Real-Time Physics Simulation Forum

Posted: **Thu Feb 03, 2022 10:45 pm**

Curious if there is a way to recalculate the local origin of a btCompoundShape? I currently have a shape consisting of two spheres and a cylinder, however the origin of the btCompoundShape seems to be the origin of one of it's child shapes and not the origin of the aabb for the compound shape.

Below is an image of the btPairCachingGhostObject which uses the btCompoundShape with it's origin set as follows (where y is up -z forward):

Code: Select all

this->ghostObject->getWorldTransform().setOrigin(btVector3(2, 0, 0));

: bbbb.JPG (23.95 KiB) Viewed 14359 times

Below is how the btCompoundShape and btPairCachingGhostObject are being generated:

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btCylinderShape* cyl = new btCylinderShape(btVector3(this->radius, (1.25f / 2.0f), this->radius));
btTransform cylTran;
cylTran.setIdentity();
cylTran.setOrigin(btVector3(0,0,0));


btSphereShape* sphereSmall = new btSphereShape(0.36f);
btTransform sphereSmallTran;
sphereSmallTran.setIdentity();
sphereSmallTran.setOrigin(btVector3(0, -0.77456, 0));


btSphereShape* blendSphere = new btSphereShape(0.4f);
btTransform blendSphereTran;
blendSphereTran.setIdentity();
blendSphereTran.setOrigin(btVector3(0, -0.62507, 0));


btCompoundShape* compoundShape = new btCompoundShape();
compoundShape->addChildShape(cylTran, cyl);
compoundShape->addChildShape(sphereSmallTran, sphereSmall);
compoundShape->addChildShape(blendSphereTran, blendSphere);


this->shape = compoundShape;


this->ghostObject = new btPairCachingGhostObject();
this->ghostObject->setCollisionShape(this->shape);
this->ghostObject->setCollisionFlags(btCollisionObject::CF_NO_CONTACT_RESPONSE);
this->collisionWorld->getDynamicsWorld()->addCollisionObject(this->ghostObject, btBroadphaseProxy::KinematicFilter, btBroadphaseProxy::StaticFilter | btBroadphaseProxy::DefaultFilter);
this->collisionWorld->getDynamicsWorld()->getPairCache()->setInternalGhostPairCallback(new btGhostPairCallback());


this->ghostObject->getWorldTransform().setOrigin(btVector3(2, 0, 0));

Posted: **Fri Feb 04, 2022 2:49 pm**

In the local-frame the origin and center of mass of the base btCollisionShape is always implicitly <0,0,0>. In other words: in the world-frame its center of mass is its position (e.g. its origin transformed by its worldTransform).

With a btCompoundShape what you're supposed to do is: compute the center of mass during the convex decomposition stage: in general it will have a non-identity transform. Then adjust the local-transforms of all the convex parts such that the center of mass is at the origin. For each subshape the math would be:

new_local_transform = inverse_center_of_mass_transform * old_local_transform

Finally, when adding each child to the compound shape you would supply its new_local_transform.

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compound_shape->addChildShape(new_local_transforms[i], sub_shapes[i]);

Posted: **Fri Feb 04, 2022 10:09 pm**

Greatly appreciated! Was able to get it worked out. Also came across this gist while researching what you had explained, which helped out as well.

Real-Time Physics Simulation Forum

Recalculate local origin of btCompoundShape?

Recalculate local origin of btCompoundShape?

Re: Recalculate local origin of btCompoundShape?

Re: Recalculate local origin of btCompoundShape?