PATH OF ACTION: 2D Animation principle

17 December, 2008

PATH OF ACTION: Animation principle

The path along which the action
follows is called Path of Action.



Animate a ball on various
paths of action:

The idea is to determine the
timing and spacing of inbetweens and where to opt for a Slow-in or Slow-out.
Path of action clearly defines the physical forces involved in the action.
Remember path of action is an imaginary line that makes animating easier. While
animating an object, an animator creates an illusion of dynamics; therefore he
must exaggerate the operation forces on the object. Since we cannot see the
force, he has to exaggerate its impact on the character or object. When an
object moves along its path of action, because of its weight it tends to stretch
towards the direction of gravity. Weight is the amount of force that pulls the
object towards the direction of gravity.

When the path is interrupted,
because of an additional force on the object, the force continues through the
distribution of volume. Always maintain the volume of the object or character,
no matter how much stretch or squash you may apply.

Objects without life
(inanimate) follow straightforward physics and dynamics, as they cannot respond
to the force applied on them. Free will is a concept you cannot apply to
inanimate object. But living characters can respond and react to the external
force that is applied on them by controlling the body. So the impact of dynamics
can be altered according to the response that comes from characters like human
and animals for example, while slipping off the stairs a human can respond and
control him from falling by applying some amount of force.

Bouncing Ball:

The bouncing ball includes many
aspects of animation that an animator uses everyday in every scene on which he
works. Animation principles such as the path of action, arcs, momentum, timing,
key drawings, inbetween drawings, weight, speed, and the substance of an object
are included in this simple exercise.




Step: 1

Let’s begin by drawing a path of action from left
to right. This is the path the ball will follow. Remember to stay “ruff” and
“loose” with your drawings. As is the case with most animation, our path of
action is made up of ARCS.

TIP: Most
objects, characters, etc. move in ARCS and NOT in straight lines.



Step: 2

Next we will plan the timing of
the ball along our path of action. Draw tick marks. This will help us figure out
the positions (on the arc) of the ball and the timing of the bounce. Tick marks
at the beginning (the top of the arc) are close together; as the ball heads
down, the tick marks become further and further apart.



The action at the beginning of
an arc is called a “slow-in”. Once the ball hits – it looses a little momentum
and then slows down (slow-out) as it approaches the top of the next arc.

TIP: Think of a ball rolling down a hill. It gains speed.

Step:  3


We “ruff” in our key drawings. Key drawings (or
key poses) are at the beginning and at the end, and wherever the character,
object, etc. changes direction. Our key drawings are #1, #7, and #13.

NOTE: Drawing #7 is where the ball makes contact, or impacts
with the ground. #7 is the “squash” drawing, i.e., where we show the illusion of
weight or impact of the object (squash).

TIP: The
illusion of weight is very important. To maintain the integrity of the ball, be
sure to maintain the same volume in drawing #7 (even though it is “squashed”) as
you have in drawings #1 and 13. Also, the faster the object, character, etc.
moves, the more “squash” you have when it hits.

Step: 4



Next we add the breakdown drawings (#6 and #8).
“Breakdowns” are really inbetween drawings which help describe the action. These
drawings (known as “inbetweens” or “tweens”) are the drawings between the pose
or key drawings. In the hierarchy of animation, first you do the key drawings
(or keys), next the breakdowns, and finally any further inbetween drawings
needed to smooth out the action. The breakdowns for the bouncing ball show the
illusion of speed. These are the “stretch” drawings. The stretch (speed) and
squash (weight) drawings are relative to each other. Generally, if you have a
lot of squash, you have a lot of stretch, too.

TIP: Notice that the ball goes from stretch directly to squash and back to stretch  
(# 6, #7 and #8) with no “transition drawings”. This gives snap to the ball’s

Step: 5



Finally, we draw in the rest of the inbetweens -
#2 to #5; and #9 to #12. Notice that as the ball falls, the shape “transitions”
from a round ball to more and more of an elliptical or stretched shape. As the
ball bounces up, it transitions from the stretched shape back to the round
shape. There you have it the plan for a bouncing ball.

Key Points:

  • Start with a path of action.

  • Plan your Timing with tick marks on the Path of action.

  • Ruff in the key drawings.

  • Use Squashed drawings to create the illusion of weight, contact.

  • Use Stretched drawings to create the illusion of speed.

  • Do your key drawings (or key poses) first, then do the breakdowns and
    finally the inbetweens.

  • Slow-in (transition drawings) builds momentum.

  • Slow-out (transition drawings) reduces momentum.


  • Most objects, characters, etc. move in Arcs and not in straight lines.

  • If something moves slowly, the tick marks are close together, if it moves
    quickly, the marks are further apart.

  • Whether you squash or stretch, a drawing always keeps the volumes
    consistent from shape to shape. Always Transition the round drawing into the
    stretched drawing.

  • No transition from stretch to squash to stretch.

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