Constraints are similar to parenting objects however, whereas a child will inherit all movements of its parent object, with constraints the object will only take on the movement of the type of constraint. For example, orient constraints make objects inherit rotations. Point constraints are for translations. Scale constraints are for scaling.
For the most part, we will use point and orient constraints.
Above is a simple test of constraints. We have two orient constraints on each tyre attached to two controllers. These controllers allow a user to grab and manipulate them in order to move the objects.
One orient controls tyre rotation and the other controls steering.
The controllers can be drawn using CV Curve Tool (Create > CV Curve Tool).
Start by making two tyres and put each one in its own group. Move the GROUP pivot to the middle of each tyre respectively. To do this open the outliner, click the group and press 'insert' to move the pivot. Press 'insert' again when it's in the right position.
Next, draw a double headed arrow controller using the CV Curve Tool. This one will be used to control steering.
Select the controller followed by either tyre, in this case I'm highlighting the left tyre, then open the Orient Constraint tool menu (Animation tool set > Constrain > Orient > click the small box).
You want 'Maintain offset' on otherwise the object will move itself. Set 'Constrain axes' to 'Y' as we want to turn the tyre only on the 'Y' axes. This means if we rotate the controller any other way than 'Y' it won't affect the tyre. Apply these settings.
Notice the red exclamation point in the outliner. This is your constraint. If something went wrong just click it and press delete and try again. Now when you rotate the controller on the 'Y' axis, the tyre should move with it. Do the same with the right tyre so they both move together.
Try doing this for the tyres on the 'X' axes now which will allow the tyre to rotate forward and back.
What is Rigging?
Rigging is a process of attaching bones and joints to a mesh which are further connected to controllers which the animator then uses. It is much like pulling the strings of a puppet. Influence is then assigned to the bones and joints so that certain bones manipulate only the vertices around its envelope.
There are two commons methods of controlling a skeleton, inverse or forward kinematics. Inverse makes the bones follow a target while forward is individually rotating the joints to achieve the desired effect.
Parenting or Grouping?
There are two main types of joining objects together, parenting and grouping. Parenting sees one object act as a 'parent' and the other attached to it are known as a 'child'. If the parent is manipulated then the child will copy its manipulation. Below is an example of two spheres, the left is the parent.
In the outliner you can see that the middle and right spheres are parented to the left sphere...
If I manipulate the left sphere (the parent), the other spheres (the child), will copy it. In this case I changed the rotation and scale of the parent...
However, if I manipulate the child independently, the parent will not react...
If I move the parent again, the child will still copy despite its changes.
Grouping is more versatile than parenting. It acts as a 'null object parent' (essentially, a non existant object is the parent). More so, a group of objects have their own independant pivots as well as a group pivot while parenting only has the objects pivot.
It's possible to have more than one group for the same objects. This means we can have multiple pivots for the same set of objects which can be useful for joints such as a foot (Heel pivot, ball pivot, toe pivot). All the objects will move based on its groups pivot.
