# $Log$
# Revision 1.1  2002/06/26 19:31:53  slevy
# Maybe this will turn into an IAU208 paper about partiview.
# Figures to appear in same partiview/doc/iau208 directory.
#

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.


 ---


Data are 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,
   color Tlog 3.2 4.5
assigns colors by using the Tlog field (assuming one had been defined)
as an index into the user-supplied color table, using a linear mapping
that associates 3.2 and 4.5 with the colormap's endpoints.

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 a given range or set of values,
or look at particles only within a given rectangular subvolume,
and can print a histogram of values of a field,
over all particles or over the selected subset.


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The same "parti" graphical and data-handling code is embedded
in multiple guises for different computing environments.
The same data and configuration scripts, and most of the same
interactive commands, work in both.  Figure [1] shows 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,
on Silicon Graphics computers with multiple graphics pipes.

This particle display system was originally written for the virtual
choreography system Virtual Director [ref] for the CAVE [ref]
virtual-reality room at NCSA, so that we could choreograph animated
flights through a couple of particle models -- a digital Milky Way
model being assembled at the Hayden Planetarium, and a galaxy catalog
created by Brent Tully -- for educational animations we had agreed to make.

At the same time, the Hayden planetarium was building a Silicon
Graphics-based display for their new dome; this turned out sufficiently
CAVE-like that the same software runs in the Hayden dome and is
regularly used there.

The Virtual Director framework also provides for networked
collaboration, where participants can share viewpoints, animation
paths, display controls, etc.; this has proven helpful for bringing
together distributed expertise, and we've used it between Illinois,
Hawaii, New York and elsewhere on several occasions, most recently
when designing some animations for the 2002 Hayden space show.

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   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 and time derivatives up to jerk [right??],
and hierarchical descriptions of interacting groups.

Partiview, coupled with the Starlab libraries to read and interpolate
traces, is adapted to display these properties as in [fig 3 of primbin16].

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.