All natural movement follows a curved trajectory, rather than a straight line. Even if you have a perfectly timed and drawn set of key poses for a hand, head or foot moving, it will feel very robotic if the in-betweens are drawn along a straight line.
This natural path is called a motion arc and it’s important to take the arcs of every part of your animation into account when creating new breakdown poses. The breakdowns are not just connecting two extreme poses but actually showing how they travel from one pose to the next. Adding just one breakdown pose on the correct curved trajectory can instantly improve a movement that feels stiff between two distant key poses.
Any time a part of the body is swinging, turning or travelling, it’s likely to follow a curved arc. Think about how a shoulder is connected to an arm – an arm moving across the body is going to swing on an arc from that shoulder joint, not move in a straight line across the body.
Other common actions that follow arcs:
- turning the head
- lifting the foot up to the next step
- throwing an object
- a ball bouncing on the floor, because the ball follows an arc up then slows down and travels back down faster as gravity pulls it towards the floor.
- bending over then standing up straight again
Remember that it’s not only the drawings along a curved trajectory that create a good movement but also the spacing of the drawings. Arcs show the direction of the travel whilst varying the spacing will determine if the movement is accelerating or decelerating.
Evenly spaced drawings can make an arc look mechanical or you could use great spacing in between drawings that travel along a very erratic arc, though neither of these would look natural.
The image below shows how to draw in-betweens along an arc when a character bends over then stands up again. This will make it look much more natural compared to a straight trajectory:
One simple way to check your animation follows a good arc is to look at your drawing with onion skins over a few adjacent frames. Choose one spot on your drawing and look at how it moves in relation to itself in the adjacent frames. If it follows a consistent and natural looking arc throughout, then your animation will too. If there’s one frame that stands out, then that’s probably the frame you need to fix to smooth over the movement. Remember, even if there’s a frame that seems to jump in the animation, you won’t necessarily need to add an additional frame, it could just need a tiny adjustment to fit the path of the movement.
To test this out for yourself, draw two sets of a circle swinging like a pendulum. Start with a few key poses, then draw the in-betweens on the curve that it will follow for the first set. Draw the exact same animation again, however draw your in-betweens on a straight line between each key pose. Compare the two animations, one should look much more natural and fluid.
Another way to check arcs and the spacing within them is to look at your animation using just one point of reference. As you can see, even when the animation looks good when watching the whole drawings, you can still tell if the path is incorrect just by watching a single point.