btGeneric6DofConstraint : rotation of local transforms ?

2013-11-27T11:02:58+00:00

Hello everyone.
Let me first thank the authors for this great engine that I'm currently
discovering, and then apologize for the long post.

I'm trying to create a chain of constrained rigid bodies, given a definition
inputed in a text file.

For instance if my definition file looks like this:

Code:

ROOT ID 0 OFFSET 0 1 0JOINT ID 1PARENT ID 0OFFSET -1 0 0MINANGLES 0 0 0MAXANGLES 0 0 0JOINT ID 2PARENT ID 1OFFSET 1 1 0MINANGLES 0 0 0MAXANGLES 0 0 0

As a result I obtain something like this :

Each joint except for the root has a parent, and an offset from this parent that defines
the length of the next rigid body to create. MINANGLES and MAXANGLES determine, as their name suggest,
the angular limits of a btGeneric6DofConstraint I want to create.

The code that follows successfully creates the rigid bodies, but my problems appear when I create the constraints.

I would like to set the rotations of the constraints so that the initial world position and orientation of each
rigid body matches with initial angle values equal to 0 for X, Y and Z.

Working code:

Code:

void BuildOneJoint2(const btTransform& currentTransform, const btVector3& origin, const joint_def_t& joint, btDynamicsWorld* m_ownerWorld, btCollisionShape** m_shapes, btRigidBody** m_bodies, btTypedConstraint** m_joints, unsigned int& id, int parentId){const btVector3 vUp(0,1,0);const btVector3 vForward(1,0,0);const btVector3 vBack(0,0,1);// retrieving joint offset from parent                // definitions from text files are contained in struct joint btVector3 offSetVector(btScalar(joint.offset[0]), btScalar(joint.offset[1]),  btScalar(joint.offset[2]));btVector3 parentOffset;//computing radius and length of capsule; radius is more or less equal to offsetbtScalar radius = offSetVector.length() * btScalar(0.1); btScalar length = offSetVector.length() - 2 * radius;if(parentId == 0){// if current joint is root, for now juste create a small capsule representing rootbtVector3 rootOffSet(offSetVector); rootOffSet.normalize();parentOffset = rootOffSet * btScalar(0.05);offSetVector += parentOffset;}else{parentOffset = btVector3(btScalar(joint.parent->offset[0]), btScalar(joint.parent->offset[1]),  btScalar(joint.parent->offset[2]));}// Setup the geometry//The total height of a btCapsuleShape is height+2*radiusm_shapes[id] = new btCapsuleShapeX(radius, length - btScalar(0.1));// Setup the rigid bodybtTransform offset;offset.setIdentity();  offset.setOrigin(origin + offSetVector / 2);// computing rotation necessary for capsulebtVector3 vToBone(offSetVector);vToBone.normalize();btVector3 vAxis = vToBone.cross(vForward);btScalar dot = vToBone.dot(vForward);btScalar angle = acos(vToBone.dot(vForward));if(angle != 0){offset.setRotation(btQuaternion(vBack, angle));}m_bodies[id] = localCreateRigidBody(m_ownerWorld, btScalar(1.), offset, m_shapes[id]);btVector3 newOrigin(origin + offSetVector);btTransform newTransform; newTransform.setIdentity();newTransform.setRotation(currentTransform.getRotation() * offset.getRotation());newTransform.setOrigin(newOrigin);int newParentId = id;// method is recursive, call it with childrenfor(unsigned int i=0; i< joint.nbChildren_; ++i){  BuildOneJoint2(newTransform, newOrigin, *(joint.children[i]), m_ownerWorld, m_shapes, m_bodies, m_joints, ++id, newParentId);}}

The code for

Code:

 localCreateRigidBody

is taken from the demos :

Code:

btRigidBody* localCreateRigidBody (btDynamicsWorld* m_ownerWorld, btScalar mass, const btTransform& startTransform, btCollisionShape* shape){btVector3 localInertia(0,0,0);if (mass != 0.f) // is dynamicshape->calculateLocalInertia(mass,localInertia);btDefaultMotionState* myMotionState = new btDefaultMotionState(startTransform);btRigidBody::btRigidBodyConstructionInfo rbInfo(mass, myMotionState, shape, localInertia);btRigidBody* body = new btRigidBody(rbInfo);m_ownerWorld->addRigidBody(body);return body;}

Considering the previous code, I now try to create my constraints:

Code:

btGeneric6DofConstraint* constraint;btTransform localA; localA.setIdentity();//localA.setRotation(???);localA.setOrigin(btVector3(parentOffset.length() / 2, btScalar(0.), btScalar(0.)));btTransform localB; localB.setIdentity();//localB.setRotation(???);localB.setOrigin(btVector3(-length / 2, btScalar(0.), btScalar(0.)));constraint =  new btGeneric6DofConstraint(*m_bodies[parentId], *m_bodies[id], localA, localB, false);m_joints[id] = constraint;constraint->setDbgDrawSize(0.5f);btVector3 limitsDown(btScalar(joint.minAngleValues[0] * DegreesToRadians),                                 btScalar(joint.minAngleValues[1] * DegreesToRadians),                                 btScalar(joint.minAngleValues[2] * DegreesToRadians));btVector3 limitsUp(btScalar(joint.maxAngleValues[0] * DegreesToRadians),                             btScalar(joint.maxAngleValues[1] * DegreesToRadians),                             btScalar(joint.maxAngleValues[2] * DegreesToRadians));constraint->setAngularLowerLimit(limitsDown);constraint->setAngularUpperLimit(limitsUp);m_ownerWorld->addConstraint(m_joints[id], true);

As I said, I would like to set the rotations of the constraints so that the initial world position and orientation of each
rigid body matches with initial angle values equal to 0 for X, Y and Z for each constraint.

I've tried several possibilities without achieving the desired goal, and adding constraints gives me something like that in the end:

It is not really easy to write on a forum, and I hope I am clear enough.
Thank you very much for your time,

Steve

Statistics: Posted by stevet — Wed Nov 27, 2013 11:02 am

Real-Time Physics Simulation Forum

btGeneric6DofConstraint : rotation of local transforms ?