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Section 2


The concept of Arcs is one of the Basic Principles of Animation (more about this in the chapter on Squash & Stretch Theory). This principle says that most animations follow a curved motion path, without sharp corners.

Movie 7.2 Arcs vs Straight Lines

Arcs and Energy

Why do things move along arcs in the real world? Evolution, and any creature that’s part of it, is under constant pressure to be efficient. Only the fittest survive, and the fittest will do their best not to waste energy. But if you want to move from point A to point B, the shortest path is always a straight line. So, shouldn’t straight lines be very popular and better than any curved detour?

Well that’s definitely the case for simple moves from A to B. But let’s take a look at a more complex move between three points, A, B and C:

The dashed path is certainly the shortest, but it’s not the most energy efficient because it would take a lot of energy to change direction so abruptly at point B. If you turn at a crossroads, you have to almost stop and then speed up again. Compared to that, the wide curves of highways are much more energy-efficient. Just like for speed changes, direction changes also usually take time. There’s no car that can go from 0 to 80 miles per hour in 0 seconds, and there’s also no car that can make an abrupt 90 degree turn without braking. Sharp corners in a motion path mean abrupt direction changes, but in practice a force needs to act on the moving object for a while to introduce the direction change gradually. The only exceptions are things like bouncing balls. During the bounce, the direction (and the speed) change instantly, since – due to its elasticity – the ball is able to preserve its momentum and use it for a new move in a different direction. So, bounces have sharp corners, but other motion paths do not.

Arcs in Character Animation

When you animate characters, you usually work pose to pose, creating a move with a handful of keyframes and then letting After Effects interpolate between them. That’s a good approach, but if your character feels a bit clumsy, check that the motion paths of these keyframes are all nicely curved. This will usually make the moves look more natural.

Movie 7.3 Carnivorous Plant - Arcs vs Straight Lines


Another good example are walk cycles. When moving the foot up and down, make sure you don’t create sharp triangles, but curved paths.

Movie 7.4 Arcs in a Walk Cycle

By the way, both the carnivorous plant and the wolf were rigged using our extension iExpressions. The plant uses the Inverse Kinematics iExpressions, and the wolf also uses the Automatic Walk Cycle iExpression. The visualization of the motion paths has been done with the Trail iExpression. If you want to exercise your character rigging and are looking for a good character to play with, the wolf vector graphic is available for free at Pixabay.

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