Modelling

Animation

Quite simple, really

• Parameters of the model are functions of time.
• parameters of moving objects
• parameters of camera, including things like depth of field
• The modeller must specify these functions.
• Constraints.
• The functions must be physically `realistic'.
• The functions must be easily specifiable.

`Specifiable': Think splines.

`Realistic': Think continuity.

1. C0 continuity: no teleportation, possible discontinuities in velocity, which violate Newtonian mechanics.
2. C1 continuity: no discontinuities in velocity, possible infinite accelarations.
3. C2 continuity: acceleration continuous, possible infinite jerks.
4. etc.

Most impotant point

Definition of `realistic' varies

1. Between camera and actors
2. As type of actor changes: compare classic Disney animation to live action, which obeys Newtonian dynamics (and other constraints, too)

Keyframe Animation

Above we described the goal of motion planning. How is it achieved in practice?

Usually by keyframe animation

Specification

1. Degrees of freedom
• very many in a typical scene
• decomposition is possible
• but not past the point at which degrees of freedom `naturally' interact
2. At t=t0
• positions
• velocities
• accelerations?
3. Same at t=t1
4. Interpolation algorithm

Editing the specification

1. Change the values at the keyframes
2. Change the interpolation algorithm
3. Insert extra keyframes
4. Delete keyframes

Without a direct manipulation interface keyframe animation is impossible,

• but being able to put calculated values into the interface is also essential

Artists are good at doing keyframe animation

• for the most part programmers are not.

Physics

What's wrong with physics?

1. Artistic problems
• physical objects
• objects that merely follow the laws of physics without any notion of intelligent agency

are not very expressive

• actors have to learn how to express themselves
• which is not easy
• mention MEL virtual reality
2. Computation problems
• Example. A ball hitting a wall.