Function StickyPreSolve

tests/cpp-tests/Source/ChipmunkTestBed/demo/Sticky.cpp:40–93

}

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++)
    {
        // Sink the contact points into the surface of each shape.
        contacts.points[i].pointA = cpvsub(contacts.points[i].pointA, cpvmult(contacts.normal, STICK_SENSOR_THICKNESS));
        contacts.points[i].pointB = cpvadd(contacts.points[i].pointB, cpvmult(contacts.normal, STICK_SENSOR_THICKNESS));
        deepest                   = cpfmin(deepest, contacts.points[i].distance);  // + 2.0f*STICK_SENSOR_THICKNESS);
    }

    // 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.
        cpVect anchorA      = cpBodyWorldToLocal(bodyA, contacts.points[0].pointA);
        cpVect anchorB      = cpBodyWorldToLocal(bodyB, contacts.points[0].pointB);
        cpConstraint* joint = cpPivotJointNew2(bodyA, bodyB, anchorA, anchorB);

        // 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).
}

static void PostStepRemoveJoint(cpSpace* space, void* key, void* data)
{