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====== HingeJoint ======

=== Misconceptions ===
  * ''HingeJoint'' does not constrain the absolute ''rotation'' or ''localRotation'' of the Transform that it is attached to. It constrains the **relative** rotation between its own RigidBody and the Rigidbody in the ''ConnectedBody'' field.

* ''HingeJoint.angle'' and ''HingJoint.axis'' do not represent the rotation of the Transform that it is attached to. Ex: ''Quaternion.AngleAxis(hinge.angle, hinge.axis)'' is not equivalent to''hinge.transform.rotation'' or ''hinge.transform.localRotation''.

These two misconceptions, taken together, can make a ''HingeJoint'' behave quite differently from what you might expect.

To quote the [[https://docs.unity3d.com/ScriptReference/HingeJoint-angle.html|official docs]]:

> The rest angle between the bodies is always zero at the beginning of the simulation.

The consequence of this is that ''HingeJoint.angle'' does not depend on the starting orientation of either the joint's ''RigidBody'' or the connected ''RigidBody''. And because ''HingeJoint.limits'' represents limits on ''HingeJoint.angle'', the limits are also independent of any starting orientation of the bodies.

An example: Consider a ''GameObject'' with rotation ''(0, 0, 0)'' and a ''HingeJoint'' on the x-axis with limits of ''[-90, 90]''. When played as-is, this ''GameObject'' will be free to rotate between ''(-90, 0, 0)'' and ''(90, 0, 0)''. However, if you rotate this ''GameObject'' in the editor such that it starts with rotation ''(90, 0, 0)'' and play it, the ''GameObject'' will then be free to rotate between ''(0, 0, 0)'' and ''(180, 0, 0)'' - even though the angle limits have not changed.

=== Things That May Solve Problems With HingeJoint: ===

In the vast majority of use cases, your ''HingeJoint'' should have a ''ConnectedBody'' assigned. This field is optional, but if left empty it causes your ''HingeJoint'' to be anchored to the global physics coordinate system. If your ''HingeJoint'' happens to be a child of some other object that moves, the joint will continue applying its limits within the global physics coordinate system - not in the joint's local coordinate system. This is usually not what you want.

A good way to ensure that your ''HingeJoint'' behaves nicely is to create a ''RigidBody'' on the parent object (if one doesn't already exist) and set that as the ''ConnectedBody''. The parent body can have ''IsKinematic'' enabled if you don't want it to act like a physics object. In this configuration, the ''axis'' and ''angle'' of the ''HingeJoint'' will actually correspond to the ''localRotation'' of the ''Transform'' (though it will still be relative to any initial ''localRotation''). This makes the joint behavior much easier to reason about!

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