Arc D' Triumph!
A Quickie Study on Adding Arcs to Your Motion
Keith Lango 8-30-04

Intro
Many people send me animated clips for critique. One of the common things I see from newer animators is the lack of concentration on the value of arcs. Many folks have successfully adapted a pose based methodology for constructing the basics of their scenes, but that's arguably not even half the battle. Along the way we need to become adept at getting into and out of those pose ideas with some flair and technical ability. One of the most effective things that we can do with our work is to pay attention to the motion arcs. If you have poses and timings that are working, but your motion looks and feels choppy and sloppy and herky-jerk, then it's a really good bet that you need to pay special attention to your motion arcs.

What Are Motion Arcs?
Long ago defined, and restated here for clarity, arcs describe the path of action (travel) that various parts and things plot out when they move. When a biological entity (a person, a character) moves a part of their body, most often that body part will move in a sweeping path of motion. Instead of going from Point A to Point B in a straight line, we move from Point A to Point C to Point B in a swooping arced path of motion. When we animate from one pose to another it is vitally important that we bring this detail to life within our characters. The degree to which we exaggerate these arcs depends upon a myriad of variables. Style of motion, design style of the character, the nature of the move, the intensity and speed (or violence) of the move, etc. But regardless of style or design sense all characters should move in an arcing fashion. Without arcs the motion lacks a sense of flow, and all great animation has at it's very core a sense of flow.

What Needs to be Arced?
Just about everything. You should build arcs into your hips for sure. A key ingredient to selling the weight of your character is to build arcs into the hips. A key to convincing and pleasing moving holds is the arc in the hips, specifically the 'settle back' from an extreme. Additionally the 'settle up' into an extreme needs to flow along an arcing path. If there's one part of your character that absolutely must have arcs it's the hips. But that's just the beginning. Your arms need arcs as well. When the arm moves it does so as the result of various joint rotations. FK arms tend to have these rotations built into them, but IK arms need to have the IK controller keyed into an arcing path of action just the same. There's no excuse for not arcing your arms. Really, any part of the body that moves should move in an arcing fashion. Track your wrists, finger ends, nose top of the head, elbows, knees, feet/ankles, toe tips, sternum. Look at these body parts in motion, track their progression frame by frame right on your screen with a dry erase marker- take care to see just how these parts move. Just setting two poses, maybe adding a single breakdown and then letting the computer in-between the rest isn't going to cut it when it comes to getting good, clean, focused and smooth motion. When it comes to building great arcs you the animator must understand that you are responsible for every frame- every drawing if you will- of your work.

Got an Example?
I never thought you'd ask! Now, before I get started, I want to re-iterate what I always say when I write one of these things: what you'll see here is just my way of dealing with the problem at hand. Everybody has their own way. Some people rely on tweaking their f-curves to get the motion paths working the way they want. I tend to be more traditionally minded and will insert a new 'drawing' (add a key) to define the motion. To each their own. The important thing to remember here is not necessarily the specifics of how I do it, but the philosophy of what I'm attempting to say here- namely, we should strive to take control of each frame and force the frames to portray the motion we want. OK, with that bit of business out of the way, let's move on. Here's a basic pose to pose blocking for a scene.

....(click image to play movie)

No arcs there. And it's pretty dull. Here's the dopeSheet for this stage of work. As you can see, we're letting the computer do too much thinking for us here.


....(it's not a movie. clicking image will accomplish nothing.)

Noah's Arc, Part Two by Two
So here I've tried to arc it up a bit.

....(click image to play movie)

The primary way I did this was to add a single breakdown key "pose" in the middle of the motion to help define an arc to the path of motion. Now if you're moderately paying attention you'll say to yourself "Hey, that's not really very good. I mean, yeah, there's a little bit of arc there, but the motion almost looks worse." At least that's what you should have said to yourself. Obviously just adding a single breakdown key for the moving parts isn't enough to give us the kind of arcing motion we desire. A quick look at the dopeSheet for this pass...


....
(it's not a movie. clicking image will only make it mad.)

So we see that we're still relying on the computer to do too much of our thinking. For great arcs, we need to take control!

Taking Charge
OK, final pass (at least final for this exercise).

....(click image to play movie)

It still needs a ton of work in many areas like overlap, force, timing, etc. And granted the arcing here is ludicrously extreme, almost to the point of going too far. But hey, what better way to illustrate a point than by pushing that point to the brink of absurdity, eh? So no, this isn't even close to great (or even good) animation, but it does (hopefully) show what an extreme example of arcs looks and feels like. I've purposefully tried to keep the motion timed as closely to the original as possible and haven't added a lot of the other goodies we like to see in animation just to help me illustrate the value of arcs. As you can tell, this motion has a lot more screen arcing of body bits than the previous version. How did this come to pass? A quick look at the dopeSheet shows what has happened here.


....
(go head and click it. don't blame me when it gets medieval on yo'.)

There are a LOT more keys on the various moving parts. Why? Because I wanted that kind of extreme arcing motion, so that's what I needed to do. So how do you add the kinds of arcs that you want to see? You set keys. Lots and lots of keys. Either that or you twiddle your f-curves to give you what you want. Either way you need to employ a mindset that declares every frame of your work as yours. Take ownership of every frame, forcing your character's motion to conform to the things you want and need and leaving nothing to the computer. This is what traditional and stop motion animators do- they draw every drawing to define the arc, they move the puppet for every frame to define the arc. They leave nothing to chance, they own it all. Will the computer sometimes do a decent job for a given frame? Maybe, but my experience tells me that in many instances the computer's in-betweens are fairly poor at giving you arcs. Arcs are an animator's job, not the computer's.

Joan of Arc Would Smile
Here's another little prize for you. Check out this screen grab of the motion paths for the various moving parts.


....
(no really, it's not a movie. your clicking the image is futile.)

These are the motion paths of the listed items from that previous (extreme) example. I used Maya's Motion Trail command to create these motion trails. Anyhow, notice the little hooks at the ends of the motion trails? That's a nice little cheat to get a good indication of weight working on your character. The key on in this image is the hook over pattern for the hips. That single bit of motion there, sometimes referred to as a "'settle back" can add tons of value to your animation. The hand/wrists also have these hook overs in their motion. Working these kinds of motion paths into your animation takes effort, but the payoff is significant as the movement has much more life going for it. Machines travel the same path forward and back, but body parts do not. (The part of the trails that seem to be linear and straight are actually adjacent frames. So the two ends of any straight part are actually back to back frames. There's no use in worrying about those straight parts since they're never seen. )

Conclusion (About time! What a windbag!)
OK, so what does all this mean? Well, here's a contrasting look at what we started with and where we're at now.

....(click image to play movie)

What we see (aside from a desperate need for some good animation) is that even just by adding one ingredient like arcs to moderately decent poses this piece has a little more zing to it. In effect, we can get into and out of our poses with more flair. Like I said, it's not a good piece of animation by any stretch. Again, this is merely a "lab exercise" in seeing how to execute arcs to improve the flavor of our animation. Hopefully it's been clear and useful for you, the reader.