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

2. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo 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. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo 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 C-LOD Technique

4. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo Height field as basis Second height field defines height of fog layer Ground Fog

5. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo Each prism is broken down to three tetrahedra Each tetrahedron is rendered and composed using the PT algorithm of Shirley and Tuchman Cell-Projection

6. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo Project each tetrahedron and decompose it into either three or four triangles (proxy geometry): PT Algorithm

7. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo Assuming homogenous scattering, the extension of Stein et al. reproduces the “correct” exponential behaviour of the ray integral: Opacity = 1- exp(l*optical_density) Ray Integral

8. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo 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 From 2D to 3D C-LOD

9. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen 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

10. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo Ray integral depends on three parameters Ray integral is pre-computed for all combinations and stored in a 3D texture Pre-Integrated Cell-Projection

11. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo 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) Lighting Pre-integration off Linear TF Lighting on Custom TF Emission Opacity

12. Fast Volumetric Display of Natural Gaseous Phenonema CGI ‘03 Stefan Roettger, University of Erlangen Tokyo Thanks for your attention! Fin