Visualization of large astrophysical simulations datasets

1
Visualization of large astrophysicalsimulations
datasets

• D. Pomarède 1,, E. Audit 2, R. Teyssier 2, B.
  Thooris 1
• 1 Service dElectronique, des Détecteurs, et de
  lInformatique, LILAS
• 2 Service dAstrophysique
• CEA/DAPNIA, Saclay, 91191 Gif-sur-Yvette, France
• Email pomarede_at_cea.fr

The SDvision graphical interface is designed to
visualize large datasets produced in numerical
simulations of astrophysical plasmas. It is
conceived for the visualization of 2D and 3D
scalar and vector fields distributed over regular
mesh grids or more complex structures such as
Adaptive Mesh Refinement data, as well as N-body
systems. Various implementations of the
visualization of the objects are simultaneously
proposed, such as 3D isosurfaces, volume
projections, hedgehog and streamline displays,
surface and image of 2D subsets, profile plots,
particle clouds. It is part of the Numerical
Simulations Software Project 1,2 engaged at the
Saclay/DAPNIA Laboratory with the objective to
provide a core of software modules useable by the
astrophysical simulation tools in development.
This application is essentially used to visualize
large data sets generated by parallelized code
running on massively parallel mainframes. The
data are written in the HDF5 Hierarchical Data
Format developed by the NCSA 4, allowing for
efficient data access. Two methods of rendering
objects are supported via a hardware graphics
accelerator or via a software rendering package.
The first solution, based on the use of the
OpenGL libraries, is highly efficient when
operating on a local machine. The second solution
applies when the application runs on a remote
computer.
The baseline technology is the object-oriented
programming offered by IDLs Object Graphics 3.
The interface is implemented as a graphical
widget providing interactive and immersive
3-dimensional navigation capabilities. The user
acts on the objects attributes through an
ensemble of menus, drop lists, buttons and dialog
fields. Complex objects such as isosurfaces are
implemented as polygon and polyline objects with
the help of optimized procedures of the IDL
library. Visualization of 3D objects benefits
from the implementation of light objects that
represents sources of illuminations.
SDvision
the Saclay/DAPNIA Visualization Interface
The SDvision program is employed to visualize
data from RAMSES 5, an AMR-based hybrid N-body
and hydrodynamical code which solves the
interplay of dark matter and the baryon gas in
the study of cosmological structures formation,
and from HERACLES 6, a radiation hydrodynamics
code used in particular to investigate
turbulences in interstellar molecular clouds.
The SDvision widget used to visualize a surface
of the density distribution in the Interstellar
Medium obtained in an HERACLES simulation. The
navigation capabilities of the widget make it
possible to rotate, displace and scale the scene
at will through mouse actions. The distribution
is also projected onto an image on which profiles
can be interactively obtained. Menus are used to
modify the attributes of the objects on display
(palette, shading, texture, lighting). Sliders
can be used to scan through the volume in either
direction.
The SDvision widget allows to visualize complex,
composite scenes. The graphical objects
lifecycles are managed through the use of
controllers (displayed on the right hand side).
Such controllers are implemented for  data
objects  (isosurface, volume projection, image,
vectors, streamlines),  geometry objects 
(boundaries, axis), and  scene objects 
(illuminations). Objects are configured through
dedicated interfaces. In the example above,
obtained in a 200?200?200 regular-grid HERACLES
simulation, the scene includes two isosurfaces,
one image and one streamline objects. Isosurfaces
are 3D contours. An histogram of the associated
data is displayed and can be used to set
interactively the desired contour value.
The SDvision widget provides simultaneous access
to the N-body data and the complex AMR octree
structures generated by RAMSES. In the example
above, the 128?128?128 Dark Matter particles are
viewed together with an image of the
hydrodynamical baryon density. An histogram of
either the DM density or velocity is displayed,
used to select a subset of the full particle
complement. On the left-hand side, an interface
is dedicated to loading data and select higher
resolution levels. The highest resolution is
reached at level 14 of the AMR, equivalent to
that of a 819281928192 Cartesian grid and
4.1107 cells . The AMR data are projected onto
regular Cartesian grids.
c)
a)
b)
d)
e)
Illustration of the immersive capabilities of the
SDvision widget. The viewing point is located
within the simulation volume and the scene is
observed with a wide-angle focal. This simulation
of cosmological structures formation for a volume
of size 100 h-1 Mpc is obtained with RAMSES with
a resolution up to the level 8 of the AMR,
equivalent to a 256?256?256 regular Cartesian
grid. From this internal viewpoint, the scene is
augmented with various graphical objects allowing
to better interpret, analyze and validate the
simulation. An important aspect is the
transparency of the objects obtained by tuning
their associated alpha-channels. In a) are
displayed the high-density Dark Matter cores
embedded in an a gray isosurface of the baryon
density. A red and a yellow lower-density
isosurfaces are also added together with a
transparent image of the density. In b) the image
is kept and the view is augmented with a display
of a maximum-intensity projection of the baryon
density. Filamentary structures are observed,
well-correlated to the grey isosurface topology
in a). The effect of the AMR algorithm can be
inferred from the varying granularity of the
graphical objects. The red Dark Matter
high-density cores displayed in c) are correlated
with the most dense baryonic regions, where
galaxy clusters are in development. In d) the
baryonic hydrodynamical velocity field is added
the streamlines are seen plunging toward the DM
cores. In e) the transparent image gives another
profile of the baryonic density which exhibits
filaments and halos. This immersive investigation
of the structures gives strong indication of the
validity of the simulation.
HERACLES simulation of turbulences in the
interstellar medium on a high-resolution
1200?1200?1200 grid with dimension 15 pc. Left
image of the plasma density on a slice. In this
very turbulent simulation, a cold dense phase and
a diluted hot phase are tightly interwoven.
Right volume projection of the density. The
brightest spots are dense protostellar cores
formed by the thermal instabilities.
Simulation of cosmological structures formation
for a volume of size 100 h-1 Mpc with RAMSES. On
the left, the density distribution is visualized
by volume projection. The hydrodynamical baryon
density is distributed in halos and filaments.
The brightest areas are associated to the
development of galaxy clusters. On the right,
Dark Matter is visualized as a particle cloud. DM
particles concentrate in halos where regions of
high baryon density develop.
Simulation of turbulences in the interstellar
medium with HERACLES. Left distribution of
internal energy and hydrodynamical velocity field
on a slice. Interactively adjustable sliders can
be used to scan through the volume box. Right
an isosurface of the density field is displayed
as a semi-transparent polygon object. The
velocity field displayed as streamlines can thus
be viewed in and out of the isosurface.

