Fundamentals of Rigging

In order to animate our 3d character models they need to be able to move in the same way people can move. Character rigging is the process of adding bones, and controllers to our characters so that they can be manipulated into different poses and therefore animated.

The following is a bried overview of the rigging process:

Creating the Skeleton

Starting with our finished 3d character models the first stage of the rigging process is to create a skeleton. The skeleton consists of many bones linked together that emulate the way the bones in the human body work.

Take the arm for example. You can lift your upper arm up and down, pivot it forwards and backwards and twist it. It has a great degree of movement. Your forearm though bends in just one direction and stops when it straightens.

So to create a skeleton for our 3d character we need to observe and study human anatomy. You need to create rigs with the same flexibility and capabilities and sometimes the same constraints of movement to be able to animate realistic and believable motions.

Custom Rigs

Maya has a joint system that allow you to create skeletons from scratch. Creating each bone in turn and defining how you’ll allow each one to move. This can be a lengthy and repetitive process, requiring the greatest degree of knowledge, but ultimately gives you the greatest flexibility and control over the rigs that you create.

Rigging Systems

Fortunately there are also many rigging systems, tools and plugins available that help speed up the skeleton creation process. They come with pre-built skeletons, that you can adapt to fit your model.

One system which we will be looking at in greater depth is Rapid Rig. This allows for the quick production of custom rigs. But there’s also many more. Human IK, The Setup Machine and Animation Rigging Toolkit, to name but a few.

Forward Kinematics and Inverse Kinematics

One of the challenges of creating custom rigs which is usually helped by using a rigging system is creating arms and legs that can be animated with FK and IK.

FK stands for Forward Kinematics, and this describes the movement of a chain of bones being animated by the rotation of each bone in turn, beginning at the parent bone and heading forward along the chain to the end. For example with an arm, in order for the arm to go from hanging by the side of a character to a waving pose, first the upper arm is rotated up, the forearm bent, the wrist rotated and fingers spread. To get to our next pose again we would start by rotating the upper arm, then the lower the arm then the wrist.

IK on the other hand stands for Inverse Kinematics, and this defines movement of a chain of bones that is determined by the position of the root and the end of the chain. So with the example of the arm again, with an IK chain in between the shoulder and the wrist, instead of rotating the upper arm, the hand is pulled into the desired position and the position of the elbow is created automatically.

This is especially helpful when animating characters standing or walking on a floor. You have an IK chain between the hip and the ankle. The foot can then be locked to the floor and the pelvis can be moved however you like and the foot stays where it should be. Trying to animate a leg with an FK chain would require meticulous rotation of the leg bones to get the feet to stay planted when the pelvis moves.

A good animator friendly character rig would therefore allow you to easily switch the limbs between FK and IK.

Creating Animation Controls

Part of creating your characters rig includes creating controls that the animator will use to be able to easily move the skeleton around. These are usually easily selectable objects or splines, visible outside of the characters mesh. Most rigging systems provide these controls or have options for them but with custom rigs you will have to make them yourself. Having obvious and easily selectable visual controls, speeds up the animation process considerably, it also allows the rigger to define which bones it's safe for the animator to animate with.

Skinning the Body

Once your skeleton is created the next stage of the rigging process is skinning, or binding the character mesh to the bones. This process defines which parts of your geometry will be moved by which bones. The vertices of the shin for example will all be controlled by the lower leg bone, and the thigh by the upper leg bone. But in order to get a believable deformation around the knee the vertices will need to be influenced by the lower and upper legs.

In future lessons we will cover techniques for skinning characters by painting weights. We will see how creating a test animation that articulates each of the rigs movements in turn allows you to check the deformation in every joint. We will also cover how to mirror, save and reload your skin settings.

Facial Rigging

One of the ways to rig a character's face is through the use of Blend Shapes. This is where you create a copy of your character mesh and move the vertices of the face around to get your desired expression. Many different expressions are created in this way and loaded as a blend shape of your character mesh. The animator can then animate the value of the blend in order to push the original mesh into the shape defined by the target, or a combination of different targets.

Another system of facial rigging is a bone based facial rig. Here bone ends are used to influence areas of the face and these are then moved either manually or through a series of controllers. Poses can be saved and recalled and therefore animated using set driven key in maya or reaction manager in 3ds max.

In order to make animator friendly controls for the face rig often a user interface is created. This is a series of sliders that when moved effect the blends or bones of the face. We will look at how to attach the sliders to the face rig using the connection editor and set driven key.