1. Technology

What is Rigging?

Preparing a 3D Model For Animation

By

When a modeler finishes building a character, it's a static 3D mesh, almost like a marble sculpture. (And if you've ever tried posing and animating a marble sculpture, you probably know that it's darn near impossible).

Before a 3D character model can be handed over to the team of animators, it must be bound to a system of joints and control handles so that the animators can pose the model. This process is typically completed by artists known as character technical directors (TDs), or riggers.

Character TDs work closely with animators to make sure any specific technical issues are accounted for, but their primary duty is to take a static 3D mesh and make it ready for animation—a process called rigging.

Rigging

A character rig is essentially a digital skeleton bound to the 3D mesh. Like a real skeleton, a rig is made up of joints and bones, each of which act as a "handle" that animators can use to bend the character into a desired pose.

A character rig can range from simple and elegant to staggeringly complex. A basic setup for simple posing can be built in a few hours, while a fully articulated rig for a feature film might require days or weeks before the character is ready for Pixar level animation.

We plan on working up a basic rigging tutorial sometime next month, but in the meantime, it's a good idea to at least familiarize yourself with some of the major concepts:

Placing the Skeleton: Placement of a skeleton is perhaps the easiest part of the rigging process. For the most part, joints should be placed exactly where they would be in a real world skeleton, with one or two exceptions.

  • Joint Hierarchy: In order for a rig to work properly, the bones and joints must follow a logical hierarchy. When setting up a character's skeleton, the first joint you place is called the root joint. Every subsequent joint will be connected to the root either directly, or indirectly through another joint.

Forward Kinematics: Forward kinematics (FK) is one of two basic ways to calculate the joint movement of a fully rigged character. When using FK rigging, a any given joint can only affect parts of the skeleton that fall below it on the joint hierarchy.

  • For example, rotating a character's shoulder changes the position of the elbow, wrist, and hand. When animating with forward kinematics, the artist typically needs to set the rotation and position of each joint individually—to achieve a desired pose the animator would work through the joint hierarchy sequentially: root → spine → shoulder → elbow → etc. The final position of a terminating joint (like a knuckle) is calculated as a function of the joint angles of every joint above it in the hierarchy.

Inverse Kinematics: IK rigging is the reverse process from forward kinematics, and is often used an efficient solution for rigging a character's arms and legs. With an IK rig, the terminating joint is directly placed by the animator, while the joints above it on the hierarchy are automatically interpolated by the software.

IK is most appropriate when the animation calls for a terminating joint to be placed very precisely&$151;a character climbing a ladder is a good example. Because the character's hands and feet can be placed directly on the ladder's rungs rather than the animator having to adjust their position joint-by-joint, an IK rig would make the animation process far more efficient. One drawback is that because IK animation uses software interpolation, there's often quite a bit of cleanup work that must be done in order to finalize the shot.

Degrees of freedom/Constraints: When rigging, keep in mind that joints like the elbows and knees limited to a single degree of freedom in the real world, meaning they can only bend along one axis. Likewise, a human neck cannot rotate a full 360 degrees. To help prevent unrealistic animation, it's a good idea to set up joint constraints when you're building your rig. We'll address this further in a tutorial.

Squash and Stretch: Another consideration that must be made is whether the rig will support squash and stretch, or whether the character will be constrained to realistic motion. Squash and stretch is an important principle in exaggerated cartoon animation, but typically doesn't look right in realistic film/VFX work. If you want your rig to maintain realistic proportions, it's important to set a constraint to lock the position of each joint in relation to the rest of the rig.

Facial Rigging: A character's facial rig is usually altogether separate from the main motion controls. It's inefficient and incredibly difficult to create a satisfactory facial rig using a traditional joint/bone structure, so morph targets (or blend shapes) are usually seen as a more effective solution. Facial rigging is a topic in and of itself, so be on the lookout for an article exploring the subject in depth.

  1. About.com
  2. Technology
  3. 3D
  4. Creating 3D
  5. What is Rigging? - Preparing a 3D Model For Animation

©2014 About.com. All rights reserved.