Collision detection between more than 2 bodies

cocos2d-x C++
1

Hello,

I was wondering that suppose a ball hits 2 bodies simultaneously, how can we give the conditions for that? Suppose a ball is hitting an L shaped 2 lines and i want have some conditions specifically for that occasion, how do i go about it?

Shashwat

2

When a collision happens you get a callback containing pointers to the two fixtures colliding.

I store a pointer to my entity in the UserData of the fixture, so I now know my two entities.

In my ‘ball’ entity I have a ‘currentlytouching’ collection into which I push the ‘wall1’ entity I just collided with.
similarly on ‘uncollide’ I remove that entity from the collection.

So I have a collection of all entities this entity is touching.

So in your case you can examine that collection &, if you just hit wall1 , test of wall2 is in the collection, and act accordingly.

3

i am understanding the logic, could u post a sample code which i can try and work around with?

4

hi @shashwatsinha

could you explain a little more what you are trying to achieve.
may be we could help better. :smile:

5

60209a4fe76a50bf.png1365×1136

Hey,

I have uploaded an image. I have different line colliders for all sides of the rectangle. I want to process the collisions when i hit the corners, that time the ball touches 2 physics bodies instead of one. So i was wondering how to solve it.

Shashwat

6

Sorry - I don’t have code that does exactly that - and no time to write it for you.

Essentially,

IN BeginContact I’d have something like …

void ContactListener::BeginContact(b2Contact* contact)
{
	b2Fixture* fixtureA = contact->GetFixtureA();
	b2Fixture* fixtureB = contact->GetFixtureB();

// I have a class PhysicalObject that represents an object that has a body
	PhysicalObject* objectA = (PhysicalObject*) fixtureA->GetBody()->GetUserData();
	PhysicalObject* objectB = (PhysicalObject*) fixtureB->GetBody()->GetUserData();

// Now I can call a method on each of the objects involved in the collision
	if (objectA)
	{
		objectA->collidedWith(objectB);
	}

	if (objectB)
	{
		objectB->collidedWith(objectA);

	}

In the collidedWith(PhysicalObject* object) method I’d do something like…

Check my Vector<PhysicalObject*> collection to see what is in it;
If there’s already an objectA, just ignore it - I’ve just triggered a double collision - happens sometimes.
If there’s not an objectA, but there is an objectX, then I know I have collided with both an A and an X - so I can act accordingly,

I obviously need to write the associated EndContact method in the listener, to call the unCollidedWith method that removes that object from the collection.

Good luck!