More Animation and (maybe) Particle Systems Procedural Animation - - PDF document

1

More Animation and (maybe) Particle Systems

Procedural Animation Motion Capture . Particle Systems Intro Differential Equation Basics Particle Systems Procedural Animation Motion Capture . Particle Systems Intro Differential Equation Basics Particle Systems

Types of Animation

• Keyframing
• Procedural Animation
• Motion Capture

2

Procedural Animation

• Define the motion using formulas

–Hand-crafted –Physically based

• The animator must be a programmer
• Keyframing starts to become procedural as

expressions are added

• At some level of complexity it becomes

easier/more efficient than keyframing.

Procedural Animation

Battle of Helm’s Deep, LOTR

3

Procedural Animation

NYU Media Research Lab (Ken Perlin, Athomas Goldberg)

Dynamics

• Generate motion by specifying mass and force,

apply physical laws (e.g., Newton’s laws)

• Simulates physical phenomena

–gravity –momentum (inertia) –muscle forces

4

Active Simulations Active Simulations: Examples

Wayne Wooten, Pixar (by way of Georgia Tech)

5

Active Simulations: More Examples

Petros Faloutsos, UCLA

Robotics Comparison

HRP2 robot, AIST (Japan)

6

Performance-based Animation (Motion Capture)

• Record the animation from live action

– simplest method - rotoscope (trace) over video of real motions

• Real time input devices

– electronic puppeteering

• Motion capture

– track motion of reference points

» body or face or hands

– magnetic – optical – exoskeletons – convert to joint angles (not always straightforward) – use these angles to drive an articulated 3-D model – These motion paths can be warped

Motion Capture

7

Motion Capture Research Motion Data Acquisition Motion Data Acquisition

8

Maze - Sketch Interface Maze - Sketch Interface Re-sequence Re-sequence

Motion Capture Region Virtual Environment Obstacles

Sketched path

9

Re-sequence Re-sequence

Motion Capture Region Virtual Environment

Data Acquisition Data Acquisition

“Poles and Holes” rough terrain “Poles and Holes” rough terrain

10

Terrain Navigation Terrain Navigation Choice Interface Choice Interface

11

Another Example

Arikan et al., (Berkeley) SIGGRAPH ‘03

Particle Systems

12

References

• W. T. Reeves, Particle Systems - a Technique for Modeling a Class of

Fuzzy Objects, ACM Transactions on Graphics, 2(2), pp. 91-108, 1983.

– http://portal.acm.org/citation.cfm?id=357320&dl=ACM&coll=portal&CFID=12858257&CFTOKEN=79986367

• Karl Sims, Particle animation and rendering using data parallel

computation, ACM SIGGRAPH Computer Graphics, v.24 n.4, p.405-413,

• Aug. 1990

– http://portal.acm.org/citation.cfm?id=97923&dl=ACM&coll=portal&CFID=12858257&CFTOKEN=79986367

• A. Witkin, D. Baraff, M. Kass: Physically-Based Modeling, SIGGRAPH

tutorial course notes. 2001 – http://www.pixar.com/companyinfo/research/pbm2001/

• Partial slide credit: Thomas Funkhouser

Kinematics and Dynamics

• Kinematics

– Considers only motion – Determined by positions, velocities, accelerations

• Dynamics

– Considers underlying forces – Compute motion from initial conditions and physics

• Today: Passive Dynamics:

– No muscles or motors – Examples:

» Smoke » Water » Cloth » Fire » Fireworks

13

Karl Sims, Particle Dreams Particle Systems

Clouds Smoke Fire Waterfalls Fireworks Reeves ’83, the Wrath of Khan Batman Returns, using Reynold’s flocking algorithms

14

Particle Systems

• For each frame:

– Create new particles and assign attributes – Delete any expired particles – Update particles based on attributes and physics – Render particles

Creating/Deleting Particles

• Where to create particles?

– Around some center – Along some path – Surface of shape – Where particle density is low

• When to delete particles?

– Where particle density is high – Life span – Random This is where user controls animation

15

Example: Wrath of Khan

Reeves

Example: Wrath of Khan

Reeves

16

Example: Wrath of Khan

Reeves

Particle System Attributes

• Creation—number, initial condiions
• position/velocity
• randomness
• surface of emitter shape
• vertex of polygonal object
• size
• color
• transparency
• shape
• lifetime
• Deletion
• Update of position/velocity
• translation
• vortex
• Rendering style – motion blur, compositing

What control handles do we want/need?

17

Equations of Motion: Gravity Only!

t V V P P t A V V g A

• 2

’ ’ ’

) , , ( z y x P

g

V

Integration: accuracy improves as step size decreases but never a perfect match

Particle System Forces

• Force fields

– Gravity, wind, pressure

• Viscosity/damping

– Liquids, drag

• Collisions

– Environment – Other particles

• Other particles

– Springs between neighboring particles (mesh) – Useful for cloth

18

Spring-Mass Systems

Cloth in 2D Jello in 3D

Spring-Mass Systems

19

Cloth

Increased Resolution of Mesh +Possible Shapes + Smoothness

• Simulation time

Breen ‘95

Modeling for Clothing

20

Collisions for Clothing

Potentially VERY expensive Bounding Box Hierarchy Partition space or objects Avoid expensive primitive tests

Polygons Primitive Level Intermediate Level(s) Top Level

Leaves in the Wind…

21

Leaves in the Wind…

uniform sink source vortex

Add together to make interesting fields…

N t t t n n n t n

Av F v A F F F F

• Wejchert&Haumann, ‘91

The Challenges of Passive Simulation

• Accurate for the situation
• What pieces of the physics are

necessary for appearance?

• How to give the animator control?

22

Dynamics, more generally

• Point mass
• Spring/mass systems
• Linkages of rigid bodies
• Other physical phenomena

–Aerodynamics –Fluids –Fracture –Explosions