Managing multiple gravities with Box2D

核心提示： The only interesting lines are line 17 where I declare the gravity vector and lines 64 and 74 where I assign names to the objects… box for bo

The only interesting lines are line 17 where I declare the gravity vector and lines 64 and 74 where I assign names to the objects… box for boxes and circle for circles

And this is the result:

As you can see, the world is ruled by standard gravity, but as I said I want boxes to be ruled by normal gravity and circles to be ruled by inverted gravity

Now, let’s see the first method, called (by myself)….

The antagonist forces method

As the name means, the principle is applying an antagonist force to circles in order to simulate a reverse gravity. Since the default gravity is (0,10), I am going to apply a (0,-20) force to all circles.

This is the updated Update function:

public　function　Update(e:Event):void　{
　var　ant_gravity　=　b2Vec2;
　m_world.Step(m_timeStep,　m_iterations);
　for　(var　bb:b2Body　=　m_world.m_bodyList;　bb;　bb　=　bb.m_next)　{
　　if　(bb.GetUserData()!=null)　{
　　　if　(bb.GetUserData().name=="circle")　{
　　　　ant_gravity　=　new　b2Vec2(0.0,-20.0*bb.GetMass());
　　　　bb.ApplyForce(ant_gravity,bb.GetWorldCenter());
　　　}
　　}
　}
}