Before any kind of advanced animation can happen, a rig must be built. A rig is the structure of joints and control objects that form the inner workings of an animated model. It allows the model to be animated quickly and precisely by the animator. Analogous to the strings on a puppet, where the puppet is the 3D model and the strings are the rig, the animator uses the strings to make the puppet move. A character rig is a virtual skeleton that is built inside of the model, it follows the anatomy of the character and is generally complete with a facial rig. The more complex the animation must be and the more freedom the artist requires, the more complex the rig will become. Rigging can be one of the more inglorious but essential steps in the process of creating an animation.
Maya’s rigging controls can be accessed through the Animation menu set and the Animation shelf. Everything starts with the joint tool. The above mentioned skeleton is made up of joints and the hierarchy of these joints is what will determine how they move in relation to each other. The word joint and it’s representation in the viewport can be misleading to novice riggers, this is because for actual control purposes, the joints (i.e: hip, knee, ankle, etc) is what matters, but it’s representation in the viewport leads to believe that what is being built are the bones (i.e: thigh, shin, metatarsals, etc). With a typical bipedal character, the whole joint structure is rooted at the hips. Both the legs and the spine are parented to the hips, and the arms and head are parented to the spine. The arms and legs of a character are almost always rigged using a technique called Inverse Kinematics (IK). IK allows for a hierarchy of joints to be animated based on the position of the last joint in the chain. This technique is in opposition to Forward Kinematics (FK), which is the animation of individual joints starting from the top of the hierarchy downwards (shoulder > elbow > wrist) to achieve a certain pose. In summary, with IK, the desired pose drives the joint position and rotation. With FK the joints have to be animated manually to achieve a certain pose. FK doesn’t require any special setup within Maya, the joint rotations are positions are simply animated directly. IK on the other hand, requires the rigger artist to connect the root of a joint chain with it’s end using the IK Handle tool. This will automatically create an IK Handle at the end of the joint chain which can be linked to a control object. Control objects are auxiliary objects to which all the chain roots and IK handles are constrained too. Typically curves are chosen to be control objects because they do not render and can be customized for easy identification.
As mentioned above, IK is typically used for a character’s legs and arms. This is because it allows for precise control over where the character puts his hands and feet, something crucial since most characters will interact with their environment. With IK arms, an animator is able to put a character’s hands on a table or wall and have them stay there while the rest of the body is free to be animated. This would be a nightmare to accomplish with FK because the position of the hands would always be a slave to the transformations of the joint chain above.
There are a few things to keep in mind while rigging a character. The transformations of the joints, handles and control objects are extremely important for a trouble free animation. All the pivots should be oriented and centered correctly to avoid unwanted offsets and to keep the IK working properly. Also, transformations should be freezed in the proper base pose position before handing the rig to an animator. Furthermore a rig needs to be as animator-proof as possible, this means that it’s the riggers responsibility to lock certain transforms that should not be animatable and to limit degrees of freedom on certain constraints. For example, a realistic elbow should only ever be allowed to rotate on one axis and no more than 180º, because this is how a real elbow functions. Ideally, a rig should remain stable no matter what the animator tries to do with it.
When the body of a character is rigged, it’s time to create a facial rig. As the name implies, a facial rig drives the animation of a characters face. There are two main techniques that are prevalent in facial rigging: joints and blend shapes. A facial joint structure, similar to the body rig, makes use of joints to deform the geometry. Rigging a face with joints can be a very intricate and complex process because a single facial expression involves many subtle deformations which have to be accounted for by setting up the corresponding joints and control objects. Blend shapes provide a simpler approach. As the name implies, blend shapes is a tool that allows for the blending between two or more shapes. Maya is able to interpolate the points between two separate and differently shaped but otherwise identical models. A facial expression can be seen as a single pose or shape (smiling, anger, yawn, etc), these shapes are created by a modeler prior to the rigging process. Several versions of the character’s head are created, each locked into one of these shapes. Maya’s blend shape tool can then interpolate between them smoothly, making the process of animating a face transitioning from a smile to an angry frown as simple as playing with a slider. Using only this technique, animators are limited to the shapes created by the modelers. Because of this, advanced rigs make use of a combination of joint facial rigs and blend shapes to give animators as much freedom as possible and get the most out of a character. At the end of the rigging process, the 3d model must be skinned to the joints using Maya’s Bind skin tool. This step allows the rig to actually deform the mesh.
Maya offers a number of advanced features such as a muscle system, which simulates the movement of muscles beneath a character’s skin as they flex and twist. Collisions can be worked into a rig, so that a character can dynamically interact with it’s environment. Squash and stretch for cartoon characters can be incorporated into the joints using expressions. Anything is possible. Rigging is a deep and technical art, it’s the veritable behind-the-scenes king of 3d animation.