static cpBool
StickyPreSolve(cpArbiter *arb, cpSpace *space, void *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.
cpFloat deepest = INFINITY;
// Grab the contact set and iterate over them.
cpContactPointSet contacts = cpArbiterGetContactPointSet(arb);
for(int i=0; i<contacts.count; i++){
// Increase the distance (negative means overlaping) of the
// collision to allow them to overlap more.
// This value is used only for fixing the positions of overlapping shapes.
cpFloat dist = contacts.points[i].dist + 2.0f*STICK_SENSOR_THICKNESS;
contacts.points[i].dist = cpfmin(0.0f, dist);
deepest = cpfmin(deepest, dist);
}
// Set the new contact point data.
cpArbiterSetContactPointSet(arb, &contacts);
// If the shapes are overlapping enough, then create a
// joint that sticks them together at the first contact point.
if(!cpArbiterGetUserData(arb) && deepest <= 0.0f){
CP_ARBITER_GET_BODIES(arb, bodyA, bodyB);
// Create a joint at the contact point to hold the body in place.
cpConstraint *joint = cpPivotJointNew(bodyA, bodyB, contacts.points[0].point);
// Give it a finite force for the stickyness.
cpConstraintSetMaxForce(joint, 3e3);
// Schedule a post-step() callback to add the joint.
cpSpaceAddPostStepCallback(space, PostStepAddJoint, joint, NULL);
// Store the joint on the arbiter so we can remove it later.
cpArbiterSetUserData(arb, 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).
}
void cArbiter::ContactPointSet( cpContactPointSet * contact ) {
cpArbiterSetContactPointSet( mArbiter, contact );
}