Studying Box2D-Lite in Javascript, Part 7: Circle Shape

In the previous post, we got as far as detecting overlap between polygons using the separating axis theorem, and the clipping for contact points, then store them in a manifold in local coordinates. Before we take a look at how to transform these contact points to world coordinates, let us create a circle shape, and the methods to detect collision and create contact points.

BVef

class b2CircleShape extends b2Shape {

	constructor(radius = 0.5) {

		super()

		this.m_type = b2Shape.e_circle;
        this.m_radius = radius;
        this.m_p = new Vec2(); // this is the center of mass local space

	}    

}

	computeMass(density) {

        this.m_mass.mass = density  b2_pi  this.m_radius  this.m_radius;
        this.m_mass.invMass = 1 / this.m_mass.mass;

        this.m_mass.center = this.m_p;

        // inertia about the local origin
        this.m_mass.I = this.m_mass.mass  (0.5  this.m_radius * this.m_radius + b2Dot(this.m_p, this.m_p));
        this.m_mass.invI = 1 / this.m_mass.I;
        
    }

	computeAABB(aabb, transform) {

		let p = Vec2.Add(transform.p, b2Mul(transform.q, this.m_p));//b2Vec2
		aabb.lowerBound.set(p.x - this.m_radius, p.y - this.m_radius);
		aabb.upperBound.set(p.x + this.m_radius, p.y + this.m_radius);
	}

	testPoint(transform, p) {
		let center = Vec2.Add(transform.p, b2Mul(transform.q, this.m_p)); //b2Vec2
		let d = Vec2.Subtract(p, center); //b2Vec2 
		return Vec2.Dot(d, d) <= this.m_radius  this.m_radius;
	}