1. Simulation Acceleration Techniques
Kelly Ward
Comp 259 – Spring 2002
March 20, 2002
March 20, 2002

2. Simulation Acceleration
Real-time performance can be necessary for many applications
Virtual Reality
Video Games
Complexity of animation environment can make it too time consuming to animate in real-time
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3. Simulation Acceleration
Represent a certain animation at different resolutions, or level of detail, depending on a number of criteria
Placement in scene with respect to viewer
In field of view
Distance from viewer
Importance to scene
Motion
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4. Dynamics Culling
“View-dependent culling of dynamic systems in virtual environments”
S. Chenney and D. Forsyth. In Proc. of ACM Symposium on Interactive 3D Graphics, 1997.
In a virtual environment, cull objects that have no effect on the view while maintaining consistency of the environment
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5. Dynamics Culling Consistency Completeness
Determine state object should be in when back in view
Completeness
Objects may come back into view on their own
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6. Conditioning Influence of initial conditions Strong Medium Weak
Accurate predictions can be made, have to integrate system as if it were in view
Medium
Qualitative predictions can be made, apply techniques that best match the properties of the system
Weak
Difficult for viewer to make prediction, but still has general knowledge of system – use general behavior of system and choose new state consistent with model
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7. Bumper Cars Simulate cars driving around ellipse
Try to avoid walls and other cars
Results in perturbed motion around ellipse
Model three different views
Static view
Walkthrough view
Random view
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8. Bumper Cars - Dynamics Integrates standard rigid body equations
Steering wheel angle
Speed
Friction from wheels and surface
Intervals over which to change drive parameter
Collisions detected using OBBTrees
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9. Bumper Cars Motion is not predictable by viewer
Actions (collision response, avoidance, random noise) make perturbations uncertain to initial conditions
Viewer will be able to detect average effect of perturbations over time
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10. Bumper Cars
Solution:
Capture distribution of spatial effect of the temporal mean of perturbations
Sample spatial effect and apply to constant elliptic motion expressed as explicit function of time
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11. Analysis
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12. Real-time Animation
“Simulation Levels of Detail for Real-time Animation”
Deborah Carlson and Jessica Hodgins. In Graphics Interface’97, pages 1–8, June 1997.
Simulate group of legged creatures using levels of detail to reduce computation cost
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13. Real-time Animation
Build a virtual world where group of legged creatures have to escape a giant puck
If creature collides with something, creature may recover balance or fall down
Two measures of success of LODs for environment
Outcome of the game must be relatively unchanged
Viewer’s perception of motion must be the same
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14. Representation of Creatures
Three levels of detail
Rigid-body dynamics – Finest detail
Hybrid kinematics/dynamics
Point-mass simulation – Coarsest detail
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15. Representation of Creatures
Rigid-body dynamics
Simulation with three bodies and four degrees of freedom
Control flight duration, body attitude (roll, pitch, yaw), and forward and sideways velocity
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16. Representation of Creatures
Hybrid kinematic/dynamic model
Simple dynamic model for motion of body
Computes motion of leg kinematically
Position of body on playing field computed using point-mass model
Height, roll, pitch of body computed using a table lookup and interpolation
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17. Choosing Representation
Dynamic behavior of creature
Area of influence around each creature and object – where interactions are likely
Creature intersects another area use highest LOD
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18. Choosing Representation
Importance of creature to viewer
Out of view: Use lowest LOD
In view but far away: Use middle LOD
In view and close to viewer: Use highest LOD
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19. Transitions Desire smooth blending of motion
Allow switch only during a particular part of flight phase during running cycle
Cyclic motion allows for this technique
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20. Effect of Puck Area of Influence
Analysis
Effect of Camera Range
Effect of Puck Area of Influence
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21. Animating Prairies “Animating Prairies in Real-Time”
Frank Perbet and Marie-Paule Cani
I3D, 2001
Use level of detail to accelerate the animation and rendering of the blades of grass on a prairie
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22. Animating Prairies Simulations:
Wind blowing through the grass
A walker moving through the scene
Due to so many blades of grass in one scene, too time-consuming to animate and render in real-time
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23. Grass Representation Three basic representations of grass
3D geometry – Finest detail
Volumetric textures
2D textures – Coarsest detail
Choose representation based on distance to viewer
Need to transition smoothly between different representations
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24. 3D Geometry Representation
Most convincing representation – Used for grass closest to viewer
Blades are represented as a chain of line-segment primitives
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25. 3D Geometry Representation
Effect of wind - each 3D blade of grass is sent
Direction of wind
Index – indicates bend of grass
Different kinds of grass have different stiffness and bending properties
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26. Volumetric Textures Referred to as 2.5D representation
Polygon strip mapped with a semi-transparent texture
Used at mid-distance from viewer
Strip is never shown to be parallel to viewer
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27. Volumetric Textures
Effect of wind – similar to 3D geometry representation
Use layers of vertical polygon strips to allow animation
Similar motion to 3D geometry allows easier transitioning between the two
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28. 2D Textures 2D texture is mapped on the terrain
Represents grass at a distance from the viewer
Textures generated by computing 2D images of 3D blades
Wind not simulated
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29. Choosing Representation
Tile terrain into squares referred to as “patches of grass”
Patches that are out of the field of view are culled since they will not appear
Representation chosen based on distance to viewer
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30. Transitions
Seamless transitions between representations desired to alleviate disturbance to viewer
Difficulty going from 3D to 2.5D to 2D and back again
Different physical representation
Different motion modeling from wind
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31. Transitions - 3D to 2.5D Cannot just switch or fade in-fade out
Blades not likely to be in exact same positions
Apply morphing technique
Each representation uses same number of blades – 3D geometry maps to position on texture
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32. Transitions - 2.5D to 2D
Since 2D representation is used at a distance, can dissolve between the two representations
Poor results on silhouettes (outline of hills)
2.5D representation progressively grows (or un-grows) from the ground
2D texture progressively disappears (or appears)
Discrepancies not as distracting to viewer
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33. Results
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34. Hair Simulation Three representations used for hair
Individual Strand Groupings – Finest detail
Clusters
Patches – Coarsest detail
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35. Transitions Patch to Cluster to Strand groupings
Same basic skeleton model – controls motion
1 patch to 3 clusters
Interpolate values of new cluster skeletons
Similar technique for reverse
Switch based on criteria
Distance from viewer
Forces acting on hair
Occlusion of hair
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36. Hair Simulation
Front View
Back View
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37. References
D. Carlson and J. Hodgins. Simulation Levels of Detail for Real-Time Animation. In Proc. of Graphics Interface 1997, 1997.
S. Chenney and D. Forsyth. View-dependent culling of dynamic systems in virtual environments. In Proc. of ACM Symposium on Interactive 3D Graphics, 1997.
D. O’Brien, S. Fisher, and M. Lin. Automatic simplification of particle system dynamics. Computer Animation 2001.
F. Perbet and M. Cani. Animating Prairies in Real-Time. Proceedings on 2001 Symposium on Interactive 3D graphics, March 2001.
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