private final void computeArea() {
// The area of a quadrilateral is 0.5*p*q*sin(theta)
// Where p and q are the length of the 2 diagonals and theta is the angle between them
// If we rotate P in the right direction, then area = 0.5*p*q*cos(theta) where theta is
// the angle between Q and the rotated P, but this is simply 0.5 * dot product of P' and Q
// Note, it is possible because of the cos to get a negative area, when this happens, it means
// that the quadrilateral has flipped and that the normally inward pointing normals are
// now pointing outward, so if we get a negative area, we should flip the force vectors...
Vector2D P = tv.getPosition().subtract(d.getPosition());
Vector2D Q = dt.getPosition().subtract(v.getPosition());
area = 0.5 * P.rotate90().dot(Q);
if (Math.signum(area) != Math.signum(desiredArea) && desiredArea != 0) {
System.err.println(
"Compartment:" + this.toString() + " has flipped inside out. Area:" + area);
}
}
