Rest Axis parameters of Ball and Socket constraint
This article explains the Rest Axis parameters of the Ball and Socket constraint.
Consider the hinge. It has one degree of freedom, which means its rest position can be specified with a single number (the "rest angle") that, in this case, varies from -180 degrees to 180 degrees.
The ball and socket joint, with two (actually, three, but we will ignore one for now) rotational degrees of freedom, would need two "rest angles." Instead of explicitly expressing two rest angles, we instead specify these angles implicitly by using a "rest axis" -- i.e. the direction to which the ball and socket joint will move when it has a joint spring attached.
The X, Y, and Z values range between -1 and 1 because we want to be able to point this rest axis in any direction (relative to the joint position), not just the positive X,Y, and Z hemisphere. The values are automatically normalized to unit length as you adjust them, which may cause some confusion.
I can't think of any examples other than to create a simple system like so:
CreatePrim "Cube", "MeshSurface" SetValue "cube.cube.length", 2 CreatePassiveRigidBody "cube" CreatePrim "Sphere", "MeshSurface" SetValue "sphere.sphere.radius", 1 Translate , 8, 0, 0, siAbsolute, siGlobal, siObj, siX, , , , , , , , , , 0 CreateActiveRigidBody "sphere" SelectObj "cube", , True AddToSelection "sphere", , True AttachRigidBodies , "cube,sphere", 4, 0, 0
Now play with the rest axis controls while the simulation plays back. It should be pretty intuitive.
