Rigging Quadrupeds

In this tutorial we are going to examine the skeletal differences that can be seen in a quadruped and look at how we can set up a quadruped rig.

First download the model (784kb) we will be using as a rigging reference.

The first issue we need to deal with in setting up a quadruped is the understanding of the anatomy of 4 legged creatures.

Essentially there is no difference between people, animals, birds etc in the makeup of bones. The difference lies in the relative length, orientation and consequently the function of the bones.

As can be seen in the image below, the human leg bone chain consists of;

- a pelvis
- a femur
- a tibia/fibula
- the metatarsals (heel to ball)
- the phalanges

It can be seen that these same functional bones also exist in the skeleton of the horse, the only difference is in their relative length and orientation.

The pelvis is also a ball joint, and the horse also has a knee, the only difference is that the knee is very high up in the leg. The equivalent ankle bone for a horse is relatively high up in the leg and the equivalent heel joint is raised off the ground (the horse always walks on tip-toe). The only joints of the hoof that touch the ground are the ball and toe joints.
 

The human leg

The leg bones of a horse

     

The back leg

Start by drawing a joint chain down the back leg using the diagram above as a guide for the joint placements.

Next we need to name our joints appropriately, and position them from a front view so that they lie correctly within the mesh.

To do this first select the pelvis joint and make sure that it is centred on the X axis.

If you select and simply translate the Hip joint you will not have to worry too much about resetting joint orientations.

Create a single joint, call it COG (centre of gravity) and point snap it to the pelvis then parent the pelvis to the COG.
     

Next lets set up the front legs. Again using the above image as a reference draw the bones for the front leg.

Again, make sure that the top joint (the scapula) is centered on the X axis, then translate the joint chain across so that it lines up nicely with the mesh.

Check your joint orientations. Mayas default orientations should work well here. If you have to reposition the joints, do so through a combination of rotating and scaling the joints. Dont translate them, it will ruin the joint orientations.

The next issue we need to look at is how this all fits together.

We need to create joint chains for the back, the tail and the head.

In choosing the number of joints we need to consider just how flexible each section has to be: the tail is very flexible, the main body is stiff and the neck is fairly flexible.

The tail


In order to be able to create light smooth flowing movements with the tail we will require a large number or joints.

Secondly, we want our joints to be oriented such that they can all rotate along the same axis in the same axis direction: ie all the joints must be oriented the same.

Drawing the joints out using the horse as a reference may result in incorrect joint orientation, so to avoid this, move away from the mesh and using grid snap, draw out a chain of joints (far more than is necessary)

Next move the root joint to the tip of the tail and progressively rotated each joint until it follows the flow of the tail. When you have oriented enough joints to cover the tail, just delete the rest of the joints.

Test the tail to make sure that when you select all the joints and rotate on only one axis, all the joints rotate together like a whip, or hey, like a tail...




The back

As the back will barely deform much at all on a horse, only a few joints will be needed between the pelvis and the scapula.

The mid section of the back will start at the same position as the COG, so start by drawing the joints just above the back, and when you are happy with them, point snap the top joint down to the COG and parent it to the COG.

With the back in place, you can now parent the scapula to the last joint in the back.
   

The head/neck

Create a joint chain, that is denser than the back, but not as dense as the tail for the neck. Take care to place a joint at the position of the jaw.

From the jaw joint, draw a couple more joints for the mouth.

To be continued...