From BlenderWiki

Using Constraints in Animation

Constraints are a way to control an object’s properties (its location/rotation/scale), using either plain static values (like the “limit” ones), or (an)other object(s), called “targets” (like e.g. the “copy” ones).

Even though these constraints might be useful in static projects, their main usage is obviously in animation. There are two different aspects in constraints’ animation:

• You can control an object’s animation through the targets used by its constraints (this is a form of indirect animation).
• You can animate some constraints’ settings, in the Constraint Ipo type datablocks – well, this is in fact a quite limited feature, as we will see below…

Controlling Animation with Constraints

This applies only to constraints using target(s). Indeed, these targets can then control the constraint’s owner’s properties, and hence, animating the targets will indirectly animate the owner.

This indirect “constraint” animation can be very simple, like for example with the Copy Location constraint, where the owner object will simply copy the location of its target (with an optional constant offset). But you can also have very complex behaviors, like when using the Action constraint, which is a sort of Ipo driver for actions!

We should also mention the classical Child Of constraint, which creates parent/child relationship. These relationships indeed imply indirect animation (as transforming the parent affects by default all its children). But the Child Of constraint is also very important, as it allows you to parent your objects to bones, and hence use armatures to animate them!

Back to our simple Copy Location example, you can have two different behaviors of this constraint:

• When its Offset button is disabled (the default), the location of the owner is “absolutely” controlled by the constraint’s target, which means nothing (except other constraints below in the stack…) will be able to control the owner’s position. Not even the object’s Ipo curves.
• However, when the Offset button is enabled, the location of the owner is “relatively” controlled by the constraint’s target. This means that location’s properties of the owner are offset from the location of the target. And these owner’s location properties can be controlled e.g. by its Loc… Ipo curves (or actions, or NLA…)!

Example

Let’s use the Copy Location constraint and its Offset button. For example, you can make your owner (let’s call it moon) describe perfect circles centered on the (0.0, 0.0, 0.0) point (using e.g. pydriven LocX/LocY Ipo curves, see this example), and then make it copy the location of a target (called, I don’t know… earth, for example) – with the Offset button enabled. Congratulation, you just modeled a satellite in a (simplified) orbit around its planet… Just do the same thing with its planet around its star (which you might call sun, what do you think?), and why not, for the star around its galaxy…

Here is a small animation of a “solar” system created using (among a few others) the technique described above:

Note that the this “solar” system is not realistic at all (wrong scale, the “earth” is rotating in the wrong direction around the “sun”, …).

You can File:ManAnimationTechsUsingConstraintsExSolarSys.blend the .blend file used to create this animation.

Animating Constraints Influence

More “classically”, you can also animate a few properties of each constraint using Ipo curves. They have their own Ipo datablock type, named Constraint.

There is one Constraint Ipo datablock per constraint (unless you manually link a same datablock to several constraints). These Ipo datablock are not linked at all to the object constrained. The Ipo Curve Editor shows the datablock of the active constraint (which panel, in the Object context and sub-context, is of a slightly lighter gray) – the last clicked one.

You only have two Ipo curves (see also this page):

• You can animate the Influence of a constraint. This is the most useful Ipo! For example, in the “solar system” example above, I used it to first stick the camera to the “moon”, then to the “earth”, and finally to nothing, using two Copy Location constraints with Offset set, and their Influence cross-fading together…
• More anecdotal, you can also, for some constraints using an armature’s bone as target, animate where along this bone (between root and tip) lays the real target point (0.0 means “full-root”, and 1.0, “full-tip”).