Without adding rigging, a three dimensional remains a static 3D mesh, which cannot be posed, etc. (which is obviously required for animating). The model thus must be connected to a “digital skeleton” before their joints can be deformed and changed in the manner needed to create an animation. The skeleton of a 3D model, much like an organic one, is bound with a series of joints and bones – acting as the points of articulation for control of the model (Pluralsight, 2014), which can be manipulated into a desired pose.
To create a successful rig, the hierarchy of the skeleton must follow a logical order, with the bones and joints tapering off the first joint – also known as the root joint – similar to an actual skeleton. From the root joint, subsequent joints either join directly or are bound through another joint (Slick, 2016). For example, the forearm is lower in the hierarchy than the upper arm, and the wrist is lower than that, like in the figure shown below.
(Chopine, 2012, p. 85)
Before the movement can be changed, the bones of the digital skeleton (or rig) must be bound to the 3D mesh itself, in a process called skinning, or binding (Pluralsight, 2014). This allows the joints on the mesh to follow the skeleton joints – and without this, the movement of the rig will have no influence on the model.
Joint movement can be calculated through two methods – forward kinematics and inverse kinematics. When animating with inverse kinematics means the child node, when moved, influences the movement of the parent joints, automatically interpolated by the software (Slick, 2016). The position of the rest of the hierarchy is calculated automatically. It is typically at this point that animators will have to use forward kinematics to tweak the pose for the final shot.
Animating with forward kinematics, at its most basic, means that the character rig will follow the hierarchal chain (Pluralsight, 2014). Despite having more control over the chain, the problem occurs when animators need to position each joint in the chain independently. For example, when moving a foot, instead of having the rest of the limb follow, the knee and ankle joints would need to be moved individually, taking more time than if one were to use inverse kinematics (Pitzel, 2011). It is comparable to animating a stop-motion armature. This process can lead to difficulties when animating a walk cycle, such as keeping feet from sliding on the ground, sinking into it, floating, or even disconnecting entirely (Ami, 2012, p. 92)
*Using FK, the object at the bottom of the hierarchy stays still whilst the objects above it are moved.
Once again drawing a parallel to a real skeleton, a rigged skeleton has a degree of constraints – joint constraints, set up whilst building the rig, can help add realism to a model. These limit the object’s radius of rotation, the amount of axis it can or can’t rotate and the status of the constraint (whether it is a parent or child). This step must be completed before control curves can be used on the rig (Ami, 2012, p. 91). Control curves (typically simple NURBS curves) are placed outside the character so that the curve can be selected to position the character, rather than individual joints (Pluralsight, 2014).
The “squash and stretch” feature allows the rig to support squashing and stretching of the model, which may be allowed or not depending on the intended realism of the animation. The squash and stretch features are typically used in cartoon animation (Slick, 2016).
Due to such a complex nature, the faces of the models are rigged using controls separate to the main motion controls, in a specialised process called facial rigging, mainly because the typical joint controls aren’t well suited. Facial rigging typically required deformers and blend shapes (or morph targets) to create the finished product. Blend shapes allow the shape of one object to change into the shape of another object, typically used for setting up facial animations, whilst deformers can move large sections of vertices on the model, often used for cheeks or eyebrows (Pluralsight, 2014).
Once the rigging is complete, the artist can move onto animating!
Chopine, A. (2012) 3D Art Essentials: The Fundamentals of 3D Modeling, Texturing, and Animation. Focal Press.
Pitzel, S., (2011) Character Animation: Skeletons and Inverse Kinematics. Retrieved from https://software.intel.com/en-us/articles/character-animation-skeletons-and-inverse-kinematics
Pluralsight (2014) Key 3D Rigging Terms to Get You Moving, Retrieved from https://www.pluralsight.com/blog/film-games/key-rigging-terms-get-moving
Slick, J., (2016) What is Rigging? Preparing a 3D Model for Animation. Retrieved from https://www.lifewire.com/what-is-rigging-2095