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Animating Four-Legged Beasts


May 2, 2012 Article Start Previous Page 2 of 4 Next
 

The Four Gaits

In the course of my career, I've learned that there is a surprising similarity in how quadrupeds move, from species to species. Eadweard Muybridge's photographic works may be a century old, but they are still relevant and extremely useful.

In his introduction to Animal Locomotion, he maintains that most quadrupeds -- be they dogs, cats, horses or rhinoceroses -- follow the same footfall pattern. This is the order in which the hooves or paws strike the ground while moving through the various gaits. Where they differ is in the flexibility of the spine. Visualize a rhino running, as opposed to a cheetah. The exceptions, according to Muybridge, are elephants, and animals like kangaroos.

The four speeds of movement, or the four "gaits", are shared amongst most four-legged animals. Almost every quadruped walks, trots, canters and gallops, and their legs move in the same manner when they do it.

As you can see in the image below, adapted from Muybridge's Animal Locomotion, the gaits have been broken down into symbols illustrating which leg strikes the ground in which order, assuming the animal is facing north with the right legs on the right and the left legs on the left.

For example, with the rotary gallop gait, if you start your cycle with the left rear foot striking the ground first, the next in sequence to hit the ground would be the right rear foot, then the right fore foot, followed by the left fore foot.

As for the transverse and rotary gallops, I've found the rotary gallop more often in reference material than the transverse, which I've mainly seen in horse footage. The canter is the roughest gait, with a lot of up-and-down movement. Elephants don't seem to follow these rules, and should be considered separately.


image adapted from:
Animal Locomotion, Eadweard Muybridge, 1887

Once you get the legs moving roughly in the order that is appropriate, you can be creative with the rest of the body.

To save time, you could animate a "vanilla" gait cycle for each gait with the leg movements blocked in on keys and breakdowns only and the body and head having the rough up-and-down motion laid in on those keys. If using a universal rig, this file could then be exported onto any beast (with minimal adjustments, depending on the disparities of beast shape) and be used as a starting point for the animations.

Several different types or speeds of walks could also be created from this base file simply by playing with the amount of frames in the animation and the distance between the legs in the stride position or the distance the beast travels in the 'leap' part of the gallop.

The Walk

In the case of the walk gait, the rear left foot strikes first, followed by the left foreleg. The rear right leg strikes third, with the right foreleg falling last. The walk is the slowest gait, and is shared by all quadrupeds. Muybridge breaks down this information into a chart for the walk, trot, canter and gallop gaits in the introduction of Animal Locomotion. Converted into a graphic table, a walk cycle would look like this:


image adapted from: Animal Locomotion, Eadweard Muybridge, 1887

Horses are great animals to study because their legs are so long and slender, creating an easily legible silhouette. Below is an example of a walk cycle broken down into 8 phases:


Image adapted from Horse Locomotion Walk 01, courtesy of www.rhinohouse.com (Click for larger image)

Once you understand what the feet are doing, it becomes easier to understand what to animate next. Think of the quadruped walk as two offset human walk cycles. I wonder how often studios have used two people in a horse costume for motion capture? Preston Blair's iconic walk cycle with the "stride" and "passing" key positions is a great illustration of the basics of a biped walk demonstrating the following: fall into the stride and recover and rise up into the passing position.


Adapted from Preston Blair, Animation, 1948

For the quadruped, the hips and chest become two offset "bouncing balls" in the same manner as the hips in a bipedal walk. Consider circled image 23 from the horse walk cycle (below), which shows the forelegs in the "stride" position and the hind legs in the "passing" position. I've added circles to show the up and down motion of the hips and chest in that phase of the movement. The head could then be animated as yet another bouncing ball, offset from the hips and chest (follow-through! overlap!)


Image adapted from Horse Locomotion Walk 01, courtesy of www.rhinohouse.com


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