Intent

Track an old and new state for multi-Model Controllers.

Problem

Some Controller systems (for example, collision physics) need to track the state of several objects simultaneously and it must be ensured that they always read the same time step state for each Model instance.
If double buffering is not used, then as the physics system traverses the Models, it will become "confused" when it reads a state which has already been updated, thus using a new state instead of an old state.
This is analogous to a variable swap. int temp = a; a = b; b = temp; Without the temp varaible, a = b; b = a; would cause a==b always.

Solution

The necessary state information is typically isolated into a separate structure. For example:
struct PhysicsState {
    Vec3 pos,vel,angles,angVel,forceAccum,torqueAccum
};
Two instances of this class are combined into either the controller or model, often in a two element array. A global or local variable is used to flip between the two states using an accessor method.
For example:
class Model {
    PhysicsState states[2];
    PhysicsState getOldState() { return state[ globalFrame   &1]; }
    PhysicsState getNewState() { return state[(globalFrame+1)&1]; }
};

Structure

Not available at this time.

Examples

None at this time.

Issues and Risks

None at this time.

Related Patterns

Controller

Uses and References

Thanks to Chris Hecker.

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Date this article was posted to GameDev.net: 6/19/2001
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Design Patterns