Physics Simulation Forum

Hi,

I am trying to use bullet in a 2D plane. I add btGImpactConvexDecompositionShape in a world (shape draw with hand).. And I wants to restricts shape to a 2D plane (X,Y movement and only Z axis rotation). I use latest 2.73SP1 bullet physics SDK. I have read "I want to constrain an object to two dimensional movement, skipping one of the cardinal axes" but it don't works ... there is no visible constraint.. I have read previous topic in forum . it seems there is a bug ? and I need to have lock axis with one value for min, and previous value + epsilon . But it's the same ? I don't understand why it's not working ? Is there a bug in SDK ?

Best regards,

kao

Hi,

I have worked a lot to find a solution... But It don't works. I am new to bullet. please help me ... all is working fine except that.

kaolite

More information and code is probably needed

some information :

I init the bullet engine :
// Build the broadphase
int maxProxies=1024;
btVector3 worldAabbMin(-2000,-2000,-1000);
btVector3 worldAabbMax(2000,2000,1000);
m_broadphase=new btAxisSweep3(worldAabbMin,worldAabbMax,maxProxies);// Sweep and Prune Standard algo

// Set up the collision configuration and dispatcher
m_collisionConfiguration=new btDefaultCollisionConfiguration();
m_dispatcher=new btCollisionDispatcher(m_collisionConfiguration); // btCollisionDispatcher : handle Convex-Concave and Convex-Convex Collision

// The actual physics solver
m_solver=new btSequentialImpulseConstraintSolver;

// The Dynamic world.
m_dynamicsWorld=new btDiscreteDynamicsWorld(m_dispatcher,m_broadphase,m_solver,m_collisionConfiguration);
m_dynamicsWorld->setGravity(btVector3(0,-10,0));

// Ground
m_groundShape=new btStaticPlaneShape(btVector3(0,1,0),1);
btDefaultMotionState* groundMotionState=new btDefaultMotionState(btTransform(btQuaternion(0,0,0,1),btVector3(0,0,0)));

// Mass : Zero (static), shape : Plane , Inertial Tensor : Null
btRigidBody::btRigidBodyConstructionInfo groundRigidBodyCI(0,groundMotionState,m_groundShape,btVector3(0,0,0));
m_groundRigidBody=new btRigidBody(groundRigidBodyCI);
dynamicsWorld()->addRigidBody(m_groundRigidBody);//,COL_GROUND,COL_SHAPE|COL_GROUND);

// Constraint to Z=0 plan
m_zerobody=new btRigidBody(0,NULL,NULL);
dynamicsWorld()->addRigidBody(m_zerobody);
m_zerobody->setActivationState(DISABLE_DEACTIVATION);

//register algorithm
btCollisionDispatcher * dispatcher=static_cast<btCollisionDispatcher*>(m_dynamicsWorld->getDispatcher());
btGImpactCollisionAlgorithm::registerAlgorithm(dispatcher);


// Then each time I add a shape to my engine :

btTriangleMesh *triangleMesh=new btTriangleMesh();
int i;
PointVert *tpoint1,*tpoint2,*tpoint3;
for(i=0;i<vectlist.count();i+=3)
{
tpoint1=vectlist.value(i);
tpoint2=vectlist.value(i+1);
tpoint3=vectlist.value(i+2);
triangleMesh->addTriangle(btVector3(tpoint1->val[0],tpoint1->val[1],tpoint1->val[2]),btVector3(tpoint2->val[0],tpoint2->val[1],tpoint2->val[2]),btVector3(tpoint3->val[0],tpoint3->val[1],tpoint3->val[2]));
}

// Create Shape
btConvexTriangleMeshShape* gimpact=new btConvexTriangleMeshShape(triangleMesh);
btShapeHull* triHull=new btShapeHull(gimpact);
btScalar margin=gimpact->getMargin();
triHull->buildHull(margin);

btConvexHullShape* simplifiedShape=new btConvexHullShape();
for (i=0;i<triHull->numVertices();i++)
{
simplifiedShape->addPoint(triHull->getVertexPointer()[i]);
}
m_physicShape=simplifiedShape;

// DELETE UNUSED SHAPE !!!!!! TO ADD !!!!!
delete gimpact;
delete triHull;

