directx12 – How to update DX12 Render Mesh from PhysX mesh


I have 2 Meshes and I want to add physics to them, the first is a primitive box, the second is a terrain

the box and the terrain made with DirectXMath

Using PhysX library, I made a copy of the terrain and used it to create a Triangle mesh then

and I created a box using physx::PxBoxGeometry as you can see here

        physx::PxTransform boxPos(physx::PxVec3(0.0f, 20, 0.0f));
        physx::PxBoxGeometry boxGeometry(physx::PxVec3(0.5f, 0.5f, 0.5f));
        physx::PxShape* shape =m_pPhysics->createShape(boxGeometry, *m_pMaterial);
        physx::PxRigidDynamic* boxRigid = m_pPhysics->createRigidDynamic(boxPos);
        boxRigid->attachShape(*shape);
        physx::PxRigidBodyExt::updateMassAndInertia(*boxRigid, .15f);
                    
        m_pScene->addActor(*boxRigid);
        m_pPhysicsActors.push_back(boxRigid);
        

I added them to the scene and I ran the simulation and I can see the box is dropped and flipped couple times using Physx Visual Debugger(though, on some runs the debugger shows artifacts, anyway)

I found out that I can get the translation and rotation by using m_pPhysicsActors[]->getGlobalPose() which is both quaternion for rotation and vector for position so I thought that’s it

but in the hello world snippets they made this function

/**
brief Retrieves the world space pose of the shape.

param[in] shape The shape for which to get the global pose.
param[in] actor The actor to which the shape is attached

return Global pose of shape.
*/
static PX_INLINE    PxTransform     getGlobalPose(const PxShape& shape, const PxRigidActor& actor)
{
    return actor.getGlobalPose() * shape.getLocalPose();
}

then they put the output in a matrix

Now I am confused, why actor.getGlobalPose() * shape.getLocalPose();

I thought the shape and the actor are the same thing, like a relationship or a property, since I assign a shape to an actor, I imagine that if I rotate a shape, I rotate the actor vice verse and it is one thing.

this is where I am lost! , so can someone walk me through on this part of how to update render items from PhysX



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