Game Development Reference
In-Depth Information
2.
The
PreSolve
function at the moment has the same code as
BeginContact
and
EndContact
, but the code is executed only if the following
if
statement
is true:
if (contact.GetManifold().m_pointCount>0) {
3.
Box2D groups contact points into a manifold structure. The
PreSolve
function is sometimes called even if the contact points aren't there in the
manifold, so this time we want it to be executed only if there is at least one
contact point.
override public function PostSolve(contact:b2Contact,
impulse:b2ContactImpulse):void {
trace("a collision has been post solved");
var fixtureA:b2Fixture=contact.GetFixtureA();
var fixtureB:b2Fixture=contact.GetFixtureB();
var bodyA:b2Body=fixtureA.GetBody();
var bodyB:b2Body=fixtureB.GetBody();
trace("first body: "+bodyA.GetUserData());
trace("second body: "+bodyB.GetUserData());
trace("impulse: "+impulse.normalImpulses[0]);
trace("---------------------------");
}
4.
The
PostSolve
function also has the same code as the other functions, with
the exception of the following line:
trace("impulse: "+impulse.normalImpulses[0]);
As you can see, the
PostSolve
function has an impulse passed as an
argument. The
normalImpulses
property of a
b2ContactImpulse
object
returns a vector with all impulses generated by the collisions. We are looking
for the first and only impulse, which in other words represents the strength
of the collision.
5. Test the movie again, and every time the ball bounces on the floor, you
should see the following output text:
a collision started
first body: Floor
second body: Ball
---------------------------
a collision has been pre solved
first body: Floor
second body: Ball
