Arcs
Part of: The 12 Principles of Animation
Arcs is one of the 12 principles of animation: the observation that nearly all natural motion follows a curved path rather than a straight line, and that animation respecting this reads as alive while animation that ignores it reads as mechanical.
Watch any natural motion, a head turning, a hand reaching, a pendulum swinging, a leaf falling, and you will see a curved path. The motion is rarely on a straight line because joints are anchors that produce arc-shaped trajectories. Even in technical motion, the path is rarely linear.
In hand-drawn animation, the discipline of arcs is taught early. Animators are encouraged to draw the arc of a moving element on a separate layer to check the trajectory before committing. A head that moves on a straight line from one pose to another reads wrong; the same head moving on an arc reads right.
In 3D, arcs come naturally to some elements (anything driven by a rig joint) and have to be designed deliberately for others (free-floating elements like a flying object or a particle). 3D animators routinely turn on motion-path display in their software to check that key elements travel on arcs.
On work like GROW for Sky, the camera moves themselves are designed on arcs to give the title sequence a sense of organic movement. Linear camera moves would have given a robotic feel; arced moves carry a botanical, growing energy that matches the show's subject matter.
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Sources
Academic papers, recognised industry standards, and canonical industry texts that back up claims in this entry.
- The Illusion of Life: Disney Animation. Thomas, F.; Johnston, O., Walt Disney Productions, 1981Supports: 12 principles arcs definition
- The Animator's Survival Kit. Williams, R., Faber & Faber, 2001Supports: arcs natural curved paths
- Timing for Animation. Whitaker, H.; Halas, J., Focal Press, 1981Supports: arcs hand-drawn technique
- Computer Animation: Algorithms and Techniques. Parent, R., Morgan Kaufmann, 2002Supports: arcs technical motion paths
Frequently asked questions
Are there any motions that should be linear?
Very few. Mechanical motions (a piston, a robot arm) sometimes use linear motion deliberately, to read as mechanical rather than organic. Even then, secondary elements (cables, attached parts) often arc. The default for character and natural motion is to arc; linear motion is the deliberate exception.
How do I check arcs in 3D?
Most 3D animation software offers a motion-path display that visualises the trajectory of any chosen element across the timeline. Animators routinely turn this on for the head and key joints during polish, to confirm the motion travels on a clean arc rather than wobbling between keys.
Does this apply to motion graphics?
Yes, especially for transitional motion. A graphic element that moves between two positions on a straight line reads as efficient but cold. The same move on a slight arc reads as organic and intentional. The rule is universal across animation disciplines, including motion design.