Resting State

[Darthfett]

As expected (by design), with the current collision resolution method, if an object is bouncing on the ground and velocity becomes very small (close to 0), the collisions will begin to stack up, and pygame will become very unresponsive.

In typical collision detection/resolution, this small velocity is referred to as coming to a resting state. After finding what causes #21, it is clear that a simple reflected-bounce resolution algorithm will cause issues, and a resting state is needed to resolve falling-through-the-ground issues, as well as laggy collision issues.

[Darthfett]

One possible way to do a resting state would be using a 'locking' behavior, which activates when an object has small velocity (below some threshold), collides with object B, and is accelerating towards object B.

Collision should be normal when object A collides with large velocity normal to B's collided segment. When A's relative velocity to B in the direction of the normal to collided segment becomes very small (below some threshold), and collision occurs:

1. Ignore future collisions between B and A (by setting them as 'locked')
2. Give both objects the average velocity (for the direction of the normal). For non-moving objects, set both velocity to zero.
3. When the colliding point of A with the line of B becomes equal to either of B's endpoints, remove the locking status.

[Darthfett]

This locking behavior will have to negate acceleration (since position is calculated), without losing the information, so that when 'unlocking' from the object, the acceleration will be reacquired.

Places with special cases for dealing with locked/resting objects:

1. Collision will need to take into account either larger mass, or will need to unlock the two objects from one another.
2. Setting an objects acceleration will need to determine whether the new acceleration is toward the object ('pushing' the object), away from the object (unlocking the object), or parallel (increasing/decreasing the time-to-unlock)

[Darthfett]

As of 3b6c33a4db, there is a very basic, frictionless resting state. When an object collides with another object such that its velocity normal to the line is very small, and its acceleration is also toward the line, it will enter into the resting state, which will negate the relative velocity and acceleration normal to the line, and ignore future collisions.

There's a bit of hardcoding for the fact that one entity is non-moving, and the other is moving.

[Darthfett]

As of 97d15da, the resting state detection now works with two moving entities (though this is untested). More work is needed to get the resting state resolution to also work for two moving entities.