public Contact popContact(Fixture fixtureA, int indexA, Fixture fixtureB, int indexB) {
final ShapeType type1 = fixtureA.getType();
final ShapeType type2 = fixtureB.getType();
final ContactRegister reg = contactStacks[type1.ordinal()][type2.ordinal()];
final IDynamicStack<Contact> creator = reg.creator;
if (creator != null) {
if (reg.primary) {
Contact c = creator.pop();
c.init(fixtureA, indexA, fixtureB, indexB);