Glossary

• Armature: A set of bones connected (i.e. parented) together, a set of bone or kinematic chains. cf. Bone.
• Auto-rig: The process of rigging an armature (or standard objects), and the name of the tool doing this process. cf. Rig.
• Bake: The process of removing a dynamic, procedural, computed value and replacing it by a fixed value, so it cannot be edited anymore, with the goal of improving the performance.
• Blocking (animation): A usual first step of the digital animation process, where there is no interpolation between keyframes. This helps the animator focus on the poses and their shapes, and the timing between these keyframes.
• Bone: In digital, rigged animation, an object used to manipulate a part of a limb (or object) between two joints. The smallest rigid part of a limb. The pivot of the bone represents the actual joint it is attached to; the end of the bone is usually the joint of the next bone in the chain. The bone is the base unit of a Kinematic Chain.
• Breakdown (animation): In traditional animation, a secondary keyframe added between key poses, to add details to a movement, but which does not represent an important pose, which is in a fast part of the motion.
• Cel (animation): In traditional, hand-drawn animation, the cel is a single drawing, usually made on transparent material, a layer used to composite a frame of the animation. The name comes from Celluloid and should be written with a single L, it is not a cell.
• Chain: see Kinematic Chain.
• Constraint: An automation which automates the movement of a part of a rig. In After Effects, it’s usually the result of using expressions, although a simple parenting can be considered a constraint.
• Controller: An object used to animate a rigged character or object. The controller is the object receiving the keyframes, while the whole rig (bones and artwork) reacts thanks to constraints.
• Degrees of Freedom: In physics, the degrees of freedom of a mechanical system is the number of independent parameters that define its configuration or state. It is important in the analysis of systems of bodies in mechanical engineering, structural engineering, aerospace engineering, robotics, and other fields.
  The position of a single railcar (engine) moving along a track has one degree of freedom because the position of the car is defined by the distance along the track. A train of rigid cars connected by hinges to an engine still has only one degree of freedom because the positions of the cars behind the engine are constrained by the shape of the track.
  An automobile with highly stiff suspension can be considered to be a rigid body traveling on a plane (a flat, two-dimensional space). This body has three independent degrees of freedom consisting of two components of translation and one angle of rotation.
• Drag (animation principle): When a character starts to move and parts of it take a few frames to catch up. These parts can be inanimate objects like clothing or the antenna on a car, or parts of the body, such as arms or hair. On the human body, the torso is the core, with arms, legs, head and hair appendices that normally follow the torso’s movement. Body parts with much tissue, such as large stomachs and breasts, or the loose skin on a dog, are more prone to independent movement than bonier body parts.
• Exposure (animation): The length of time that an image stays on the screen. In digital animation, the exposure depends on the frame rate, and each image is displayed the duration of a frame. But in traditional animation (or tradigital animation), the animator can choose for each image how long it stays on screen (usually using a dope sheet or exposure sheet).
• Extrapolation: Method for estimating new data outside of a known data set. Applied to animation, it is a method to generate some motion after (or before) existing keyframes, based on the existing animation between the keyframes.
• FK (Forward Kinematics): The mathematical process of calculating the position of the end of a Kinematic Chain, such as a character’s armature, from the values of the joint parameters (usually rotation angles). For example if the object to be animated is an arm with the shoulder remaining at a fixed location, the location of the tip of the thumb would be calculated from the angles of the shoulder, elbow, wrist, thumb and knuckle joints. The reverse process, that computes the joint parameters that achieve a specified position of the end-effector, is known as Inverse Kinematics.
• Follow through (animation principle): Loosely tied parts of a body should continue moving after the character has stopped and the parts should keep moving beyond the point where the character stopped only to be subsequently “pulled back” towards the center of mass or exhibiting various degrees of oscillation damping. Frank Thomas and Ollie Johnston explained in their book on Disney animation The Illusion of life:
  • A character might have a coat or long ears, and these parts would keep moving once the figure had stopped moving. The ears, or coat, would “follow through” even after the main action had stopped.
  • The completion of an action - how the action “follows through” - is often more important than the action itself.
  • The “moving hold”. A character might come to a complete halt, but the fleshy parts might follow through the main action in order to convey weight and believability.
