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!