Animation I - Principles

Overview

Most natural action tends to follow an arched trajectory, and animation should adhere to this principle by following implied "arcs" for greater realism. This can apply to a limb moving by rotating a joint, or a thrown object moving along a parabolic trajectory. The exception is mechanical movement, which typically moves in straight lines.

As an object's speed or momentum increases, arcs tend to flatten out in moving ahead and broaden in turns. In baseball, a fastball would tend to move in a straighter line than other pitches; while a figure skater moving at top speed would be unable to turn as sharply as a slower skater, and would need to cover more ground to complete the turn.

An object in motion that moves out of its natural arc for no apparent reason will appear erratic rather than fluid. Therefore when animating (for example) a pointing finger, the animator should be certain that in all drawings in between the two extreme poses, the fingertip follows a logical arc from one extreme to the next. Traditional animators tend to draw the arc in lightly on the paper for reference, to be erased later.

Arcs in Motion

Arcs are everywhere when it comes to motion. Most of this is due to the way the forces of nature work. Newtons first law sets our framework here nicely.

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

Wait?!?! Didn't we just learn from the slow in and out principle that things don't ever move at a constant velocity very much in nature?

The answer is yes and these two statements are not contradictory.

The root of this is that objects themselves wish to remain constant unless forces are exerted on them. The reason the motion isn't constant is because the forces themselves are not constant. The gravity from one object (lets say the Earth) is always pulling on another object (lets say a tennis ball). The ball sitting on the floor is being pulled towards the center of the earth constantly. The ground is pushing up on the ball to keep it from moving toward the center so the ball appears to remain still. However, if I apply a force on the ball (pick it up), raise it to a height and release it the ball still has the force of gravity trying to move it and now nothing (except wind resistance) is in the way from gravity moving the ball. Gravity accelerates objects it affects so those objects speed up the longer they are under that force. The equation for that happens to be F=Ma^2 for those interested. Basically it is a parabolic curve and not a constant one. Therefore if I throw a ball it will be pulled toward the center of the Earth at a gradually higher velocity (speed) until another force changes its motion like a wall or the floor or tennis racket.

There is a lot more science that goes into this which I find fascinating, but this is a visual art class so we won't go much more into that here. The important take away here is that objects naturally move in arcs all around us as forces are exerted on them. We of course can move things around on our own in very erratic patterns that do not represent arcs, but that requires more energy. Energy exertion has a cost and we are inherently lazy so when we don't think about (or want to be really efficient) it we will definitely move in smooth arcs and work with the natural forces around us and not against them.

Let's see some visual examples