Physics with Bullet - jMonkeyEngine 3.0 - page 417

2. The constructor takes three of these ( size , speed , and orbit ) as the input, as
shown in the following code:
public StellarBody(float orbit, float speed, float
size)
3. We override the setSpatial method and add RigidBodyControl to the
supplied spatial with SphereCollisionShape , using size as the radius and
0 for mass:
RigidBodyControl rigidBody = new RigidBodyControl(new
SphereCollisionShape(size), 0f);
rigidBody.setGravity(Vector3f.ZERO);
spatial.addControl(rigidBody);
4. In the controlUpdate method, we make it move along its orbit by increasing
the speed of the cycle by multiplying it by tpf , as follows:
cycle += (speed * tpf) % FastMath.TWO_PI;
5. Then, we set the actual position of the planet along the orbit using the sin and
cos methods of the FastMath class:
float x = FastMath.sin(cycle);
float z = FastMath.cos(cycle);
6. We multiply the result by the orbit and set localTranslation of the spatial
to the new location as follows:
spatial.setLocalTranslation(x * orbit, 0, z * orbit);
7. Then, we also need to set physicsLocation of RigidBodyControl to the
same location.
8. We need a new method, getGravity , that will take the position of the ship as
an input Vector3f .
9. The method begins by subtracting the input position by worldTranslation ,
to get the position of the ship relative to the StellarBody class, as follows:
Vector3f relativePosition =
spatial.getWorldTranslation().subtract(position);