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Double Buffered State
By
Zachary Booth Simpson
| Published Jun 19 2001 10:23 PM in General Programming
© 2000 - Zachary Booth Simpson. Copied with permission from http://www.mine-control.com/zack. If you find any of this work useful, please sign Zack's guest book: http://www.mine-cont...i/gbook-zbs.cgi.
Intent
Problem
Solution
Structure
Examples
Issues and Risks
Related Patterns
Uses and References
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 {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.
Vec3 pos,vel,angles,angVel,forceAccum,torqueAccum
};
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.
