The Principles of Animation, Arcs
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Expressive Motion
The visual path of action from one extreme to another is always described by an arc. In nature, arcs are the most economical routes by which a form can move from one position to another.[1]

Such arcs are used extensively in animation, since they create motion that is more expressive and less stiff than action along a straight path.

Relevance in CGI
In Computer Animation, motion is usually represented in a timeline view using splines(arcs). The arcs represent the values of an objects parameters at a specific moment in time. The method used for calculating interpolated keyframe values determines the characteristic of the arc(motion).

A linear interpolation creates motion that is rather dull and stiff. While a spline interpolation creates motion that is more expressive.

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