Large Scale Structure of the Universe

1
Large Scale Structure of the Universe

• Sameshan Perumal
• and
• Carl Hultquist

2
Motivation

• Two major repositories of galaxy information the
  Sloan Digital Sky Survey and the 2dF Galaxy
  Redshift Survey.
• Galaxies group to form large-scale structures.
• By studying these, astronomers hope to develop a
  better understanding of the devlopment of the
  universe.
• Due to large amounts of data, suitable software
  is needed to conduct these studies.

3
The Bath-Sponge

• Prototype visualisation of 2dF data

4
Data Representation

• Image taken from 2dF Movie II

5
Data Representation

• Potentially huge volume of data to represent
• Large overhead to process
• Must be efficiently used by OpenGL Renderer
• Memory constraints must be considered 100 000
  galaxies ? 256 bytes 24 Mb

6
Data Representation

• Use minimal spanning trees
• Split space into neighbourhoods
• Store only critical information
• Keep coordinate data close to OpenGL standard

7
Structure Identification

• Attempt to extract structure from raw data
• Density calculations offer quick approximation
• Percolating spheres are accurate
• Grow sphere of given radius around galaxy
• Compute intersections between neighbours
• Connect intersecting galaxies

8
Percolated Spheres
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Density Calculations

• Image taken from 2dF Movie II

10
Surface Generation

• Uses results from Structure Identification
• Construct surfaces around structures
• Triangulate surfaces
• Pass data through to rendering engine

11
Surface Generation I

• Image generated by SurfGen, astro-ph/020136 v1,
  6 October 2002.

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Surface Generation II

• Image generated by SurfGen, astro-ph/020136 v1,
  6 October 2002.

13
Statistical Feedback

• Use generated surfaces
• Compute various statistical measures
• Volume, Area, Density
• Can be used to refine results
• Most useful to compensate for Diminishing Data
  Density

14
N-Body Simulations

• Simulate the evolution of the Universe
• Uses finite number of bodies
• Evolution occurs within a bounded box
• Attempt to analyse data using system
• Possibly run N-Body simulations

15
Graphical User Interface (GUI)

• Allow for flying around the universe.
• Clearly depict galaxies, large-scale structures
  and spheres of percolation.
• Allow for customisation of view
• Choosing which types of objects should be
  displayed
• Choosing what method should be used to display
  objects
• Allow for individual structures of galaxies to be
  selected and queried for data.
• Allow for parameters affecting structure
  identification to be modified.

16
GUI ? Aims

• Usable
• Accessible
• Sound HCI principles
• Realistic and adaptive
• Shading techniques
• User immersion
• Efficient
• Varying levels of detail
• Maintaining a target frame-rate
• Bill-boarding
• Programmable vertex and pixel pipelines

17
GUI ? Usability

• Accessible
• Cross-platform
• C
• OpenGL
• wxWindows toolkit for dialogs, buttons, etc. that
  are native to the environment
• Sound HCI principles
• Familiar icons
• Intuitive interface
• Possibly develop more than one interface (or make
  interface adaptive) and assess usability by means
  of a poll amongst several users.

18
GUI ? Realism and Adaptivity

• Shading techniques
• 3D surfaces
• Probability of accurate structure identification
• User immersion
• Use of ChromadepthTM

A pair of ChromaDepth lenses
A ChromaDepth image of the Death Valley
Earthquake Fault
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GUI ? Efficiency (1)

• Varying levels of detail (Clark, 1976)

Images from Progressive Meshes by Hugues Hoppe
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GUI ? Efficiency (2)

• Maintaining a target frame-rate (Funkhouser and
  Séquin, 1993)
• Fixed level of interactivity
• Maximises level of detail by placing an upper
  bound on the cost of rendering a scene

Images that appear larger contribute more to
the image.
21
GUI ? Efficiency (3)

• Bill-boarding

Left explanation of bill-boarding. Courtesy of
Lighthouse 3Ds Bill-boarding Tutorial.
Right an prototype developed by us that renders
the 2dF galaxy data by drawing each galaxy as a
bill-board, using the image of a sphere as a
texture.
22
GUI ? Efficiency (4)

• Programmable vertex and pixel pipelines
• Vertex shaders
• Morph continuously between different levels of
  detail (Southern Gain, 2002)
• General purpose GPU-based transformations rather
  than CPU-based
• Pixel shaders
• Shading of surfaces

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Outcomes

• Create library to
• Efficiently identify large-scale structures
• Generate surfaces around structures
• Refine structures, provide statistical feedback
• Effective GUI which uses the above to
• Interactively visualise large-scale structures
• Allow the user to manipulate structure
  identification
• View information about structures