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[go]

I ran into what is possibly a really easy problem, but I can't seem to find the solution.

The Box2D forums are down, so I'll try my luck here.

I want move a Box2D body in a circular pattern. It's a pretty trivial thing to do.

This works perfectly with SetXForm :

Code:

//orbitAngle loops from 0 to 360
float dX = cos( orbitAngle * PI / 180.0 ) ;
float dY = sin( orbitAngle * PI / 180.0 ) ;
			
b2Vec2 moveVec = b2Vec2( body->GetPosition().x + dX / scale, body->GetPosition().y + dY / scale);
body->SetXForm( moveVec , body->GetAngle() );

The problem is SetXForm doesn't work very well with the outside forces applied to the body.

I know I have to use impulses, however this doesn't produce what I want :

Code:

float dX = cos( orbitAngle * PI / 180.0 ) ;
float dY = sin( orbitAngle * PI / 180.0 ) ;

b2Vec2 impulseVec = b2Vec2( 0.2 * dX , 0.2 * dY );
body->ApplyImpulse( impulseVec , body->GetWorldCenter() );

The body makes half circles and never returns to origin.

What is the obvious thing I'm missing?

[bateleur]

What is the obvious thing I'm missing?

Initial velocity?

For a body to follow a circular path the forces on it (or equivalently, impulses) vary as you have them. However, that will only result in a circular path if at any given point the force is centripetal. If the body in your example is initially stationary that won't be the case.

[encoder]

either case, you need an initial speed.

register a force pointing towards a center and set a tangent initial velocity.
if your body moves in a circular fashion you are lucky, most likely will have an elliptical path. you must do the math to get it exactly right. trial-and-error works to for some degree.
  

[go]

Yeah, it works fine with an initial velocity.

Thanks.