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Control Systems for Computer Animation

The issue of how to control animation systems is an important. Systems must offer flexibility and yet must maintain an ease of use that allows users to define exactly what they want without resorting to complex programming.

Object-oriented control systems

Work on the REALISM animation system included development of a rule and constraint system to control object behaviour. The implementation of object control in a system that maintains a high degree of encapsulation is problematic. If an object offers external agents access to its structure, then it is easy to exert control over the object but the security of the object's data is compromised. If access is only permitted through the object interface then explicit control will be computationally expensive.

To overcome this dilemma, the REALISM and ACE systems use a system of rules and constraints. Each object has associated with it a list of each of these types of object, and it is through these lists that object behaviour is controlled.

Rules

These affect behaviour of an object in a global sense, i.e. they are independent of its state or position in space. Examples might be simple rules of gravity, or rules affecting the behaviour of an object when it comes into contact with other objects.

Constraints

 As their name suggests, constraints are used to limit the motions or behaviours of an object to a certain subset of all those possible. A simple example might limit the motion of object to that of rotating around a single point.

Some examples

 An example animation that uses constraints and rules is shown here. It models a `pool table' and has `rules' of gravity and object interaction applied to all the objects to create the correct motion. A constraint limits the balls to motion in contact with the surface of the table until they move beyond its bounds. The constraint is then no longer valid and the rule of gravity causes the ball to fall.

MPEG, 20K

Another example shows a simulation of Newton's cradle (MPEG, 33K). This has`invisible' supports for the balls to give a better view of their motion. Constraints are use to keep the balls a constant distance from the support (to model the strings) and a gravity rule is included to model the forces present when the balls are displaced from the rest position.

Publications

State-transition systems

 The work is this area is the result of research carried out with the Formal Methods Group. It aims to provide a formal framework for generating animated sequences that will increase the ability to define precise behaviours. An example of this is the generation of an animation of Newton's cradle (MPEG, 18K) from an analysis of the essential elements of its behaviour.

Publications

Author: I.J.Palmer
Last update: 16/8/95
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