Tricks for Cartoon Snappy Motion
An Energy Crisis
© 2003 Keith Lango
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.....
First Look: Two Lonely Keyframes....
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.
Second Look: Better
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 closer examination, I'll break it down by body part:
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. 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.
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.
Third Look: Zoinks! Now that's got some zip!
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. 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:
Upper Arm: the energy building anticipation move is 5 frames. The upper arm then hits its 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.
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.
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 builds 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.