Blender Documentation: Last modified July 24 2003 bv
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# Constraint Evaluation Rules

Constraints can be applied to objects or bones. In the case of constraints applied to bones, any constraints on the armature OBJECT will be evaluated before the constraints on the bones are considered.

When a specific constraint is evaluated, all of its dependencies will have already been evaluated and will be in their final orientation/positions. Examples of dependencies are the object's parent, its parent's parents (if any) and the hierarchies of any targets specified in the constraint.

Within a given object, constraints are executed from top to bottom. Constraints that occur lower in the list may override the effects of constraints higher in the list. Each constraint receives as input the results of the previous constraint. The input to the first constraint in the list is the output of the ipos associated with the object.

If several constraints of the same type are specified in a contiguous block, the constraint will be evaluated ONCE for the entire block, using an average of all the targets. In this way you can constrain an object to track to the point between two other objects, for example. You can use a NULL constraint to insert a break in a constraint block if you would prefer each constraint to be evaluated individually.

Looping constraints are not allowed. If a loop is detected, all of the constraints involved will be temporarily disabled (and highlighted in red). Once the conflict has been resolved, the constraints will automatically re-activate.

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