Hi Abhinav,
The reason you're not getting the impulse applied in your contact point callback is that this callback is fired BEFORE the collision is resolved. It is designed this way so that the listener can make changes to the rigid body properties that could affect the way the collision will resolve. It sounds like you want access to the resulting values AFTER the collision is resolved. Havok is not designed to query the collision forces directly for individual contact points, so the solution will be a bit more involved. If your collision lasts more than a single frame, the impulse applied will correspond to the collision response for the previous frame. If your collision only lasts one frame, though, this will not work. In this case, you may have to recreate the forces by hand. You can capture the net force applied to a rigid body each frame capturing its linear and angular velocities and differentiating over the time step. If you need the force for a particular contact point, you can compute it using the separating velocity (if interested, please look at this article describing the mathematics involved). These approaches are mathematically complex, computationally expensive, and approximate, though. I'll keep looking for a serviceable workaround, and let you know what I find.

Hi Abhinav,

I recommend that you look at 1.3.1.8. Limiting Constraint Force in the Havok Physics help (Creating a Simulation -> Creating Physics Objects ->Constraints -> Introduction -> Limiting Constraint Forces). In particular, take a look at what it says about hkpContactImpulseLimitBreachedListener. For ragdoll constraints, you can specify a maximum impulse that each constraint can apply. Conceptually, this corresponds to the amount of force that a joint can safely withstand along a direction in which it doesn't move freely. When this limit is exceeded, you can use this callback to respond appropriately. In the case of a ragdoll constraint, this could mean notifying the gameplay logic that the character has sustained an injury. This won't give you the applied forces directly, but it may provide a useful substitute. Keep in mind that this will cap the constraint impulses, allowing the joints to break apart if this limit is exceeded.

Here's an example of how to set this up for a ragdoll constraint using the Physics -> Api -> Constraints -> Ragdoll demo:

add in Demos\Physics\Api\Constraints\Ragdoll\RagdollDemo.h:

> #include <Physics/Dynamics/Constraint/Contact/hkpContactImpulseLimitBreachedListener.h>

> class MyListener : public hkpContactImpulseLimitBreachedListener, public hkReferencedObject
> {
> public:
>     HK_DECLARE_CLASS_ALLOCATOR(HK_MEMORY_CLASS_DEMO);
>     virtual void contactImpulseLimitBreachedCallback( const hkpContactImpulseLimitBreachedListenerInfo* breachedContacts, int numBreachedContacts ) ;
> };

add in Demos\Physics\Api\Constraints\Ragdoll\RagdollDemo.cpp:

> void MyListener::contactImpulseLimitBreachedCallback( const hkpContactImpulseLimitBreachedListenerInfo* breachedContacts, int numBreachedContacts )
> {
>     hkprintf("ouch!");
> }

//...

RagdollDemo::RagdollDemo(hkDemoEnvironment* env)
{

    //...

    //
    // CREATE RAGDOLL CONSTRAINT
    //
    {
        hkReal planeMin = HK_REAL_PI * -0.2f;
        hkReal planeMax = HK_REAL_PI * 0.1f;
        hkReal twistMin = HK_REAL_PI * -0.1f;
        hkReal twistMax = HK_REAL_PI * 0.4f;
        hkReal coneMin = HK_REAL_PI * 0.3f;

        hkpRagdollConstraintData* rdc = new hkpRagdollConstraintData();

        rdc->setConeAngularLimit(coneMin);
        rdc->setPlaneMinAngularLimit(planeMin);
        rdc->setPlaneMaxAngularLimit(planeMax);
        rdc->setTwistMinAngularLimit(twistMin);
        rdc->setTwistMaxAngularLimit(twistMax);
>         rdc->setBreachImpulse(10.f); // this is the maximum impulse that can be applied per physics iteration

        hkVector4 twistAxis(1.0f, 0.0f, 0.0f);
        hkVector4 planeAxis(0.0f, 1.0f, 0.0f);
        pivot.set(0.0f, 0.0f, 0.0f);
        rdc->setInWorldSpace(moveableBody->getTransform(), fixedBody->getTransform(), pivot, twistAxis, planeAxis);
        
        //
        //    Create and add the constraint
        //
        {
            hkpConstraintInstance* constraint = new hkpConstraintInstance(moveableBody, fixedBody, rdc );
            m_world->addConstraint(constraint);
            constraint->removeReference();
        }    

        rdc->removeReference();

>         m_world->addContactImpulseLimitBreachedListener( &m_myListener );
    }
    

Hi Abhinav,
You can set up a rag doll for your character using the Havok Maya plugin. Please look at the Havok Content Tools help under Maya Tools -> Tutorial: Rag Doll Setup for more information on how to do this.

There are many ways to model a bomb using Havok Physics. Depending on how you choose to model it, you can apply explosion forces however you like. One way to do it would be to create a custom action to simulate a shockwave. For example, a simple shockwave action could have a position, energy, radius, and expansion velocity. Each frame, you would increase the radius by the expansion velocity and look for objects in the sphere formed around the explosion epicenter. Then you could apply impulses to these objects based on their mass, distance from the epicenter, and explosion energy. In building this, you could build in ways to track the forces applied by the shockwave. For more information on how to create a custom action, please look at Havok Physics Help under Creating a Simulation -> Simulating a Physics World -> Populating a World -> Actions -> Creating a Custom Action.

Hi Abhinav,
It sounds like you're already using the physics -> api -> dynamics -> Actions -> WindAction -> Explosion demo, which is what I would recommend. The explosion demo does essentially everything you need already, though it uses hkpWind rather than an hkpAction. If you have extra time and are curious about the process, you can build the same functionality using an action instead of a wind. You would need to derive from hkpNullAction because this action isn't tied to a particular rigid body. You would then need to override getEntities to get all rigid bodies that intersect the shockwave. You would also need to override applyAction to get and apply the vector to each rigid body from getEntities. This calculation should look like Explosion::getWindVector, followed by a call to hkpRigidBody::applyForce(). There are no examples in the demos of hkpNullActions, but the action interface is shared between all action types. For an example, look at Demos\Physics\Api\Dynamics\Actions\GravityAction\GravityAction.h