PreSolve and PostSolve
Both PreSolve and PostSolve give you a b2Contact pointer, so we have access to the same points and normal information we just looked at for BeginContact. PreSolve gives us a chance to change the characteristics of the contact before the collision response is calculated, or even to cancel the response altogether, and from PostSolve we can find out what the collision response was.
Here are the alterations you can make to the contact in PreSolve:
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void SetEnabled(bool flag);//non-persistent - need to set every time step
//these available from v2.2.1
void SetFriction(float32 friction);//persists for duration of contact
void SetRestitution(float32 restitution);//persists for duration of contact
Calling SetEnabled(false) will disable the contact, meaning that the collision response that normally would have been applied will be skipped. You can use this to temporarily allow objects to pass through each other. A classic example of this is the one-way wall or platform, where the player is able to pass through an otherwise solid object, depending on various criteria that can only be checked at runtime, like the position of the player and which direction direction they are heading, etc.
It's important to note that the contact will revert back to being enabled in the next time step, so if you want to disable contacts like this you'll need to call SetEnable(false) every time step.
As well as the contact pointer, PreSolve has a second parameter from which we can find out info about the collision manifold of the previous time step. If anybody has an idea about what this could be used for, let me know
PostSolve is called after the collision response has been calculated and applied. This also has a second parameter, in which we can find information about the impulse that was applied. A common use for this information is to check if the size of the collision response was over a given threshold, perhaps to check if an object should break, etc. See the 'sticky projectiles' topic for an example of using PostSolve to determine whether an arrow should stick into a target when it hits.
