1. Jeff Lait Inside Fire Rebuilding Houdini 9.5's Fire Simulator

2. Overview  Flame fronts  Inflation with divergence  Contraction with fixed flame speed  Curvature dependent flame speed  DSD-style flame speed  Flame generation  Track particles inside fluid  Track crossing age of particles  Flame sources  Gas sources  Solid sources  Combustion models

3. Inflating Flame Fronts

4. What are Signed Distance Fields (SDFs)? ● Voxels store 1 for fluid, 0 for none ● SDFs store negative for fluid, positive for none ● Zero crossing not restricted to voxel boundaries, allows sub-voxel detail

5. Divergence from the Flame Front

6. What is Divergence? ● Measure of imbalance of velocity field ● Divergence at center can be measured by comparing in going and outgoing velocties of boundary of cell ● Incompressible fluids should have zero divergence ● Consists of “swirl”, “shear”, “translate” factors. Does not have “scale” factor

7. Velocity from Divergence

8. Inflation by Divergence  Divergence calculated from flame front  Flame front advected by velocity

9. Collapsing Flame Fronts  Burning of fuel along the flame front has it move in the direction normal to its surface

10. Collapsing SDF by Adding Constant

11. Flame Front without Rebuilding SDF  Rebuild SDF is bypassed  Flame front holds its own and then collapses  Divergence field drops first, removing inflation  But divergence is a direct function of the flame front!

12. Flame Front, SDF view  False color of SDF  Note that the center washes out first, becoming constant  Then the field collapses inwards as it loses the inflation  Flat areas of color are no longer distance fields

13. Flame Front with Rebuilding SDF  SDF Rebuilding term reintroduced  System enters a steady state where collapse is matched by inflation

14. Flame Front with Rebuilding, SDF View  Note the interior gradient stays strong as it is rebuilt every frame.

15. Collapsing Flame Front by Curvature  Instead of constant reduction, vary according to local curvature of flame front  Note projection bypassed so movies will be only curvature effects

16. What is Curvature?  Measure of how quickly surface is turning  1 / radius  Zero curvature is flat  Infinite curvature is sharp bend  Signed curvature tracks which way it is bending  3d is more confusing  Saddle points  Sphere vs Cylinder R1 R2

17. Curvature Calculation

18. Blur Curvature

19. Negative Curvature Update  Areas bulging out move farther out  Areas bulging in move farther in  Unstable. Curvature blur is what keeps it from blowing up

20. Positive Curvature Update  Areas bulging out move farther in  Areas bulging in move farther out  Stable. Flattens into lines and collapses circles

21. Detonation Shock Dynamics  Simplified Gas DSD Solver  Add history to Flame speed  Flame speed  Flame speed speed

22. Detonation Shock Dynamics Model  Spring model  Detonation Speed (rest)  Elasticity, Damping  Dv/Dt = Dx  Curvature Spring  Curvature Forcing, Damping  Dv/Dt alpha Kappa  Material vs Time derivatives

23. No Dynamics  Curvature Forcing 0, so no perturbations of flame speed  Flame speed starts at “rest” speed so elastics do nothing

24. Elastic Dynamics  Strong elastic force  Zero default speed, so accelerates to goal flame speed, overshoots, and returns.  Damping reduces oscillations

25. Curvature Forcing, Weak Elastics  Curvature forcing triggers a perturbation of the field  Weak elastics let curvature dominate

26. Curvature forcing, Strong Elastics  Strong elastics increase response to curvature term.

27. Passive Particle Tracking  Pop Object Tool merges in a particle system to our Dop network

28. Passively Advect Particles  “Advection” is movement of a property by a velocity field  Advect By Volumes to passively advect particles

29. Particles Overlaying Curvature Flame Speed  Flame front appears stationary  Moving particles bely actual motion of fluid

30. Tracking Flame Front Crossing Times  Attribute “heat” created to track time since particle crossed boundary  In spirit of advectbyvolume, made a subnetwork to provide a clean interface.  Color Pop remaps heat into a black body ramp

31. Glue Layer  Ensure heat attribute exists  Use Sop network to fetch the volume from Dops  Build actual tool out of Vops

32. Heat Tracking  Track current side of volume by comparison  Use history to flag when particle crosses flame front – ie, ignites  Heat tracked in separate attribute and constantly cooled  Note particles inside the flame front are unheated and unburnt

33. Heat as Blackbody  Particles crossing flame front get heat of 1 that decays  Matches Houdini's Fire Solver, but that uses a scalar field for heat rather than particles

34. Gas Source – No Collision

35. Solid Source – Source Matches Collision

36. Creation of Offset Surface for Source

37. Solid Source, Collisions and Offset  Divergence boosted to account for lower area  Gas sticks to the surface, only leaving due to divergence

38. Paint Fuel Attribute

39. Auto-Build Source Surface  Microsolvers to build source surface  Build SDF  Add to SDF fuel value to make offset

40. Build Signed Distance Field

41. Fuel Field from Attribute

42. Add Fuel and SDF for Source

43. Fuel Consumption

44. Effect of Fuel Consumption  Fuel slowly consumed  Offset decreases as fuel attribute decreases

45. Create Temperature Field  Temperature field initialized from Vop Sop

46. Vop Based Temperature Animation

47. Burn Based Fuel  Copy temperature field onto temperature point attribute  Create burn attribute based on temperature and fuel values  Use burn attribute for offseting

48. Fuel Ignition in Vop Network

49. Ignition Based Fuel  Very similar to what is packaged in Gas Burn Object Dop

50. Conclusion  Burst Into Flame's fire is  Flame front based  Renders based on time-since-flame crossing  Uses ignition temperature for fuel combustion model  Houdini's fire is  Configurable  Modular  Open  Microsolvers are sufficient to build complicated effects

51. Wavelet Turbulence for Fluid Simulation  Siggraph 2008 paper by Theodore Kim, Nils Thürey, Doug James, Markus Gross  Overview  Run a low resolution simulation  Re-advect smoke density in higher resolution simulation with modified upsampled velocity field  Ideas  If noise added to velocity field is bandwidth limited, bulk motion of low res sim stays true  Amount of noise can be predicted by examining energy in highest frequencies of low res velocity fields

52. Linear Upscaling

53. Adding bandwidth limited turbulence

54. Curl based turbulence