that pair of nodes.}
\end{figure}

\begin{figure}
\psfig{figure=0607.ps,height=2.5in}
\caption{Star cluster dispersing in a tidal field}
\end{figure}

Each star's dynamical state is sufficiently finely sampled in time
to allow accurate interpolation, generally at some fixed multiple of the
internal simulation timestep.   Thus stars in dense regions
may have far more frequent trace entries than isolated stars.
The Starlab libraries offer functions to interpolate the state of the
simulation at any time.

\end{document}
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