Tricks for Cartoon Snappy Motion [Archive]

Aatur

14-10-2005, 02:09 AM

Tricks for Cartoon Snappy Motion An Energy Crisis ? 2003 
Keith Lango

Enron Who? A common problem with many attempts at snappy animation is the old "hit &
stick" jerky feel to the motion. For many beginning animators, they think that
if they just simply move an object faster (over fewer frames) they will achieve
snappy motion. But I'm here to show you that faster is not snappier. In fact,
I'll prove to you that the snappier motion actually takes longer to perform
correctly. The trick is in managing the energy release of the move. Let's have a
look.....

<hr>
First Look: Two Lonely Keyframes....

<a href="http://www.keithlango.com/tutorials/old/toonSnap/linear01.avi" target="_blank">
<img src="http://www.keithlango.com/tutorials/old/toonSnap/anim01.jpg" border="0" height="143" width="200" border="0"></a>

 In this animation, there's 2 keyframes. A start key and an end key 4 frames
later. No breakdowns, nuthin. The upper arm, lower arm and hand all start on the
same frame and end on the same frame. There's no weighting to the motion, the
arm moves at a linear constant rate from beginning to end. Basically...it's
ugly. And even though it's only 4 frames long, the move feels slow and lifeless.
Obviously, just using fewer frames to make a move is not the answer to
snappiness in a cartoony way.  There's just no texture to the energy as
it's being released. Here's a graph of the energy as it's being released by the
various parts of the arm. Energy levels are on the vertical, while the time
elapsed is on the horizontal. 

<img src="http://www.keithlango.com/tutorials/old/toonSnap/energy_01.jpg" border="0" height="200" width="400" border="0">

<hr>
Second Look: Better 

<a href="http://www.keithlango.com/tutorials/old/toonSnap/linear02.avi" target="_blank"><img src="http://www.keithlango.com/tutorials/old/toonSnap/anim02.jpg" border="0" height="143" width="200" border="0"></a>

 In this animation I've added a few things. I've added some breakdown keys to
weight the motion and play with the energy release speed. I also pushed the
poseB pose to be an extreme. this helps sell the notion that more energy has
been released than in example 1. I've also added a settle back keyframe roughly
5 frames after the arm hit's it's extreme pose. This is a recoiling move, a way
of dissipating any left over energy from the arm's motion. For a coser
examination,  I'll break it down by body part:

<blockquote>
UpperArm: the upper arm move is still just 4 frames from poseA to poseB. But
there's a breakdown key that weighs the ease in to the slower end of the
spectrum. Meaning this: the upper arm's rotation at what would normally be frame
2 has been shifted to frame 3. Let's say for the sake of argument that the upper
arm moves 40 degrees over 4 frames (it moves more than that here). That means
that at frame 0 (poseA) it's 0 degrees. At frame 1 it's 10 degrees. At frame 2
it's 20 degrees. Frame 3, 30 degrees and frame 4 (poseB) it would be the full 40
degrees. Now, let's say that instead of being at 20 dgerees at frame 2, it's 20
degrees at frame 3. That means it's a slow in (going from 0-20 degrees over 3
frames for a ratio of roughly 7 degrees per frame) and a fast out (going from
20-40 degrees over 1 frame, or a ratio of 20 degrees per frame). The immediate
effect is the move feels snappier. (comparative ratio table here) The upper arm
settleback from the extreme is 5 frames. The settle back is weighted to be fast
in/slow out. Meaning that the upper arm has recoiled about 70% of the way by the
second of those 5 frames. It recovers the last 30% over the last 3 frames for a
nice slow ease out. 
LowerArm: this begins it's move at the same frame as the upper arm. But I've
offset it so that it hits it's extreme poseB one frame later than the upper arm.
This offset helps give a sense of the arm unfolding successively. The breakdown
of the lower arm is also offset by a frame. In effect, the lower arm has an even
slower in and a faster out to the move than even the upper arm. So by stretching
the move to 5 frames, but managing my breakdown, I've made it snappier than the
original 4 frames. Longer in time, but snappier in effect. The settle back from
the extreme is the same 5 frames as the upper arm, with the same fast in/slow
out weighting.
Hand: much the same as the upper arm, the hand has been offset even a frame
later than the lower arm (2 frames later than the upper arm). The break down is
also shifted. So now the hand has a 6 frame move, with it's breakdown at frame 4
of it's move. The hand has the slowest ease in and the fastest out. It's very
snappy. So it's 6 frames long, but by placing the breakdown at the right spot,
it feels snappier than the original 4 frame move. The settleback from the
extreme for the hand is the same fast/slow 5 frames as the upper & lower arms.
It just happens to come to a stop 2 frames later than the upper arm because of
the offsets. 
</blockquote>

So overall, this move has been made longer by 7 frames (2 for the offset, 5
for the settle back from the extreme), yet it feels snappier and has more life
than the simple 4 frame move in example 1. By the way, the fCurve tangents for
all the arm parts in this clip are still linear. Here's the energy flow graph
for the arm for Example 2. You'll notice there's some texture to it now, we're
beginning to manage the energy release. 

