Learning to Walk
The Theory and Practice of 3D Character Motion
by George Maestri
If you want to move your 3D Characters to use their feet, it helps to understand the mechanics behind walking. Although we learned to walk as toddlers, translating our personal experience into animation techniques is more complex than it first may seem
Walking requires much balance and coordination, yet we can do it without even thinking about it. Walking also conveys a great deal about ones personality. The next time youre in a crowded place, notice all the different types of walks that people have. Some people waddle, others saunter, and some drag their feet. Its amazing how almost everyone you see has their own unique walk. Youve probably noticed the distinct way in which Mae West, Groucho Marx, John Wayne, and Charlie Chaplin all walk. If you want to peg a character, figure out how they walk.
Computer animators have a number of tools available for animating walks. It seems as though software vendors have focused a considerable amount of effort on technology for animation walks, and quite a few new and innovative tools are available for automating all or part of the walking process. These sophisticated tools can be both a help and a hinderance. As the animator, you should still understand exactly how characters walk and how you want your characters to do it. If a piece of software helps you to get this done in half the time, thats fantastic. Just be sure that you, not a piece of software, are controlling the animation process.
Understanding the mechanics of walking
Walking has been described as "controlled falling." Every time you take a step, you lean forward and fall slightly, and are caught by your outstretched foot. If you failed to put your foot forward, you would fall flat on your face. After your foot touches the ground, your bodys weight is transferred to it and your knee bends to absorb the shock. The front leg then lifts the body and propels it forward as the rear leg swings up to catch you again, and the cycle repeats.
Before you read any further, get up and walk around the room for a bit. Pay attention to how each part of your body moves. Youll soon notice that every part of your body, from your feet to your arms to your head, has its own unique set of motions. As you walk around, notice how you lean forward into the walk, and how your legs neatly catch your body to prevent it from falling.
Walking is complex. Not only do the feet have to move across the ground, but the hips, spine, arms, shoulders, and head all move in sync to keep the system in balance. Although these movements are complex, if you break them down joint by joint, the mechanics of walking becomes clear.
The following sections break down a basic walk, step by step. For clarity, Ive animated a simple skeleton so you can see exactly how each joint moves.
Animating a walk
Now that you understand the underlying mechanics, you can attempt to animate a walk. Using traditional animation techniques, a walk cycle is tough to create; it can be just as tricky on a computer.
The first thing you need to concern yourself with is the timing of the walk. How many frames does it take? Thats not an easy question to answer. Is your character large and lumbering, or small and scrappy? Is your character running or walking? Happy or sad? All of these factors will determine the time it takes your character to take a step.
A normal walking gait will take anywhere from one-third to two-thirds of a second per step, (8-16 frames at 24fps, or 10 to 20 frames at 30fps) with a half second per step being about average. A full cycle (both right and left steps) about a second per cycle. Larger characters tend to walk slower and smaller characters walk faster. In general, men have slightly slower gaits than women, and sad people walk slower than happy people.
One nice thing about working with a computer is that many programs allow you to scale the length of your animation. If your character is walking too slow, you can speed him up a bit by reworking the keys.
Keeping your feet on the ground
The most important thing to remember when animating a walk is to keep your characters feet firmly locked to the ground. The friction between the feet and the ground propels him forward. If the feet slide around, the illusion of friction is lost and the animation will not seem realistic (though, if hes walking on banana peels or an oil slick, sliding may seem hilarious.)
How do you keep the feet locked to the ground? It really depends on your software and its feature set. Many packages have tools to assist you in this task; others do not. The following are a few of the more popular methods:
To cycle or not to cycle?
Because walking is a cyclical motion, it may behoove the animator to create walking motion as a cycle rather than as straight ahead animation. If done properly, a cycle can save an inordinate amount of animation time. One cycle can be applied to a number of different environments. Your character and his cycle can just as easily be placed in a cityscape as a country road because the walk motion is essentially the same. Why duplicate your efforts? Classical animators use this trick a lot, simply repeating a sequence of drawings and swapping only the background painting to place the character in a different location. In 3D, you have the flexibility to change the cameras, lighting, and environment to make the shot look completely different.
There are downsides to using cycles. First, because the cycle is repetitive, it can seem sterile and flat, particularly when viewed for an extended period of time. Second, cycles work best on level terrain. If your character has to walk around a corner or over a hill, the cycle might not match up properly. It can be difficult in some programs to stop a cycle once its started. For example, if your character walks into a room and stops, the cycle must be stopped and the character keyframed from that point. Animating a cycle is similar to making your character walk on a treadmill. The body does not move forwardthe feet simply move beneath it. To maintain the illusion of walking, the entire character must be moved across the ground (or the ground moved past the character) at the exact same rate that the feet are moving. Otherwise, the characters feet appear to slip. Also, the foot on the ground needs to move the exact same distance on each frame. Again, if the length of the steps vary, the feet look like theyre slipping.
The following exercise illustrates one way you can go about animating a two- legged walk. This is certainly not the only way to animate, but it does touch on many of the major points discussed in this article. You can use these techniques as jumping- off points for creating your own techniques.
When youre animating multiple characters, its always tempting to make them walk at the same rate to simplify the animation processnot a good idea. Giving your characters the same gait makes your characters look like theyre marching in unison, which can detract from the shot. Its always best to stagger walk cycles and give characters different gaits. If one character is walking at 12 frames per step, give the other a gait thats slightly slower or fastermaybe 10 or 15 frames per step. By mixing it up, you make your shots more varied and interesting.