Prepare a 3D Model for Animation With Rigging

3D models need to be rigged before they can be posed

A 3D model being rigged

DajaGellerova/Pixabay 

In computer graphics, when a modeler finishes building a character, it's a static 3D mesh, almost like a marble sculpture. Before it's handed over to the team of animators, it must be bound to a system of joints and control handles so 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.

What Is 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 acting as a "handle" animators use to bend the character into the 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.

Placing the Skeleton

Skeleton placement 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, 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 the 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 it's often used as 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 placing a terminating joint very precisely. A character climbing a ladder is a good example. Because the character's hands and feet are placed directly on the ladder's rungs, rather than the animator adjusting their position joint-by-joint, an IK rig makes the animation process far more efficient.

One drawback is that because IK animation uses software interpolation, quite a bit of cleanup work 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 are 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.

Squash and Stretch

Another thing to consider 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.