Lecture 20: PhysicallyBased Animation

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Lecture 20Physically-Based Animation
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Physically-Based Animation

• Idea to obtain a physically plausible animation,
  simulate the laws of Newtonian physics. In
  contrast to kinematics, this is called a dynamics
  approach
• Have to control objects by manipulating forces,
  either directly or indirectly
• Example bouncing balls, wall of bricks fall
  down, etc
• Inverse dynamics is difficult

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Inverse dynamics

• Finding forces to move a physical object along a
  desired path
• Useful for secondary effects in keyframe
  animation
• A mouse characters ears should flap when it
  shakes its head
• A feather in a cap should wave when wind blows on
  it
• A wall of bricks should fall down when a
  superhero crashes on it

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Type of Physically-Based animation

• Procedural Animation
• Representational Animation
• Stochastic Animation
• Behavioural Animation

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Type of animation - Procedural

• Control over motion by explicitly define the
  movement as a function of time.
• Objects are animated by a procedure -- a set of
  rules.
• The animator specifies rules and initial
  conditions and runs simulation.
• Rules are often expressed by mathematical
  equations.

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Type of animation - Procedural

• recursive algorithm to build the
  binary tree

add leaves to the ends of the branches
3D - A more complex and realistic tree
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Type of animation -Representational

• Specifying how a rigid object is to move in space
  by allowing the object itself to change shape
• Animation of articulated object
• Forward Inverse Kinematic
• Animation of soft object
• Animating on formation deformation of object
• Eg. stretching face muscles to create different
  expressions
• the realistic facial expressions of the creature
  Gollum in "Lord of the Rings.
• Morphing

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Type of animation - Stochastic

• Features that generate large amounts of low-level
  details
• Eg. particle system
• often are used to simulate natural phenomena like
  clouds, running water, fire, etc.

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Type of animation - Behavioural

• Animate how object behave or interact with
  environment naturally.
• Eg animating birds flying
• Eg animating fishes living in their environment.

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Type of animation - Behavioural
A flock of birds
A group of bugs - avoiding hitting the box
A group of fish reaching for oxygen
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Particle Systems

• How do we model fuzzy objects? Object with
• soft boundary
• Changing boundary
• chaotic behaviour
• Eg clouds, fire, water, grass, fur

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Particle Systems

• Basic idea model fuzzy or gaseous objects as
  large collection of particles
• Animate fuzzy or gaseous objects as a changing
  cloud of particles
• Aim for overall effects by having many, simple
  particles

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Particle Systems

• Each particle has properties
• Geometry (point, line, sphere)
• Position
• Velocity vector
• Size
• Colour
• Transparency
• State
• lifetime

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Particle Systems

• Animating Particles Systems
• New particles are born, old ones die
• State information describes type of particle
• At each time step, based on state
• Update attributes of all particles
• Delete old particles
• Create new particles
• Display current state of all particles

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Particle Systems

• Particle Systems Details
• Good for describing objects that change over time
  by flowing, spattering or expanding
• Often will model gravity when updating particles
• Render fire etc using transparency brighter
  when particle system denser
• As each particle moves, its path is plotted and
  displayed in a particular color.

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Particle Systems

• Example 1
• A fireworks pattern can be displayed by randomly
  generating particles within a spherical region of
  space and allowing them to move radially outwards
  as shown in next figure. The particle paths can
  be color from red to yellow, to simulate the
  temperature of exploding particles.

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Particle Systemseg Fireworks
Source http//www.maylin.net/Fireworks.html
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Particle Systems

• Example 2
• A realistic displays of grass clumps have been
  modeled with trajectory particles that are shot
  up from the ground and fall back to earth under
  gravity.
• In this case, the particle paths can originate
  within a tapered cylinder, and might be color
  from green to yellow

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Particle Systemseg Plant
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Particle Systems

• Example 3
• A waterfall system
• The water particles fall from a fixed elevation,
  are deflected by an obstacle, and then splash up
  from the ground.

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Particle Systemseg Waterfall