const gravity = new b2Vec2(0, 10);
world = new b2World(gravity);
const bd_ground = new b2BodyDef();
const ground = world.CreateBody(bd_ground);
const shape = new b2EdgeShape();
var xx=new b2Vec2(3, 4)
shape.SetTwoSided(xx, new b2Vec2(6, 7));
ground.CreateFixture(shape, 0);