Coordinate system

From Physics Simulation Wiki

Coordinate systems in Bullet are defined so the xz plane is horizontal, while the +y axis is meant to point "up".

Collision Shapes are defined in local shape coordinates, some shapes such as btSphereShape and btBoxShape are centered at the origin of this coordinate system. When shapes are attached to a btRigidBody a rigid world transformation is provided (see btTransform). No scaling or shearing is supported, although complex shapes can be efficiently scaled using a btUniformScalingShape.