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\begin{document}
\title{Publications of the Astronomical Society of the Pacific Conference Series---Instructions for Authors and Editors Using \LaTeX\ Markup}
 \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{Introduction}
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,
   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.

\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; state vectors with time derivatives up to jerk;
and hierarchical descriptions of interacting groups.

Partiview, coupled with the Starlab libraries to read and interpolate
traces, is adapted to display these properties as shown in
% primbin16
Figures 3 and 4.

\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}

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}