public bool StickyPreSolve(cpArbiter arb, object data)
{
// We want to fudge the collisions a bit to allow shapes to overlap more.
// This simulates their squishy sticky surface, and more importantly
// keeps them from separating and destroying the joint.
// Track the deepest collision point and use that to determine if a rigid collision should occur.
float deepest = cp.Infinity;
// Grab the contact set and iterate over them.
cpContactPointSet contacts = arb.GetContactPointSet();
for (int i = 0; i < contacts.count; i++)
{
// Sink the contact points into the surface of each shape.
contacts.points[i].pointA = cpVect.cpvsub(contacts.points[i].pointA, cpVect.cpvmult(contacts.normal, STICK_SENSOR_THICKNESS));
contacts.points[i].pointB = cpVect.cpvadd(contacts.points[i].pointB, cpVect.cpvmult(contacts.normal, STICK_SENSOR_THICKNESS));
deepest = cp.cpfmin(deepest, contacts.points[i].distance); // + 2.0f*STICK_SENSOR_THICKNESS);
}
// Set the new contact point data.
arb.SetContactPointSet(ref contacts);
// If the shapes are overlapping enough, then create a
// joint that sticks them together at the first contact point.
if (arb.GetUserData() == null && deepest <= 0.0f)
{
cpBody bodyA, bodyB;
arb.GetBodies(out bodyA, out bodyB);
// Create a joint at the contact point to hold the body in place.
cpVect anchorA = bodyA.WorldToLocal(contacts.points[0].pointA);
cpVect anchorB = bodyB.WorldToLocal(contacts.points[0].pointB);
cpConstraint joint = new cpPivotJoint(bodyA, bodyB, anchorA, anchorB);
// Give it a finite force for the stickyness.
joint.SetMaxForce(3e3f);
// Schedule a post-step() callback to add the joint.
space.AddPostStepCallback(
(s, o1, o2) => PostStepAddJoint(joint, null),
joint, null);
// Store the joint on the arbiter so we can remove it later.
arb.SetUserData(joint);
}
// Position correction and velocity are handled separately so changing
// the overlap distance alone won't prevent the collision from occuring.
// Explicitly the collision for this frame if the shapes don't overlap using the new distance.
return(deepest <= 0.0f);
// Lots more that you could improve upon here as well:
// * Modify the joint over time to make it plastic.
// * Modify the joint in the post-step to make it conditionally plastic (like clay).
// * Track a joint for the deepest contact point instead of the first.
// * Track a joint for each contact point. (more complicated since you only get one data pointer).
}
public void StickySeparate(cpArbiter arb, object data)
{
cpConstraint joint = (cpConstraint)arb.GetUserData();
if (joint != null)
{
// The joint won't be removed until the step is done.
// Need to disable it so that it won't apply itself.
// Setting the force to 0 will do just that
joint.SetMaxForce(0.0f);
space.AddPostStepCallback((s1, o1, o2) =>
PostStepRemoveJoint(o1, o2), joint, null
);
// Perform the removal in a post-step() callback.
// NULL out the reference to the joint.
// Not required, but it's a good practice.
arb.SetUserData(null);
}
}