// Set Motion State
btMotionState* MotionState=new OgreMotionState(m_node);

// Create Rigid Body
btScalar mass=1000;
btScalar restitution=1;
btScalar friction=0;
btVector3 localInertia(0,0,0);
m_physicShape->calculateLocalInertia(mass,localInertia);
// Mass : mass, shape : , Inertial Tensor : Null
btRigidBody::btRigidBodyConstructionInfo rigidBodyCI(mass,MotionState,m_physicShape,localInertia);
m_rigidBody=new btRigidBody(rigidBodyCI);
m_rigidBody->setRestitution(restitution);
m_rigidBody->setFriction(friction);
m_rigidBody->setActivationState(DISABLE_DEACTIVATION);
layer()->dynamicsWorld()->addRigidBody(m_rigidBody);//,COL_SHAPE,COL_SHAPE|COL_GROUND);

// Constraint
m_constrict=new btGeneric6DofConstraint(*m_rigidBody,*layer()->zeroBody(),btTransform::getIdentity(),btTransform::getIdentity(),false);
// Use limit(axis, a, b) where a>b to disable limits on that axis
m_constrict->setLinearLowerLimit(btVector3(-2000,-2000,0.00001));
m_constrict->setLinearUpperLimit(btVector3(2000,2000,0));
m_constrict->setAngularLowerLimit(btVector3(0.00001,0.00001,0.00001));
m_constrict->setAngularUpperLimit(btVector3(0,0,0));
layer()->dynamicsWorld()->addConstraint(m_constrict,true);

Your lower limit on the Z axis is higher than the upper limit on that axis, which effectively disables limits on that axis at all

Gary (-;

sorry it was old code, I use this code without success:

m_constrict->setLinearLowerLimit(btVector3(1,1,0));
m_constrict->setLinearUpperLimit(btVector3(0,0,0));
m_constrict->setAngularLowerLimit(btVector3(0,0,1));
m_constrict->setAngularUpperLimit(btVector3(0,0,0));

I don't know what's to do... It seems that it's possible to constraint bullet in 2D.

Well, FWIW I wrote that code snippet for constraining bullet to 2d, and it's working for me. I do note that you're using set{Linear,Angular}{Upper,Lower} limit, and that's not what I actually use. The code snippet I put on the wiki uses 6dof->setLimit, perhaps you could try that?

Gary (-;

thanks for the answer ... but I have tryed with your wiki add ... using setLimit ... but I have the same trouble

hello,
After loosing a lot of time trying to use bullet in 2D... It seems that it's better for me to change engine ... perhaps box2d is best for that...

I am back .... after trying to using box2d, not working with triangle mesh,
Physx : no collision between triangle mesh....
I am back to Bullet, the winner .... triangle mesh with collision detection between triangle mesh....
but I don't know what's wrong to constraint to a 2D world ... I don't know if I do some mistake or if it's a bug in Bullet engine...

Best regards

Did you find a solution to this? I've tried both the 2D constraint code from the Wiki and from the user manual without success.

No ... it's always not working. I have lost a lot of time on this without success. It seems that there is a bug in bullet or the wiki is wrong. I am not the only one.

kao

have you tried a simple cube/sphere?

seems like there was some tricks i had to do to get it work, but here's my relevant code:



This was for a skateboarding game, so i constrained it to the Y-Z plane instead of X, but it's all the same. I also had to update the position of the zerobody every frame, but this is only cause i was rotating the plane about the Y-Axis, so you shouldn't have to do this.

I think it don't works when you try to restrict an object translation to one plane, and only one axis of rotation (a true 2D movement)

I have just retry with the last svn bullet. It seems better, but when two object collides they move on the Z axis (It's better because I have no more rotation the X and the Y axis ). I have the following constraint :

m_constrict->setLimit(0,1,0); // Disable X axis limits
m_constrict->setLimit(1,1,0); // Disable Y axis limits
m_constrict->setLimit(2,0,0); // Set the Z axis to always be equal to zero
m_constrict->setLimit(3,0,0); // Disable X rotational axes
m_constrict->setLimit(4,0,0); // Disable Y rotational axes
m_constrict->setLimit(5,1,0); // Uncap the rotational axes

hopes it help to fixe the issue ...