• IK (Inverse Kinematics): The mathematical process of calculating the variable joint parameters (usually rotation angles) needed to place the end of a Kinematic Chain, such as a character’s armature, in a given position and orientation relative to the start of the chain. Given joint parameters, the position and orientation of the chain’s end, e.g. the hand of the character, can typically be calculated directly using multiple applications of trigonometric formulas, a process known as Forward Kinematics. However, the reverse operation is, in general, much more challenging. This is also true when animating: using Inverse Kinematics means animating the position of the end of the chain (the hand), and positions are notoriously more difficult to animate than rotations, as the animator has to take care of two (or three) dimensions and the trajectory, whereas rotations are usually more constrained and have less degrees of freedom, thus easier to animate.
• Inheritance (parenting): To form a Kinematic Chain, objects are parented together: the start of the chain is the ancestor, and the end is the (grand…) child. Child objects inherits the transformation of their parent (and can have only one parent): all rotations, translations or scales of the parent is inherited by the child which moves with its parent.
• Interpolation: Method for estimating new data between known data in a set. Applied to animation, it is a method to generate the motion between the keyframes, to control the movement and velocity between two (or more) keyframes.
• Kinematic Chain: In mechanical engineering, a kinematic chain is an assembly of rigid bodies connected by joints to provide constrained motion that is the mathematical model for a mechanical system. Applied to armatures in animation, a Kinematic Chain, or Bone Chain is a set of bones parented together, from an end to a root. For example, an arm is a Kinematic Chain where the hand is parented to the forearm, which is parented to the arm, which is parented to the shoulder.
• Kinematics: Subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a field of study, is often referred to as the “geometry of motion” and is occasionally seen as a branch of mathematics.
• Locator: An object to expose the location (i.e. the absolute coordinates) of another object.
• Master: An object or a property controlling another property, leaving it without any degree of freedom, through a connection, such as one made with Duik’s connector. Don’t be offended by the use of this vocabulary, we’re talking about abstract properties and layers, there’s no harm.
• Meta-rig: A predefined Armature which already includes all needed information for it to be completely rigged once applied on a given character.
• Numerical Property: A property defined by a number. In After Effects, most properties are numerical properties, if we consider multi-dimensional properties to be numerical too. In this sense, colors are numerical properties (with four dimensions, Red, Green, Blue and Alpha), whereas the source text property of a text layer is not a numerical property, nor the path property of a mask or a shape.
• Overlap / Overlapping animation: Overlapping animation is one of the most important animation principles. It is the tendency for parts of the body to move at different rates, for example when the hand rotates after the forearm which itself rotates after the arm. Frank Thomas and Ollie Johnston explained in their book on Disney animation The Illusion of life:
  • Bodies in motion do not move all at once, rather different parts of a body may move at different speeds.
  • Loose flesh, such as a dog’s floppy jowls, might move at a slower speed than the more solid parts of the character. These parts might drag behind the main action.
• Parent: see Inheritance.
• Procedural Animation: Animation generated with parameters, such as the weight, the energy, some character traits, instead of keyframes. Such an animation is generated from the parameters without the need to add any keyframe to the properties. In Duik, the Walk Cycle, the Wiggle, the Swink, etc. are all procedural animations.
• Rig: The process of preparing a character (or any object) to be animated more easily than directly manipulating the artwork. More specifically the process of adding constraints and controllers to the armature controlling the artwork.
• Simulation: The computation of a movement using mathematical functions based on laws of physics, and some parameters or in continuity (extrapolation and interpolation) of an existing animation. This usually costs a lot in terms of performance, and may lead to poor animation; for example, a simulated bouncing ball may touch the ground between actual frames of the video, and in this case the contact would not be seen by the audience and the ball would seem to float instead of bouncing.
• Slave: An object or property connected to a Master property, which movement is completely constrained (automated) by this connection.
• Squash and Stretch (animation principle): This principle is based on observation that only stiff objects remain inert during motion,  while objects that are not stiff, although retaining overall volume, tend to change shape in an extent that depends on inertia and elasticity of the different parts of the moving object. For example, a ball would change shape, compressing (squash) as it hits the ground, then extending (stretch) as it bounces up again.
• InbeTweening: The process in animation that involves creating intermediate frames, called inbetweens, between two keyframes. The intended result is to create the illusion of movement by smoothly transitioning one image into another.
• Weight (parameter): A mathematical function to give some elements more influence on the result than other elements in the same set. The result of this application of a weight function is a weighted sum or weighted average. Usually given in percent in animation parameters to “weigh” some values.

Some of these definitions are sourced from Wikipedia.