<img src="http://www.keithlango.com/tutorials/old/toonSnap/energy_02.jpg" border="0" height="200" width="400" border="0">

<hr>
Third Look: Zoinks! Now that's got
some zip! 

<a href="http://www.keithlango.com/tutorials/old/toonSnap/smoothed03.avi" target="_blank"><img src="http://www.keithlango.com/tutorials/old/toonSnap/anim03.jpg" border="0" height="143" width="200" border="0"></a>

 Here we're really pushing things faster, but we've also made things longer.
I've added a subtle anticipation that builds the tension & energy. By building
up this energy, we can release it much faster and it will feel perfectly
natural. If we were to move the arm this fast without building up this energy,
the move would look very herky-jerky (show a clip of the move without the build
up). By building this extra pent up energy into the arm, we magnify the ability
for our arm to release that energy. So we can build it up, hold it and then let
it literally explode on out through the back end of the move. Once again, I'll
break it down by part:

<blockquote>
Upper Arm: the energy building anticipation move is 5 frames. The upper arms
then hits it's extreme target in just 2 frames. It holds this target until the
lower arm hits. By freezing the elbow here, essentially waiting for the rest of
the arm to unfold, we don't steal any energy from the move. If we didn't hold
the elbow in place while the rest of the arm unfolds and instead started to come
back up toward our settleback key, we would be draining some snap energy from
the arm. Locking the upper arm helps keep the energy focused on the parts that
are still moving in the move. The settle back for the upper arm has been
shortened to just 3 frames. It doesn't need more than that.
Lower Arm; the energy building anticipation for the lower arm is also about 5
frames. But the trick here is that we hold that energy all pent up for 2 frames
longer than the upper arm. We've taken that built energy, and like holding a
breath, we refuse to let it out. So while the upper arm is releasing it's
energy, the lower arm is storing it's energy. Then once the upper arm has hit
it's extreme, then we're unleashing that pent up energy for the lower arm. If
you step through you'll see that the lower arm goes from fully compressed to
fully extended in just one frame. But it feels pretty natural. That's because
we've managed the energy, held onto it for as long as we could and then let that
energy explode out in a single frame. The arm does the same fast/slow settleback
over 5 frames as as in example 2. 
Hand/Fingers: like the lower arm, we're keeping the energy in the hands all
pent up for as long as possible. Step through the animation and you'll see that
we are forcing the hand to stay curled back even while the upper arm is hitting
it's extreme pose position. Then just like the lower arm, the hand explodes that
pent up energy out in a single frame, with one exception- the fingers. We're
offsetting the finger opening by a frame to give some overlap and help soften
the blow. When you blow out that much energy that quickly you need to find
little ways to keep the move from sticking badly. Offsets are the grease that
keeps things flowing in this instance. the settle back is the same fast/slow,
offset from the upper arm and lower arm.
</blockquote>

Overall this last move, when you include the energy building anticipation and
the recoiling settleback from the extreme, is 13 frames long. Yet it has way
more snap than a similar move that was only 4 frames long, and a good deal more
snap than a move that is similar, but still only 11 frames long. The curves for
this move have been converted to spline, but there's no weighting of the
tangents. Just the default ease filters have been applied. If you look at the
energy flow chart below, you can definitely see where the snap comes in.

<img src="http://www.keithlango.com/tutorials/old/toonSnap/energy_03.jpg" border="0" height="200" width="400" border="0">

 

 

Pure Diesel Power, Baby! To sum it all up: to release energy quickly (snap)
and have it feel organic and natural (not jerky), you need to manage that energy so that it
build intensity over time, creating the proper sense of anticipation for the
event to follow. There's so much in animation that can be thought of
in terms of the build up & release of energy or tension. This principle
relates to everything from the emotional
gears of a character's inner motivation right on out to the technical nature of
their very movement. One of these days I hope to explore even more areas where
this energy management principle can be put to use to help craft better
performances and better animation. I hope this little glimpse has been helpful.
 

-k