Doc:Manual/Constraints/The Stack

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Constraints are accessed via the object buttons (F7) in the Constraints panel. After you press the Add Constraint button and select the desired constraint type from the menu, a constraint UI is added to the panel. The constraint will be linked to the active object or the active selected bone, indicated by the label To Object: or To Bone: on the right of the Add Constraint menu.

Constraints are evaluated in a specific order: from top to bottom of the constraint stack. This order can be viewed and changed inside the Constraints panel. Each constraint has a pair of arrow buttons on the right of the constraint name in its top right corner, which can be used to move the constraint up or down the constraint stack. The little cross on the right of these buttons can be used to delete the constraint from the stack.

NOTE: The name field of a newly added constraint will appear red, which indicates that the constraint is not functional. In the case of a newly added constraint this is because the constraint does not yet have a target specified. All constraints require a target object (a normal world object or an armature bone). You should enter the name of the desired target object in the Target: OB field. When you want an armature bone as target, enter the name of the armature object as target. A new field with BO: will appear, where you can place the name of the target bone.

The name field of a constraint will also turn red if the settings for the constraint are invalid or if the constraint conflicts with another constraint. For example: A Track To constraint of which the To and Up vector are set to Z.

[edit] Influence

The influence of a constraint on the actual location/rotation/size of the object can be defined with the Influence value slider. This can be linked to an Ipo Curve, which can be evoked in the Ipo Curve Editor with the button Show on the right of the Influence value slider. The Key button beside it can be used to add a key to the Ipo Curve.

You can key any constraint, and an object can have multiple constraints keyed to exert a different influence at different times.

[edit] Multiple Influence Example

Suppose that you want a camera to follow one path for awhile and then drift off and follow another path. You apply a follow path constraint to a camera for one circle/curve/path (make sure you have curve path enabled and enter its name), then open up an IPO curve window. In the buttons window (constraint panel) you can either hit I and insert an Influence value by sliding the slider (to vary the force strength or force fall off) or LMB click the "key" button at the bottom of the constraint panel. Once you have inserted the initial key, up arrow or change to the frame where you want the first influence to start falling off. Go to the IPO window and and CtrlLMB to insert a key. Go forward a few frames, where you want its influence to be zero, and insert another key, and edit it so that its Y value is zero. At that frame, the constraint in the stack will not influence the camera motion at all. Now insert a second Follow Path constraint, and enter the name of the second path. Now position back a few frames and insert a key to where you want its influence to start (probably where the first constraint starts dropping off). Edit this second influence curve to come up from zero to one over time. You can imagine that the IPO curves for each circle should cross over each other so that when one is at full influence (max 1.0) the other is at zero etc. You might have to play with the curves a bit to get the transition smooth.

[edit] Constraints on Bones

Concerning bones with constraints: the color of the bones in the 3D Window indicate what type of constraint is used:

• Grey: No constraint.
• Yellow: A bone with an IK constraint.
• Orange: A bone with an IK constraint but no target.
• Green: A bone with any other kind of constraint.
• Blue: A bone that is animated with keyframes.
• Purple: The Stride Root.