computer modelling of fallen snow
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Computer Modelling Of Fallen Snow. Paul Fearing University of British Columbia Vancouver, Canada. Goal. Goal. Introduction. Related Work Snow Accumulation Snow Stability Implicit Function Validation Future Work Conclusion. Decomposition of Gravity. Global of the Snow Model. - PowerPoint PPT PresentationTRANSCRIPT
Computer Modelling Of Fallen Snow
Paul FearingUniversity of British ColumbiaVancouver, Canada
Goal
Goal
Introduction
• Related Work• Snow Accumulation• Snow Stability• Implicit Function• Validation• Future Work• Conclusion
Decomposition of Gravity
Global of the Snow Model
• Snow Location• Snow Stability• Snow Surface• Wind
Snow Location
• Snow bridge across gaps• Cornice and Overhang
Snow Location
Related Work
• Snows– Metaballs
• Stochastic Motion• Snow Shadows• Flow and Change• Dust Accumulation
Related Work
• Three Major Models– Volume-based model
– Surface-based model
– Hybrid-based model
Volume-based model
Surface-based model
Hybrid-based model
Contribution
• Accumulation Model
• Stability Model
snow pipeline• Overview of the snow pipeline
• Commercial software– Alias Wavefront 96 (Shader libraries, Rendering)
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
Entities• Launch site• Subdivision area (or Launch area)
Entities
• Edge group• Drops
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.
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 )
Launch sites
• Shoot particles– This approach allow launch sites on each
surface to emit a series of particles aimed upwards towards a sky bounding plane.
Launch sites
– Whenever a launch site has a sufficiently different sky occlusion from an adjacent neighbor, a new launch site is added at the perturbed midpoint to be refine the transition.
– Likewise, launch sites can be merged whenever all surrounding neighbors have identical sky occlusions.
Launch sitesThere is no stability in this example
Occlusion Boundary• Transition Zone
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
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.
Importance Ordering• Limits
– Prevent launch sites from increasing very complex occlusion boundaries.
• Steepness– Launch sites that are too steep to support
much snow.
Importance Ordering• Camera
– Sites closer to the camera receive more particles, greater refinement and accuracy.
• User– “Boring”– “Interesting”
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.
Launch Site Meshing
• Launch sites are connected in the Delaunay triangulation, where each launch site is responsible for its own immediately surrounding Voronoi area.
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)
Edge Groups• Edge groups are primarily used for
– Avalanche– Denoting sharp boundary– Snow may slide off from one edge group to
another
Edge Groups
• Drops• Bordered by
XY silhouette edge (in red)
Edge Groups• This graph show a model (knot ) that
our meshing algorithm considers hard.
Initial Particle Distribution
• Final mesh
• Initial launch sites
• Final mesh
• Final launch sites
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.
Snowflake Motion
• Circumference(swirl)
• Radius(wiggle)
• Z step resolution
rf
h
Snowflake Motion• Changing a flake’s Z incremental test change
the flake’s direction.
Snowflake Motion• At each step
– The value of is randomly chosen from a normal distribution.
– “Area of effect ”
rf
increases from 1 cm to 4 cm to 7 cm from left to right. = 1 cm
F F
Wind
• The “wind influence ” is essentially a velocity vector for every point x, y, z in space.
Intersection Bucketing• Dividing the XY plane into a regular grid of buckets.
Locating Particles in the Sky
Writing in the Sky
Snow Stability• All launch sites are initially stored by Z height
plus accumulation.• Angle of Repose (AOR)
• Fresh snow => 90
o
• Slush snow=> 15
o
Stability Test1. Compute AOR between s and all neighbors ni lower than s.2. For each i with an AOR to steep to support snow, perform an obst
acle test between s and ni . 3. Evenly shift snow from s to all neighbors ni .4. Repeat steps 1 to 3 until no unstable neighbors left, or s is bare of
snow.
Moving Snow over Edges
Moving Snow over Edges
Implicit Function
• Each snow volume is converted into one of several different implicit function types.– Gap bridging, Edge bulges, Wind cornices
Implicit Function
Implicit Function
Validation• Validation of snow-covered scenes is hard.
– Uncontrollable– Unknown environmental factors
Future Work
• Physically realistic• Animation• Time
– Large model
Result
Result
Result