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 in Pictures
• A Walking Excercise

Walking requires much balance and coordination, yet we can do it without even thinking about it. Walking also conveys a great deal about one’s personality. The next time you’re in a crowded place, notice all the different types of walks that people have. Some people waddle, others saunter, and some drag their feet. It’s amazing how almost everyone you see has their own unique walk. You’ve 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, that’s 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 body’s 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. You’ll soon notice that every part of your body, from your feet to your arms to your head, has it’s 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, I’ve animated a simple skeleton so you can see exactly how each joint moves.

• The feet and legs.
  • The feet and legs propel the body forward. To keep your charcter looking natural, you should always keep the joints bent slightly, even at full leg extension.
• The hips, spine, and shoulders.
  • The body’s center of gravity is at the hips; all balance starts there, as does the rest of the body’s motion. During a walk, it’s best to think of the hips’ motion as two separate, overlapping rotations. First, the hips rotate along the axis of the spine, forward and back with the legs. If the right leg is forward, the right hip is rotated forward. Second, at the passing position, the free leg pulls the hip out of center, forcing the hips to rock from side to side. These two motions are then transmitted through the spine to the shoulders, which mirror the hips to maintain balance.
• The arms.
  • Unless the character is holding something or gesturing, its arms hang loose at the sides. When walking, they act like pendulums, dragging a few frames behind the motion of the hips and shoulders. Even at full extension, try keeping the arms slightly bent at the elbows, to make them look natural.
• The head and spine from the side.
  • If you look at a character’s spine from the side, you can see its general posture, which may be very stiff or slouched over, depending on the character’s attitude. The spine also absorbs some of the shock transmitted to the hips from the legs, making it flex from front to back a bit. In a standard walk, the head tries to stay level, with the eyes pointing in the direction of the walk, but it bobs around slightly to stay balanced. If a character is excited, this bobbing will be more pronounced. The head may also hang low for a sad character, or may look around if the scene requires it.

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? That’s 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 character’s 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 he’s 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 it’s 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:

• Inverse Kinematics (IK) with Locks.
  • Many packages with IK allow you to "pin" or "lock" an object, such as a foot, to a point in space or to another object—such as the floor. IK is one of the preferred walk-animation methods because it ensures that a character’s grounded foot stays in the same spot for the duration of the step. Knowing that the feet are locked frees the animator’s mind from annoying details and enables him to concentrate on animating the character.
• Forward Kinematics with Locks.
  • When you use inverse kinematics, you move the end points of the chain, and the comput-er determines a "solution," rotating some joints automatically to keep the skeleton together. Sometimes this solution is not optimal in that it forces you to rotate the joints manually. Plug-ins such as Meme-X’s Lock&Key for LightWave and Kinetix’ Biped for 3D Studio MAX enable you to lock the feet to the ground and manipulate the skeleton with or without IK.
• Footstep Generators.
  • Some packages have what I call "footstep generators." The most popular of these are 3D Studio MAX’s Biped and Softimage’s Multiped. These plug-ins enable you to toss down footprints on the ground, up hills, down ladders, wherever you want. The program then automatically moves the character’s legs and feet to match. This is not ‘animation’ in the true sense of the word. Typically the motion is generated by an algorithm that computes the character’s physical weight and dynamics.
  • Using such a plug-in is a great way to get the first pass of a walk into the computer, but that’s about all it will do. As the animator, you still have to control the process and add personality to the walk. You must go back over the animation and tweak the poses and motion on a second and third pass to give your character it’s individuality.
• Inverse or Broken Hierarchies.
  • This method does not require IK and works with any package that supports forward kinematics. In a normal skeleton, the hips are at the top of the tree, with the spine and legs as children. If you don’t have a way to lock the feet, this setup can cause problems, because moving the hips will move the spine, legs, and feet.
  • In the Inverse Hierarchy method, the hierarchy is turned upside-down and broken at the hips. The feet become the parents of the legs, enabling you to place the feet where you want them. Because they’re the parents, you can rest assured that they won’t move unless you absolutely want them to move.
  • What about the hips? In a hierarchy, because an object can’t have two parents, the hips must be disconnected from the legs and are forced to float freely above them. The hips are then animated so that they match up with the legs. Because exactly matching the hips to the legs can be problematic, this method works best for characters whose hips are hidden —a soda can with feet, or a character in a skirt. It also works well when using bones, as the character’s skin tends to obscure most hip-alignment problems. Besides, viewers usually notice the feet slipping long before they ever notice that the hips are not quite connected; it’s the lesser of two evils.
• Using Guides.
  • If you’re unable to use the preceding methods, you can always use guides to aid in the placement of feet. In any package you can place null objects, dummies, or transparent objects along the character’s feet and use them as virtual place-holders. If the foot moves inadvertently, you have a reference point that enables you to put the foot back where it belongs.
  • Some packages allow you to simulate an "onion-skin" effect by ghosting the previous frame which provides a good simulation of the traditional animator’s light table. Because you can see where the feet are on the previous frames, it’s easy to align them on the current frame.
  • Finally, some animators use wet-erase overhead projector pens and draw the guides directly on the computer screen. The marks can then be erased with a damp cloth. Still, I’d hate for you to grab the wrong pen and make permanent marks on the screen, so be careful! A good substitute might be a sticky note placed on the screen and marked with a pencil.

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 it’s 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 forward—the 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 character’s 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 they’re 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.

Tip

When you’re animating multiple characters, it’s always tempting to make them walk at the same rate to simplify the animation process—not a good idea. Giving your characters the same gait makes your characters look like they’re marching in unison, which can detract from the shot. It’s 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 that’s slightly slower or faster—maybe 10 or 15 frames per step. By mixing it up, you make your shots more varied and interesting.