Hello World why does it go down?

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noatom
Posts: 12
Joined: Thu Nov 08, 2012 2:03 pm

Hello World why does it go down?

Post by noatom » Thu Nov 08, 2012 2:06 pm

In the hello world demo,there is an dynamic rigidbody.When i run the demo,I can see the y value of it getting lower and lower.

I thought it had something to do with gravity.But if I set the default y value to -56,IT STILL Goes down!

Why is it going down?!

odinsbane
Posts: 6
Joined: Thu Nov 08, 2012 8:51 pm

Re: Hello World why does it go down?

Post by odinsbane » Thu Nov 08, 2012 9:03 pm

It isn't clear what you mean when you say you set the default y value to -56. The gravity is set on line 43.

dynamicsWorld->setGravity(btVector3(0,-10,0));

If you set the gravity to a positive y value then it will 'fall' upwards.

noatom
Posts: 12
Joined: Thu Nov 08, 2012 2:03 pm

Re: Hello World why does it go down?

Post by noatom » Thu Nov 08, 2012 10:24 pm

Look at the output:

x 0 y -56 z 0
x 0 y -57 z 0
x 0 y -58 z 0
x 0 y -59 z 0
x 0 y -60 z 0
x 0 y -61 z 0
x 0 y -62 z 0
x 0 y -63 z 0
x 0 y -64 z 0
x 0 y -65 z 0
x 0 y -66 z 0
x 0 y -67 z 0
x 0 y -68 z 0
x 0 y -69 z 0
x 0 y -70 z 0

Gravity is set to:
dynamicsWorld->setGravity(btVector3(0,-10,0));

How is it possible to go down?! Should it get higher and higher untill it reaches -10?

Here's full code:

Code: Select all

///-----includes_end-----

	int i;
	///-----initialization_start-----

	///collision configuration contains default setup for memory, collision setup. Advanced users can create their own configuration.
	btDefaultCollisionConfiguration* collisionConfiguration = new btDefaultCollisionConfiguration();

	///use the default collision dispatcher. For parallel processing you can use a diffent dispatcher (see Extras/BulletMultiThreaded)
	btCollisionDispatcher* dispatcher = new	btCollisionDispatcher(collisionConfiguration);

	///btDbvtBroadphase is a good general purpose broadphase. You can also try out btAxis3Sweep.
	btBroadphaseInterface* overlappingPairCache = new btDbvtBroadphase();

	///the default constraint solver. For parallel processing you can use a different solver (see Extras/BulletMultiThreaded)
	btSequentialImpulseConstraintSolver* solver = new btSequentialImpulseConstraintSolver;

	btDiscreteDynamicsWorld* dynamicsWorld = new btDiscreteDynamicsWorld(dispatcher,overlappingPairCache,solver,collisionConfiguration);

	dynamicsWorld->setGravity(btVector3(0,-10,0));

	///-----initialization_end-----

	///create a few basic rigid bodies
	btCollisionShape* groundShape = new btBoxShape(btVector3(btScalar(50.),btScalar(50.),btScalar(50.)));
	
	//keep track of the shapes, we release memory at exit.
	//make sure to re-use collision shapes among rigid bodies whenever possible!
	btAlignedObjectArray<btCollisionShape*> collisionShapes;

	collisionShapes.push_back(groundShape);

	btTransform groundTransform;
	groundTransform.setIdentity();
	groundTransform.setOrigin(btVector3(0,-5,0));

	{
		btScalar mass(0.0f);

		//rigidbody is dynamic if and only if mass is non zero, otherwise static
		bool isDynamic = (mass != 0.f);

		btVector3 localInertia(0,0,0);
		if (isDynamic)
			groundShape->calculateLocalInertia(mass,localInertia);

		//using motionstate is recommended, it provides interpolation capabilities, and only synchronizes 'active' objects
		btDefaultMotionState* myMotionState = new btDefaultMotionState(groundTransform);
		btRigidBody::btRigidBodyConstructionInfo rbInfo(mass,myMotionState,groundShape,localInertia);
		btRigidBody* body = new btRigidBody(rbInfo);
		
		//add the body to the dynamics world
		//dynamicsWorld->addRigidBody(body);
	}


	{
		//create a dynamic rigidbody

		//btCollisionShape* colShape = new btBoxShape(btVector3(1,1,1));
		btCollisionShape* colShape = new btSphereShape(btScalar(1.));
		collisionShapes.push_back(colShape);

		/// Create Dynamic Objects
		btTransform startTransform;
		startTransform.setIdentity();
		startTransform.setOrigin(btVector3(0,-56,0));
		

		btScalar	mass(1.f);

		//rigidbody is dynamic if and only if mass is non zero, otherwise static
		bool isDynamic = (mass != 0.f);

		btVector3 localInertia(0,0,0);
		if (isDynamic)
			colShape->calculateLocalInertia(mass,localInertia);

			//startTransform.setOrigin(btVector3(2,10,0));
		
			//using motionstate is recommended, it provides interpolation capabilities, and only synchronizes 'active' objects
			btDefaultMotionState* myMotionState = new btDefaultMotionState(startTransform);
			btRigidBody::btRigidBodyConstructionInfo rbInfo(mass,myMotionState,colShape,localInertia);
			btRigidBody* body = new btRigidBody(rbInfo);

			dynamicsWorld->addRigidBody(body);
	}



/// Do some simulation


	///-----stepsimulation_start-----
	for (i=0;i<100;i++)
	{
		dynamicsWorld->stepSimulation(1.f/60.f,10);
		
		//print positions of all objects
		for (int j=dynamicsWorld->getNumCollisionObjects()-1; j>=0 ;j--)
		{
			btCollisionObject* obj = dynamicsWorld->getCollisionObjectArray()[j];
			btRigidBody* body = btRigidBody::upcast(obj);
			if (body && body->getMotionState())
			{
				btTransform trans;
				body->getMotionState()->getWorldTransform(trans);
			
				printf("world pos = %f,%f,%f\n",float(trans.getOrigin().getX()),float(trans.getOrigin().getY()),float(trans.getOrigin().getZ()));
			}
		}
	}

odinsbane
Posts: 6
Joined: Thu Nov 08, 2012 8:51 pm

Re: Hello World why does it go down?

Post by odinsbane » Thu Nov 08, 2012 11:22 pm

The gravity is a force with direction and magnitude. The object is being pushed with a force of 10 in the -y direction. It will go down no matter where you put it, unless you change the direction of the force.

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