“Computer Modelling of Fallen Snow” by Paul Fearing Presented by Luv Kohli COMP238 October 29, 2002

The Problem Want to generate realistic “snowy worlds” Need to determine: –how much snow falls on a scene –where this snow accumulates

Previous work Premoze, et al. Mostly concerned with far away landscapes Uses digital elevation model enhanced with aerial photo Much larger scale

Basic Algorithm (Fearing) Two stages Snow accumulation stage –How much snow accumulates per surface Snow stability stage –Resolves unstable snow surfaces

Snow accumulation stage Attempt to simulate “flake flutter” Shoot snow particles from “launch sites” towards sky –Like ray tracing, but not straight lines –Piecewise linear path towards sky

Flake flutter Circles of varying radius Flake path Random points on circles

Snow accumulation stage flakes from each launch site shot upwards –until blocked – ‘hit’ –or until sky reached – ‘miss’ Gives an idea of launch site’s occlusion from sky Snow accumulation is used to add snow as 3D surfaces above model

Importance ordering Each launch site given a priority based on several factors –Completeness –Area –Neighborhoods –etc.

Occlusion boundaries

Launch site meshing Sites represented as triangles generated from original base scene models Initially at least one site per upward- facing triangle Sites can be merged or refined

Sky buckets Snowfall should be fairly equal across sky Sky divided into grid of equal-area buckets Flake’s representative area spread across one or more buckets on a ‘miss’

Sky buckets

Flake dusting Thin ‘dusting’ of snow difficult to represent as 3D objects Semi-transparent procedural noise textured polygons used instead Dusting polygons placed slightly in front of 3D surface

Flake dusting

Snow stability stage Redistribute accumulated snow into stable configuration Use angle of repose –Measure of static friction of a pile of granular material

Angle of repose (AOR) 90º for fresh snow 15º for slush Can model probability of stability around AOR

Stability test Compute angle between snow surface on site s and neighbors n i lower than s If angle too steep to support snow, perform obstacle test between s and n i Shift snow from s to n i if not blocked Repeat until no unstable neighbors or s is empty

Obstacle test (a) avalanche blocked by scene object (b) avalanche blocked by snow on object (c) avalanche partially blocked by snow on object

Stability termination Simulation runs out of time All launch sites are stable Only a small amount of snow moved during last pass Most unstable snow resolved during first few passes

Rain, flour, wind Rain can be simulated by setting AOR = 0º and not allowing any flake flutter Feasible for other materials, like flour Framework in place for basic wind effects

Rain

Recent work “Modeling the Accumulation of Wind- Driven Snow” – Bryan E. Feldman and James F. O’Brien

References Fearing, P Computer modelling of fallen snow. In Proceedings of ACM SIGGRAPH 2000, Feldman, B. E., O’Brien, J. F Modeling the accumulation of wind- driven snow. ACM SIGGRAPH 2002 Technical Sketch.