Animating a Four-Legged Walk
A quadrupedal skeleton can be made to walk quite easily. It is a simply matter of getting the back legs to walk much like a two-legged character's and then adding in the front leg motions. When you're animating a four-legged walk, it is important that both pairs of legs move the same distance with each step. If the back legs have a larger stride than the front, for example, the back of the creature will soon be ahead of the front.
In this exercise, you will animate a simple four-legged walk using an IK skeleton. The timing of a walk will vary widely, depending on an animala mouse will step much more quickly than an elephant. In this case, you will animate 1 step every 12 frames.
Start with positioning the back legs, as shown in Figure 8. Rotate the back hips and place the right back foot forward. Exactly how far forward depends on the animal; a creature with longer legs might step as far as halfway up the body.
Position the front legs by mirroring the pose on the back legs, placing the right front foot forward (see Figure 9). This means that, for this step, the forward feet (the right front and the left rear) will remain planted.
Both pairs of legs should have the feet approximately the same distance apart. This distance is the stride length. Take a note of this distance as a reference for the next step by estimating it or measuring it exactly, as shown in Figure 10.
Animate the first step. In a four-legged walk, you will need to make sure that both pairs of feet move the same distance. Move the time slider forward to frame 12. Set keys for the forward feet (the right front and the left rear.)
Rotate the shoulders and hips to mirror the pose on the first frame. Move them forward by the stride length. This will cause the feet to lift off the ground, as you can see in Figure 11.
Move the back feet forward by double the stride length, and set a key (see Figure 12). Scrubbing the animation will have the skeleton perform a walk in which the feet simply slide.
Go to the middle of the stride (frame 6) and create the middle pose. Lift the two legs that are moving forward off the ground, as shown in Figure 13.
As in a bipedal walk, lifting the legs throws the hips and shoulders out of balance slightly. Rotate the hips and shoulders so that the side with the moving leg is lower than the side with the planted leg.
Finally, the hips and shoulders will also rise and fall vertically, much like a biped's hips. In this step, the recoil position, in which the body absorbs the weight of the planted foot, will be at approximately frame 3. Move to that frame and lower both the hips and the shoulders. At frame 6, they'll be at their peak; move to that frame and lift the hips and shoulders so that the planted leg is at full extension.
Scrub the animation. This is just the first step. Repeat this procedure for each additional stride.
Figure 8 Start with the back legs.
Figure 9 Move on to the front legs.
Figure 10 The stride length is the distance that a character moves in one step.
Figure 11 Make sure that the character's shoulders move properly.
Figure 12 Move the back feet forward.
Figure 13 In the middle pose, the legs lift off the ground.
This will get you a basic walk. You might want to play with the timing of the feet, however, to get a little bit more realism in the walk. You can do this by offsetting the motion of opposite feet by a frame or two so that the feet don't hit at exactly the same time.