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