Simulation and Rendering of Liquid Foams Hendrik K

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Simulation and Rendering of Liquid FoamsHendrik
Kück (UBC, Vancouver)Christian Vogelgsang (FAU
Erlangen, Germany) Günther Greiner (FAU Erlangen,
Germany)

• Graphics Interface 2002

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Motivation

• Liquid foams can be found in many places in the
  real world
• Very difficult / impossible to recreate using
  standard techniques
• extremely complex microscopic structures
• unique optical properties
• complex dynamic behaviour

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Goals

• Visually convincing simulation and rendering of
  liquid foams
• Not Physically accurate
• But Efficient
• Interaction with external objects
• Integration into existing raytracing systems

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Outline

• Structure and dynamics of liquid foams
• Previous work
• Simulation of foam dynamics
• Shading
• Results
• Future Work

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Structure of liquid foams
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Structure of liquid foams
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Dynamics of liquid foams

• Viscoelastic, can behave like
• Solids (elastic deformation)
• Fluids (viscous flow)
• Film rupture
• Rising bubbles

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Previous work

• Physics
• Durian, 19952D foam dynamics

• Computer Graphics
• Almgren Sullivan, 1993Surface Evolver,
  Interference Colours
• Icart Arquès, 19992D foam, Interference
  colours
• Glassner, 2000Soap bubbles, Interference
  colours
• Durikovic, 2000Soap bubble dynamics,Mass-spring-
  damper system

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General approach

• Use simple model in simulation step
• Fixed size spheres
• No explicit computation of foam micro geometry
• Forces acting on spheres
• Output sphere geometry
• In ray-tracing step
• Reconstruct liquid films and Plateau borders
• Use appropriate shading models

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Bubble ? Bubble Forces

• Soap films minimize surface area due to surface
  tension

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Bubble ? Bubble Forces

• Soap films minimize surface area due to surface
  tension

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Bubble ? Bubble Forces

• Soap films minimize surface area due to surface
  tension

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Bubble ? Bubble Forces

• Soap films minimize surface area due to surface
  tension

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Bubble ? Bubble Forces

• Model with 2 spring forces per pair of
  overlapping spheres
• Attractive force
• Repulsive force

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Simulation

• Forces acting on spheres due to
• Contact with other spheres/bubbles
• Viscosity
• Air resistance
• Gravity
• Contact with external objects
• Assumption Bubbles have no mass
• ? Forces have to add up to 0 for each bubble
• Results in 1. order ODE system

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Simulation

• Start with randomly generated bubbles
• Initial simulation to get a stable configuration
• For each animation frame
• Randomly add/remove spheres
• Numerical integration to compute sphere positions
  for that point in time
• Generate sphere geometry
• Flatten spheres at external objects

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Rendering

• Special shader
• Invoked at every ray/sphere intersection
• Has to
• Decide if intersection corresponds to Plateau
  border or liquid film
• Perform shading using corresponding shading model

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Shading model selection

• Base decision on the order in which the ray
  enters and leaves spheres
• Shading only for some intersections
• Approximate separating films by averaging of
  adjacent intersections

Bubble 1
Bubble 2
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Plateau Borders

• 2 different cases
• Overlap of 3 spheres
• Empty space between spheres

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Liquid Film Shading

• Fresnel reflection
• (Interference Effects)

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Plateau Border Shading

• High curvature
• Refraction total reflection randomize light
  direction
• Our shading model
• Simple light diffusion approximation (multiple
  scattering)
• Single scattering

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Results

• Implemented for Mental Ray as combination of
  geometry shader and material shader

Resolution 800x630 700 bubbles 4 s.
simulation 40 s. rendering
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Future work

• Improve shading models
• Interference effects
• Simulation of multiple scattering
• Level of detail approach
• Efficient simulation and rendering of arbitrary
  dense foams at arbitrary scale

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Questions
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?
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• Acknowledgements
• This project was supported by Animation/VFX (SZM
  Studios, Munich, Germany)
• Special thanks to Horst Hadler and Michael
  Kellner