Computer Modelling Of Fallen Snow

1. Computer Modelling Of Fallen Snow Paul Fearing University of British Columbia Vancouver, Canada

2. Goal

3. Goal

4. Introduction • Related Work • Snow Accumulation • Snow Stability • Implicit Function • Validation • Future Work • Conclusion

5. Decomposition of Gravity

6. Global of the Snow Model • Snow Location • Snow Stability • Snow Surface • Wind

7. Snow Location • Snow bridge across gaps • Cornice and Overhang

8. Snow Location

9. Related Work • Snows • Metaballs • Stochastic Motion • Snow Shadows • Flow and Change • Dust Accumulation

10. Related Work • Three Major Models • Volume-based model • Surface-based model • Hybrid-based model

11. Volume-based model

12. Surface-based model

13. Hybrid-based model

14. Contribution • Accumulation Model • Stability Model

15. snow pipeline • Overview of the snow pipeline • Commercial software • Alias Wavefront 96 (Shader libraries, Rendering)

16. Entities • World • Sky, Ground, wind, Original input model and allocated snow • Model • The set of input polygons • Connected and Non-connected component • Face • Primary structure

17. Entities • Launch site • Subdivision area (or Launch area)

18. Entities • Edge group • Drops

19. Entities • Snow planes • Top snow planes (Triangular ) • Edge snow planes (Quadrilateral ) • Avalanche • Avalanche Flake • When an avalanche hits a drop, it is converted into a number of particles.

20. Snow Accumulation • Occlusion Boundary • The “Flake Flutter” effect eventually produces an occlusion boundary between completely blocked and unblocked areas. • Influence • Amount of snow • Closeness of the occlusion to the ground • Fluttering effect (wind )

21. Launch sites • Shoot particles • This approach allow launch sites on each surface to emit a series of particles aimed upwards towards asky bounding plane.

22. Launch sites • Whenever a launch site has a sufficiently different sky occlusion from an adjacent neighbor, a new launch site isadded at the perturbed midpoint to be refine the transition. • Likewise, launch sites can be merged whenever all surrounding neighbors have identical sky occlusions.

23. Launch sites There is no stability in this example

24. Transition Zone Occlusion Boundary

25. Importance Ordering • Resolution • How many launch sites the face needs. • How many particles each site should shoot. • Determination • Order of site testing • Improve the resolution

26. Importance Ordering • Completeness • Global approximation • Area • To prevent missing occlusion, large area may need more particles per launch site and more initial sites. • Neighborhoods • Add or remove the launch site.

27. Importance Ordering • Limits • Prevent launch sites from increasing very complexocclusion boundaries. • Steepness • Launch sites that are too steep to support much snow.

28. Importance Ordering • Camera • Sites closer to the camera receive more particles, greater refinement and accuracy. • User • “Boring” • “Interesting”

29. Launch Site Meshing • Launch site surfaces are represented as triangles.(the original base models) • All upwards-facing triangles are initially allocated at least one launch site. • Additional launch sites are allocated base on the importance ordering of the surface.

30. Launch Site Meshing • Launch sites are connected in the Delaunay triangulation, where each launch site is responsible for its own immediately surrounding Voronoi area.

31. Launch Site Meshing • In practice, many surface are small and isolated (such as pine needle) • Significant meshingoccurs on large, connected surface (such as the ground)

32. Edge Groups • Edge groups are primarily used for • Avalanche • Denoting sharp boundary • Snow may slide off from one edge group to another

33. Edge Groups • Drops • Bordered by XY silhouette edge (in red)

34. Edge Groups • This graph show a model (knot ) that our meshing algorithm considers hard.

35. Initial Particle Distribution • Final mesh • Initial launch sites • Final mesh • Final launch sites

36. Snowflake Motion • Have no experimental data • How flakes of various sizes and shapes move when dropped from a significant height. • Provide some parameters to simulate snowflake motion.

37. Snowflake Motion • Circumference(swirl) • Radius(wiggle) • Z step resolution

38. Snowflake Motion • Changing a flake’sZ incremental test change the flake’s direction.

39. Snowflake Motion • At each step • The value of is randomly chosen from a normal distribution. • “Area of effect ” increases from 1 cm to 4 cm to 7 cm from left to right. = 1 cm

40. Wind • The “wind influence ” is essentially a velocity vector for every point x, y, z in space.

41. Intersection Bucketing • Dividing the XY plane into a regular grid of buckets.

42. Locating Particles in the Sky

43. Writing in the Sky

44. Snow Stability • All launch sites are initially stored by Z height plus accumulation. • Angle of Repose (AOR) • Fresh snow => 90o • Slush snow=> 15o

45. Stability Test • Compute AOR between s and all neighbors ni lower than s. • For each i with an AOR to steep to support snow, perform an obstacle test between s and ni . • Evenly shift snow from s to all neighbors ni . • Repeat steps 1 to 3 until no unstable neighbors left, or s is bare of snow.

46. Moving Snow over Edges

47. Moving Snow over Edges

48. Implicit Function • Each snow volume is converted into one of several different implicit function types. • Gap bridging, Edge bulges, Wind cornices

49. Implicit Function

50. Implicit Function