Transcript Fast Volumetric Display of Natural Gaseous Phenonema

Fast Volumetric Display of
Natural Gaseous
Phenonema
Stefan Roettger
Thomas Ertl
University of Erlangen
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)
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
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
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
Ground Fog
• Height field as basis
• Second height field defines height of fog layer
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
Cell-Projection
• Each prism is broken down to three tetrahedra
• Each tetrahedron is rendered and composed using the
PT algorithm of Shirley and Tuchman
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
PT Algorithm
• Project each tetrahedron and decompose it into either
three or four triangles (proxy geometry):
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
Ray Integral
• Assuming homogenous scattering, the extension of
Stein et al. reproduces the “correct” exponential
behaviour of the ray integral: Opacity = 1exp(l*optical_density)
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
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
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
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
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
Pre-Integrated Cell-Projection
• Ray integral depends on three parameters
• Ray integral is pre-computed for all combinations and
stored in a 3D texture
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo
Lighting
• Transfer function setup for preintegration: dark opaque inside
and bright translucent on the
“surface”
• Additional “diffuse lighting” with
modified PT algorithm (decrease
opacity on the back of a cloud)
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
Lighting on
Custom TF
Emission
Pre-integration off
Linear TF
Opacity
CGI ‘03
Tokyo
Fin
Thanks for your attention!
Fast Volumetric Display of Natural Gaseous Phenonema
Stefan Roettger, University of Erlangen
CGI ‘03
Tokyo