Bullet physics collision detection

Question

I'm using bulletsharp (C# wrapper of bullet physics) to do some collision detection. Everything went fine until i tried to change the collision shape of an existing rigid body from box to compound at runtime. I'm doing this to simulate the collision again with a higher accuracy. The problem is: collision of compound shapes is not detected.

Scenario:

• DicreteDynamicsWorld containing some RigidBody with BoxShape
• Collision of two kinematic RigidBody with BoxShape happens (and is detected)
• Change the shapes of these two RigidBody to CompoundShape using the HACD algorithm for convex decomposition
• Remove RigidBody from DicreteDynamicsWorld
• RigidBody.CollisionShape = CompoundShape
• Set Position using RigidBody.MotionState.WorldTransform
• Add RigidBody to DicreteDynamicsWorld
• Undo the movement which caused the collision (one object doesn't contain the other)
• Repeat the movement
• Collision is not detected

Remarks:

• Changing the CollisionShape from BoxShape to CompoundShape was successful (correct CollisionShape and correct position)
• For collision detection i'm using DicreteDynamicsWorld.Dispatcher.NumManifolds > 0 after a DicreteDynamicsWorld.StepSimulation(...)

Some code snippets as requested:

If you need something particular, please tell me. My solution is too big and too complexe to post the complete code...

RigidBody creation:

// Create rigid body
MotionState motionState = new DefaultMotionState(startTransform);
RigidBodyConstructionInfo rbInfo = new RigidBodyConstructionInfo(0.0f, motionState, collisionShape);
RigidBody rigidBody = new RigidBody(rbInfo);
rbInfo.Dispose();

// Kinematic body: mass=0 -> static/kinematic -> use flag
bool isKinematicBody = (compModel.Children[i].Type ==...) || ... ;
rigidBody.CollisionFlags = isKinematicBody ? CollisionFlags.KinematicObject : CollisionFlags.StaticObject;
rigidBody.ActivationState = ActivationState.DisableDeactivation;

Basic steps:

// Get old collision data
if (compModel.Children[i].Container.TryGetValue(ContainerType.Collision, out container))
    collisionData = ((ContainerCollision) container).CollisionData;

// Get geometry
if (compModel.Children[i].Container.TryGetValue(ContainerType.Geometry, out container))
{
    verticesGeo = ((ContainerGeometry) container).GeometryData.Vertices;
    trianglesGeo = ((ContainerGeometry) container).GeometryData.Triangles;
}

// Remove rigid body from world
_world.RemoveRigidBody(collisionData.RigidBody);

// Create new shape
List<Vector3> vertices = Utility.ToBulletVector3List(verticesGeo);
List<int> indices = Utility.ListIntArrayToListInt(trianglesGeo);
CompoundShape collisionShape = ConvexDecomposition(compModel.Children[i].Id, vertices, indices);

// Set collision shape
collisionData.RigidBody.CollisionShape = collisionShape;

// Set position
collisionData.RigidBody.MotionState.WorldTransform *= collisionData.PositionDifference;

// Add rigid body to world
_world.AddRigidBody(collisionData.RigidBody, collisionData.CollisionGroup, collisionData.CollisionMask);

CollisionContainer:

public interface IContainer
{
    ContainerType Type { get; }
}

public struct ContainerCollision : IContainer
{
    public ContainerType Type
    {
        get { return ContainerType.Collision; }
    }

    public CollisionData CollisionData;
}

Structure CollisionData:

public struct CollisionData
{
    public BulletSharp.RigidBody RigidBody;
    public BulletSharp.Matrix PositionDifference;
    public BulletSharp.Vector3 ZeroPosition;
    public short CollisionGroup;
    public short CollisionMask;
}

Any ideas, what i'm doing wrong?

Thanks.

Answer 1

Assuming that all your change in shape is happening here

CompoundShape collisionShape = ConvexDecomposition(compModel.Children[i].Id, vertices, indices);

// Set collision shape
collisionData.RigidBody.CollisionShape = collisionShape;

// Set position
collisionData.RigidBody.MotionState.WorldTransform *= collisionData.PositionDifference;

    // Add rigid body to world
_world.AddRigidBody(collisionData.RigidBody, collisionData.CollisionGroup, collisionData.CollisionMask);

Probably you will have to recalculate the entire RigidBody from start, I have no clue how you do that in the first place so I will show you an example how I do it.

public virtual RigidBody LocalCreateRigidBody(float mass, Matrix startTransform, CollisionShape shape)
        {

            //rigidbody is dynamic if and only if mass is non zero, otherwise static
            bool isDynamic = (mass != 0.0f);

            Vector3 localInertia = Vector3.Zero;
            if (isDynamic)
                shape.CalculateLocalInertia(mass, out localInertia);

            //using motionstate is recommended, it provides interpolation capabilities, and only synchronizes 'active' objects
            DefaultMotionState myMotionState = new DefaultMotionState(startTransform);

            RigidBodyConstructionInfo rbInfo = new RigidBodyConstructionInfo(mass, myMotionState, shape, localInertia);
            RigidBody body = new RigidBody(rbInfo);
            rbInfo.Dispose();


            return body;
        }

With that being said, take a note at your "collisionData" (Which I guess its a class?) and the naming of RigidBody (it might have a conflict with the BulletSharp class?)

Best reguards with your progress and feel free to contact me if you have any questions!