= Bullet Physics Pitfalls
:revnumber: 2.0
:revdate: 2020/07/27


Bullet physics is not without its problems. Unfortunately, many of those are outside of the control of the jMonkeyEngine Core Team and thus cannot be fixed.


== Sweep Test Issues

.  When using link:{link-javadoc}/com/jme3/bullet/PhysicsSpace.html#sweepTest(com.jme3.bullet.collision.shapes.CollisionShape, com.jme3.math.Transform, com.jme3.math.Transform)[PhysicsSpace.sweepTest()], ensure that the distance between the transforms is at least 0.4wu or greater.
.  Note that the sweep will not detect collisions if it done inside of a collision shape. It must be on the edge of a collision shape to detect any collisions.


== Ghost Control AABB Collision only

As the javadoc for link:{link-javadoc}/com/jme3/bullet/objects/PhysicsGhostObject.html[PhysicsObjectControl] says, the ghost object collision detection uses AABB (Axis-aligned bounding box) collision only, regardless of the collision shape it has been assigned.

*Workaround:*+
Please use PhysicsSpace.sweepTest() instead, or kinematic physics objects with link:{link-javadoc}/com/jme3/bullet/PhysicsSpace.html#addCollisionListener(com.jme3.bullet.collision.PhysicsCollisionListener)[collision listeners].


== Rigid bodies fall through ground

This usually happens if the ground physics object has large triangles or consists of a large BoxCollisionShape.

*Workaround:*+

*  For meshes with large triangles - Subdivide the mesh in a model editor such as Blender.
*  For large boxes - separate into smaller boxes or use a MeshCollisionShape for terrain instead of BoxCollisionShape.