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visualization of large astrophysical simulations datasets

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.

Visualization of large astrophysicalsimulations datasets

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 IDL’s 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.


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.

D. Pomarède 1,*, E. Audit 2, R. Teyssier 2, B. Thooris 1

1 Service d’Electronique, des Détecteurs, et de l’Informatique, LILAS

2 Service d’Astrophysique

CEA/DAPNIA, Saclay, 91191 Gif-sur-Yvette, France

*Email :

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 8192×8192×8192 Cartesian grid and 4.1×107 cells .The AMR data are projected onto regular Cartesian grids.

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.






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 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.

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.

  • 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).


[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

[3] IDL The Data Visualization & Analysis Platform,

[4] The National Center for Supercomputing Applications HDF Home Page,

[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).