• Perspective of developments
• optimization of memory management to access
  larger data sets synchronous spatial and
  resolution zoom.
• parallelism to improve speed for some specific
  processes (AMR projection into regular grids,
  computation of volume projections) using the
  fastDL/mpiDL solutions for IDL 3.
• application to new simulation algorithm
  (multiple-grid HERACLES) and in new fields (solar
  dynamics, proto-planetary systems).

References 1 Numerical Simulations of
Astrophysical Plasmas status and perspectives of
the Saclay/DAPNIA software project, E. Audit, D.
Pomarède, R. Teyssier, B.Thooris, Proceedings of
the First CalSpace-IGPP International Conference
on Numerical Modeling of Space Plasma Flows, Palm
Springs CA, USA, March 27-30, 2006, to appear in
the Astronomical Society of the Pacific
Conference Series. 2 The SNOOPY Project Web
Site http//www-dapnia.cea.fr/Projets/SNOOPY/ 3
IDL The Data Visualization Analysis Platform,
http//www.ittvis.com/idl/ 4 The National
Center for Supercomputing Applications HDF Home
Page, http//hdf.ncsa.uiuc.edu/ 5 Cosmological
Hydrodynamics with Adaptive Mesh Refinement A
New High Resolution Code Called RAMSES, R.
Teyssier, Astronomy and Astrophysics, 385, 2002,
337-364 6 HERACLES a new, parallelized,
multi geometry and tridimensional RHD code, M.
González, E. Audit, P. Huynh, to be submitted to
Astronomy and Astrophysics (2006).