***********
Constraints
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The **Constraints** tab in **Properties** editor exposes constraints setups for current 
object.

Currently in UPBGE only the *Rigid Body Joint* constraint can be set through the user 
interface. 

===========================
Rigid Body Joint Constraint
===========================

The *Rigid Body Joint* constraint is very special, it is used by the physics part of
UPBGE to simulate a joint between its owner and its target. It offers four joint types: 
hinge type, ball-and-socket type, cone-twist type, and generic six-\ :abbr:`DoF (Degrees 
of Freedom)` type.

The joint point and axes are defined and fixed relative to the owner. The target moves as 
if it were stuck to the center point of a stick, the other end of the stick rotating 
around the joint/pivot point.

.. note::

   In order for this constraint to work properly, both objects
   (so the owner and the target object) need to have *Collision Bounds* enabled.


Options
=======

.. figure:: /images/editors-properties-constraints-rigid_body_joint.png

   Rigid Body Joint panel.

Target
   Object used to select the constraints target, and is not functional (red state) when 
   it has none.

Pivot Type
   Ball
      Works like an ideal ball-and-socket joint, i.e. allows rotations around all axes 
      like a shoulder joint.
      
   Hinge
      Works in one plane, like an elbow: the owner and target can only rotate around the 
      X axis of the pivot (joint point).

      Limits
         .. figure:: /images/editors-properties-constraints-rigid_body_joint-hinge.png
         
         Angular limits for the X axis.
         
   Cone Twist
      Similar to *Ball*, this is a point-to-point joint with limits added for the cone 
      and twist axis.

      Limits
         .. figure:: /images/editors-properties-constraints-rigid_body_joint-cone_twist.png
         
         Angular limits.
         
   Generic 6DOF
      Works like the *Ball* option, but the target is no longer constrained at a fixed 
      distance from the pivot point, by default (hence the six degrees of freedom: 
      rotation and translation around/along the three axes). In fact, there is no longer 
      a joint by default, with this option, but it enables additional settings which 
      allow you to restrict some of these DoF:

      Limits
         .. figure:: /images/editors-properties-constraints-rigid_body_joint-generic_6_dof.png
         
         Linear and angular limits for a given axis (of the pivot) in Blender Units and 
         degrees respectively.

Child Object
   Normally, leave this blank. You can reset it to blank by right-clicking and selecting 
   Reset to Default Value.

Linked Collision
   When enabled, this will disable the collision detection between the owner and the 
   target (in the physical engine of the BGE).

Display Pivot
   When enabled, this will draw the pivot of the joint in the 3D Views. The most useful, 
   especially with the *Generic 6DOF* joint type!

Use Breaking
   Allow breaking of constraint on high impulse.
   
   Breaking Impulse Threshold
      Break constraint on impulse greater than threshold.

Pivot
   These three numeric fields allow you to relocate the pivot point, *in the owner's space*.
   
Axis
   These three numeric fields allow you to rotate the pivot point, *in the owner's space*.