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Motivation
Movie Game Engineering
Introduction
Ideally Looks good Fast simulation
Looks good? Look plausible Doesn’t need to be exactly correct doesn’t suffer from excessive numerical
dissipation
Overview
Simulation Compressible/Imcompressible Equation Vortex particle method Using octree data structure
Rendering Thick smoke: plain particles Thin smoke: adaptive particles Using Compensated Ray Marching
Compressible Fluids Yngve, G. D., O'Brien, J. F., Hodgins, J. K., 2000,
Animating Explosions. The proceedings of ACM SIGGRAPH 2000, New Orleans, July 23-28, pp. 29-36.
Operator Splitting
Stam used the technique in his Stable Fluids paper, 1999.
Allows us to solve the Navier-Stokes equation in Parts.
As long as each part is stable the technique will be stable.
Semi-Lagrangian Convection
Trace each point p in the field backwards in time. The new velocity at x is therefore the velocity that the particle had a time step ago at the old location.
Smoke Viscous Diffusion
In the simulation of smoke, it is reasonable to consider the fluid inviscid. Therefore this term is zero and need not be solved for.
In fact, the implicit solver that is used will take energy away from the system anyway, so there is a numerical dampening that can look like a non-zero viscosity.
Body Forces
Gravity, user forces, and object interaction.
The visible smoke is from a density field.
Areas of high density fall in the direction of gravity.
Heat moves against gravity (hot air rises).
Incompressibility
At each time step we start with a velocity field that is divergence free. We need this to be true at the end of the time step because the fluid is incompressible.
After solving for convection, and body forces the velocity field is not divergence free.
Incompressibility
Fedkiw, R., Stam, J. and Jensen, H.W.,"Visual Simulation of Smoke",SIGGRAPH 2001, 23-30 (2001).
Incompressible Euler Equations
self-advection Forces
incompressible
(Navier-Stokes without viscosity)
Additional Equations
smoke’sdensity
temperature
We now know all the finite differences we need to add the forces for heat and density…
Vorticity Confinement
The numerical dissipation, and the coarse grid size, cause the fine scale detail of turbulent swirling smoke to vanish.
Identify where the curl is highest, and add back in a rotational force there.
“Vorticity Confinement” force preserves
swirling nature of fluids.
What is vorticity?
A measure of the local rotation in a fluid flow.
Pressure Boundary Conditions A Neumann boundary condition is a
restriction on the derivative of a function.
Velocity Boundary Condition A Dirichlet boundary condition is a
restriction on the value of a function.
Vorticity Confinement
Vorticity Confinement
Vorticity Confinement
Solving the System
Need to calculate:
Start with initial state
Calculate new velocity fields
New state:
Smoke Simulation
While (simulating) Get external forces (if any) from UI Get density/heat sources (from UI or init
grid cells) Update velocity Update density Update temperature Display density
Smoke Simulation
While (simulating) Get external forces (if any) from UI Get density/heat sources (from UI or init
grid cells) Update velocity Update density Update temperature Display density
Update Velocity
Equation: First term:
Advection Move the fluid through its velocity field
(Du/Dt=0) Second term:
external forces Final term:
pressure update
Update Velocity
Add external forces (fbuoy + forces from UI + fconf)
Advection (semi-lagrangian step – trace particles back)
Pressure update (solve linear system)
Smoke Simulation
While (simulating) Get external forces (if any) from UI Get density/heat sources (from UI or init
grid cells) Update velocity Update density Update temperature Display density
Update density
Equation First term:
Advection Move the temperature through velocity field
Second term: Diffusion
Can skip this term
Second term: external sources
Update density
Add Sources (pick grid cells or from UI)
Advection (semi-lagrangian step – trace particles back)
Diffusion (solve linear system – can skip this step)
Smoke Simulation
While (simulating) Get external forces (if any) from UI Get density/heat sources (from UI or init
grid cells) Update velocity Update density Update temperature Display density
Update temperature
Equation First term:
Advection Move the temperature through velocity
field
Second term: Diffusion
Can skip this term
Update Temperature
Add Sources (grid cells or objects or UI)
Advection (semi-lagrangian step – trace particles back)
Diffusion (solve linear system – can skip this step)
Smoke Simulation
While (simulating) Get external forces (if any) from UI Get density/heat sources (from UI or init
grid cells) Update velocity Update density Update temperature Display density
Display density
Use any approach you want Visualize the density field: just opengl render a bunch of cubes
(corresponding to grid cells) that have alpha values based on how dense the smoke is.
Numerical Dissipation
‘ Stable Fluids’ method dampens the flow Typical with semi- Lagrangian methods
Improve using “Vorticity Confinement” force
Total Forces
User supplied fields
Buoyancy force
New confinement force
Results
Results
Other Methods
Vortex particle method Using octree data structure
Vortex particle method-Lagrangian primitives
Curves carry the vorticity
Each local vortex induces a weighted rotation
dx
xp
xp3
4
)(
ω
v dlxp
xp
3
4
)(
ω
v
Vortex particle method-Lagrangian primitives
Curves carry the vorticity
Each local vortex induces a weighted rotation
Method of simulation
Vortex particles (for motion) organized as curves. = tangent
Smoke particles (for visualisation)
Curves carry vortices Vortices induce a velocity field velocity field deforms curves & smoke
At every step: Advect the curves Stretch Advect the smoke
Method of simulation
Vortex particles (for motion) organized as curves. = tangent
Smoke particles (for visualisation)
Curves carry vortices Vortices induce a velocity field velocity field deforms curves & smoke
At every step: Advect the curves Stretch Advect the smoke
Method of simulation
Vortex particles (for motion) organized as curves. = tangent
Smoke particles (for visualisation)
Curves carry vortices Vortices induce a velocity field velocity field deforms curves & smoke
At every step: Advect the curves Stretch Advect the smoke
Video
Smoke Rendering Thick smoke: plain particles Thin smoke: adaptive particles [AN05]
accumulate stretching
01nTOT JJJJ ... 01nTOT JJJJ ...
Smoke Rendering
Thin smoke behaves like a surface
[ William Brennan ][ William Brennan ]
ee
nn
ll
eJJn 1T eJJn 1T
Smoke Rendering
Using Compensated Ray Marching
References