Radial gravity, orbital gravity, orbital mechanics, whatever you want to call it there is no denying it can be useful for many kinds of games. Unfortunately, neither Box2D nor Chipmunk Physics Engines for the iPhone had support for it. I had a hell of a time trying to figure this out but in the end it was so simple. Here is how to do it.
First, create a new Cocos2D / Box2D Hello World project from the supplied Cocos2D templates. Add the following code to HellowWorldScene.mm and HelloWorldScene.h:
HelloWorldScene.mm
-(id) init { //... Add at end of init after body->CreateFixture(&fixtureDef); // Create our static "Planet" b2CircleShape shape; shape.m_radius = 1.0f; shape.m_p.Set(8.0f, 8.0f); b2FixtureDef fd; fd.shape = &shape; planet = groundBody->CreateFixture(&fd); } -(void) tick: (ccTime) dt { int32 velocityIterations = 8; int32 positionIterations = 1; world->Step(dt, velocityIterations, positionIterations); for (b2Body* b = world->GetBodyList(); b; b = b->GetNext()) { b2Body* ground = planet->GetBody(); b2CircleShape* circle = (b2CircleShape*)planet->GetShape(); // Get position of our "Planet" b2Vec2 center = ground->GetWorldPoint(circle->m_p); // Get position of our current body in the iteration b2Vec2 position = b->GetPosition(); // Get the distance between the two objects. b2Vec2 d = center - position; // The further away the objects are, the weaker the gravitational force is float force = 250.0f / d.LengthSquared(); // 150 can be changed to adjust the amount of force d.Normalize(); b2Vec2 F = force * d; // Finally apply a force on the body in the direction of the "Planet" b->ApplyForce(F, position); if (b->GetUserData() != NULL) { CCSprite *myActor = (CCSprite*)b->GetUserData(); myActor.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO); myActor.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle()); } } }
HelloWorldScene.h
@interface HelloWorld : CCLayer { b2World* world; GLESDebugDraw *m_debugDraw; b2Fixture* planet; // This will be a static body }
And here is the result!
Enjoy!
Download Source Code for this tutorial:
Radial Gravity Box2d & Cocos2d Example Source Code – 692k
