Animating a Quadruped Walk

In these lessons, you'll animate a four-legged character, a dog, to walk in a continuous way. You’ll use the ForeFeet option to make the fingers of the biped hands behave like toes on forefeet.

A Quick Review of a Biped Walk

If you don’t use Biped to create a walk for you, it helps to know that a human walk cycle is defined by two steps: left foot to right foot, followed by right foot to left foot (or vice versa). The two steps break down into four states:

Left to right:

1. Contact

2. Down

3. Passing

4. Up

5. Contact again (same as 1, but with legs reversed)

1. Contact: Both feet are on the ground. At this point, the stride is at its longest: this is known as an extreme pose.
2. Down or “Recoil”: After contact, the weight goes down on the front leg. The body lowers, and both legs bend.
3. Passing or “Breakdown”: The front leg straightens and the back leg passes it. The body raises to a point that is higher than in the contact position.
4. Up or “High Point”: The back foot is now the front one, and is about to make contact. The other foot pushes up and forward, raising the body to its highest position.
5. Contact: The same as pose 1, but with the opposite leg forward.

You can start animating the cycle at any of these poses. Animators often prefer to begin with the contact pose, as that pose (in general, any extreme pose) is a good reference to build from.

You have to decide how many frames the walk cycle will use. 12 frames yields two steps per second: this is a natural pace, which we will use in this tutorial. Cartoonists sometimes use an 8-frame cycle to create a fast, humorous walk. A 24-frame cycle would give (for film) one step per second, suitable for a slow-moving character.

The Walk Cycle for Quadrupeds

A quadruped walk is essentially two biped walks linked together, but out of phase with each other. When a biped walks, the shifting weight on the pelvis causes the up-down motion just described. For a quadruped, the same weight shifts occur for the pelvis and the shoulders.

Quadrupeds have different proportions than human bipeds. In particular:

• The rib cage is elongated downwards, unlike the flatter human rib cage.
• The shoulder blades lie along the side of the rib cage, not on the back.
• There are no collarbones.

The lack of collarbones gives the shoulder blades more freedom. This affects weight distribution on the front legs.

When you use Biped to animate a quadruped, its “clavicle” parts behave more like shoulder blades.

In spite of these differences, and some others we will mention later, a 3ds Max Biped can model a quadruped quite well.