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Fast Volumetric Display of Natural Gaseous Phenonema

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Fast Volumetric Display of Natural Gaseous Phenonema CGI 03 ... for pre-integration: dark opaque inside and bright translucent on the 'surface' ... – PowerPoint PPT presentation

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Title: Fast Volumetric Display of Natural Gaseous Phenonema


1
Fast Volumetric Display of Natural Gaseous
Phenonema
  • Stefan Roettger
  • Thomas Ertl
  • University of Erlangen

2
Introduction
  • Use unstructured volume rendering to display
    volumetric clouds and ground fog
  • C-LOD terrain rendering approach (2D)
  • Ground fog extension (2.5D)
  • Volumetric clouds (3D)

3
C-LOD Technique
  • View-dependent approximation of a height field
  • Coarser approximation for distant objects
  • Finer approximation in areas with high local
    curvature
  • Construct triangle mesh using a quadtree
  • Use top-down refinement (WSCG 98)
  • Implements geomorphing to supress popping

4
Ground Fog
  • Height field as basis
  • Second height field defines height of fog layer

5
Cell-Projection
  • Each prism is broken down to three tetrahedra
  • Each tetrahedron is rendered and composed using
    the PT algorithm of Shirley and Tuchman

6
PT Algorithm
  • Project each tetrahedron and decompose it into
    either three or four triangles (proxy geometry)

7
Ray Integral
  • Assuming homogenous scattering, the extension of
    Stein et al. reproduces the correct exponential
    behaviour of the ray integral Opacity
    1-exp(loptical_density)

8
From 2D to 3D C-LOD
  • From 2D height field to 3D scalar volume
  • Scalar field defines optical density
  • Data structure is a octree not quadtree
  • Primitives not triangles but rather tetrahedra

9
Volumetric C-LOD
  • Refinement of octree depends on second derivative
    of scalar field and view distance
  • Requires pre-integrated cell-projection (Roettger
    et al. IEEE VIS 00)
  • Nodes with zero optical density are culled

10
Pre-Integrated Cell-Projection
  • Ray integral depends on three parameters
  • Ray integral is pre-computed for all combinations
    and stored in a 3D texture

11
Lighting
  • Transfer function setup for pre-integration dark
    opaque inside and bright translucent on the
    surface
  • Additional diffuse lighting with modified PT
    algorithm (decrease opacity on the back of a
    cloud)

Pre-integration off Linear TF
Lighting on Custom TF
Opacity
Emission
12
Fin
  • Thanks for your attention!
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