SLOW-IN & SLOW-OUT : 2D Animation principle

17 December, 2008

SLOW-IN & SLOW-OUT : 2D Animation principle

Any action should happen in
such a way that it should look believable. To attain this, timing is applied by
placing inbetweens in different positions. In the case of a slow-in, most
inbetweens are near the last key drawings and deceleration is very evident.

In the case of slow-out, most
inbetweens are near the first key and gradual acceleration is evident. Animating
any action requires not only good drawing skills, but an understanding of
dynamics and kinetics from physics. As action starts, we have more drawings near
the starting pose, one or two in the middle, and more drawings near the next
pose. Fewer drawings make the action faster and more drawings make the action
slower. Slow-ins and slow-outs soften the action, making it more life-like. For
a gag action, we may omit some slow-out or slow-ins for shock appeal or the
surprise element. This will give more snap to the
scene.

Animation without Slow In and
Slow Out between the key frames appears abrupt, mechanical, and stiff. In the
animation of the ball, you can see that its path of action, the bounce, already
has some of this fluidity, but it’s too rubbery at the top of its bounce and the
interpolation needs to be adjusted to create a more real appearance.

Slow In and Slow Out also
relates to the weight or mass of an object. An oil tanker takes longer to get up
to full speed and longer to stop than a speedboat will. How quickly or slowly an
object acts and reacts to an external force tell you visually a lot about the
object. The way the ball moves in space and reacts to the ground when it
squashes is what tells you that it’s a ball. Slow in and out deals with the
spacing of the inbetween drawings between the extreme poses. Mathematically, the
term refers to second- and third-order continuity of motion.

In early animation, the action
was limited to mainly fast and slow moves, the spacing from one drawing to the
next fairly even. But when the poses of pose-to-pose animation became more
expressive, animators wanted the audience to see them. They found that by
grouping the inbetweens closer to each extreme, with only one fleeting drawing
halfway between, they could achieve a very spirited result, with the character
zipping from one attitude to another. “Slowing out” of one pose, then “slowing
in” to the next pose simply refers to the timing of the inbetweens.

The animator indicates the
placement of the inbetweens, the slow in or slow out, with a “timing chart”
drawn on the side of the drawing. This tells himself, or his assistant who will
be doing the inbetweens later, how he wanted the timing to be and where he
wanted the inbetween drawings placed.

Interpolation:

By default, interpolation of an
objects inbetween positions between extreme poses are evenly spaced, and graphed
as a straight line from one value to another. “Slowing out” (ease out) of one
pose, then “slowing in” to the next pose generate inbetweens clustered on either
end of the distance between the extremes, with less inbetweens toward the
center. When graphed, an ease in and out is graphed as a spline from one pose to
the next.

  

   

 

Speed:

Eases can be used to create
acceleration and deceleration. As a ball bounces, it accelerates and
decelerates. When you drop the ball, it gains speed as it approaches the ground.
After the impact on the ground it bounces and begins to loose speed as it
reaches the apex of its bounce. The graph of the balls acceleration would show
an increasing distance between the balls positions as it came closer to the
ground. Similarly, the graph of the balls bounce off the ground would indicate a
decreasing distance between the balls positions as it reaches the apex.

 

   

 

Tangency:

With this type of spline
interpolation, it is common to have spline overshooting at extreme poses when
there is a large change in value between them over a small number of frames.
Tangency handles can be used to manipulate the tension of the spline, reducing
the overshooting and achieving the desired inbetween.

Timing chart for ball
bounce:

In most 3D keyframe computer
animation systems, the inbetweening is done automatically using spline
interpolation. Slow in and slow out is achieved by adjusting the tension,
direction or bias, and continuity of the splines. This works well to give the
affect of slow in and out, but a graphical representation of the spline is
required to see the effect of tension, direction, and continuity have on its
shape.

With this type of spline
interpolation, a common problem is the spline overshooting at extremes when
there is a large change in value between them, especially over a small number of
frames. This also happens when the direction control of an extreme is adjusted.
The danger is that, depending on the variable the spline controls (translate,
rotate, or scale), the value will shoot in the wrong direction just before (or
just after) the large change in value. Sometimes this effect works out well when
it occurs just before a large movement it may appear to be anticipation.
However, more often than not, it gives an undesirable effect.

In Luxo Jr., there was an
example of this problem of overshooting splines, Jr.’s base was very heavy and
when he hopped, we wanted the base to start stationary, then pop up in the air
from the momentum of his jump, arc over, then land with a thud, suddenly
stationary again. For the up translation, there were three key frames, the two
stationary positions and the highest point of his jump. The spline software
forced continuity, so that his base would move down under the surface of the
floor just before and after the jump. The solution was to put two new extremes,
equal to the two stationary extremes, on the frames just before and just after
the extremes. This “locked” down the spline, so that the up translation stayed
the same value, popped up in the air, landed and then stayed the same value
again. This gave the desired feeling of weight to his little base.

The same solution can be
achieved by breaking the spline using its continuity parameter at the two
stationary extremes. This solution requires a graphical display of the spline so
that the correct shape can be achieved.


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