In cel animation, lead animators often create keyframes for the significant poses a character assumes during an action or sequence.

Assistant animators then create drawings to fill in the steps in-between keyframe poses to complete the animation.

For expressive, lifelike motion to result, both the lead animators and the in-betweeners need to have an awareness of of how things move - weight, inertia, momentum, etc. - in addition to how characters may act under various emotional states.

Such awareness informs the ``rules'' governing the creation of inbetween frames.

In 3D computer animation, keyframes also represent extremes or significant positions that are critical to the motion or deformation of objects in the scene.

In 2D computer animation, keyframes represent significant positions or parameter values for compositing or image processing functions (e.g. transparency of an overlay, pan positions across an image, distortion or morphing values, etc.).

In-betweens are created by numerical interpolation between key parameters.

Numerical algorithms form the ``rules'' by which the in-betweens are created.

The awareness of the animator informs the location of keyframes and the choice of interpolation algorithms in order to acheive lifelike or expressive motion.

The same expressive principals that apply in cel animation can be successfully applied in computer animation (see Lasseter/SIGGRAPH 87 article).

The numerical values for animated parameters change over time and can be described by a curve which plots changes in the parameter's value over time.

Sometimes these are called to as motion channels.

Either keyframes or the interpolation method (motion curves) can be altered to adjust the final motion in a scene.

Since the parameters controlling motion are numerical values that change over time, arithmetic or algebraic expressions can be used to define the successive frame values for motion channels.

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This file was last modified on September 18, 2000.