Let's All Go To The Hop
by Adam Watkins

I. Introduction-The Computer Hop

One of the big advantages of using computers in animation is the ability to easily find rhythms. Frames are neatly labeled on a palette, and if you can count, you can make events occur at very even intervals; your running character’s foot hits the ground every x frames, your bouncing ball hits the ground every 2 seconds, etc....The problem is, within those segments of evenly spaced instances are fairly unevenly spaced series of motions. Perhaps one of the biggest instances is the ‘hop.’

One sure sign of a beginning animator is the ‘computer hop;’ the object jumps, crests, and lands all in the same fluid motion at the same speed. It’s easy to do. All the animator has to do is key frame the moment the object leaves the ground, crests, and then when it hits. Too often, this same evenly hopping object also remains fixed, hard, and looking more like a ball-bearing than an animate object.

So, the purpose of this tutorial is to examine what the anatomy of the ‘hop’ really is. The platform or animation application you use don’t really matter; the idea is to understand the motion of effective movement.

II. Motion Analysis

First of all, let’s look at an example of what a ‘computer-hop’ looks like. Animation 1 shows a chess pawn making just such a hop. Notice how smoothly the hop goes; all perfectly one speed, perfectly timed (Image 1), and perfectly phony. Notice how the pawn has such a rigid look, that it doesn’t appear to have anything that’s actually driving it up and driving it forward. It still looks as if the animator is moving the piece rather than the piece actually hopping. Although, this may be the look an animator is going for, it usually makes for very unconvincing motion.

Animation 1

Image 1

Eadweard Muybridge was a 19th century photographer who in 1850 set up shop in San Francisco and worked on landscape pictures. In 1872, railroad tycoon Leland Stanford, former Governor of California, asked him to photograph his famous horse, Occident to settle a long time debate over whether or not a running horse ever had all four feet off the ground. Muybridge used a series of glass plated cameras triggered by rubberbands and wire to photograph the animal. His photographs of the ‘flying horse’ made him an international sensation. As his setup became more complex, he was able to capture more and more series of motion, including human motion. His subsequent discovery that these series of images, when viewed in rapid succession, produced what looked to be motion, earned him the title of "Father of Motion Pictures." His exhibits inspired the likes of Thomas Edison who soon began his own work on motion picture photography.

Among Muybridge’s photography are several series of people in the act of jumping. Through analysis of these series of photographs, we can get a good idea of the mechanics behind a body in motion. This sort of study is very important in depicting a self propelled body.

Animations 2 and 3 give composited animations (built by Charl Lucassen from Muybridge photographs at http://web.inter.nl.net/users/anima/chronoph/), that give a good idea of a couple of different kinds of hops. Although neither of these animations show the exact same type of hop that we are creating in the Pawn animation, we can extrapolate from the start of one and the end of the other the kind of information that we need.

III. Object Orientation in Motion

Animation 2

Image 2

Some very important concepts can be learned from the non-composited Muybridge series of photographs. Image 1 shows a breakdown of Animation 2. An aspect that seems rather intuitive, but often forgotten is the idea that an object orients itself in the direction that it is propelling itself. One key element behind good motion, is that when an object is in motion, it will orient itself in the direction that it is moving. Image 2 shows that as the figure takes off, his head is pointed in the direction that it is soon to travel. We can see in Image 3 that as the figure reaches its crest, its orientation shifts so that its feet are facing the direction the body is traveling.

Animation 3

Image 3

Animation 4 shows our trusty pawn utilizing this concept and Image 4 shows a breakdown of the pawn’s orientation in different points of its flight. This motion is a good start and certainly better than the original pawn animation, but still, the pawn is simply too rigid to be believable.

Animation 4

Image 4

IV. Stretch-Elongation and Squash-Recoil

An important part of Stretch and Squash is the idea of weight. That is, in order to make an object appear as though it has weight there must be an anticipation and appearance of stress. In Image 2, the first five images show the jumping man, crouching, bending like a spring in anticipation of his leap. This kind of movement signals to the viewer that there is weight that needs to be propelled, and thus the crouching anticipation called the "Squash".

Likewise, when the motion is stopped, or the object lands as the figures in Images 2 and 3 both do, there is a similar squashing as the weight comes to bear against the surface that stops its motion.

Just as important in Stretch and Squash is the stretch that occurs between "lift off" and "land fall." Notice in frames 5 and 6 of Image 2 (lift off) and frames 8 thru 11 of Image 3 (landfall) the elongation of the figure, or the "Stretch" in anticipation of the coming action.

That’s basically the idea behind stretch and squash. An object squashes in anticipation of propelling itself forward/upward, then stretches in anticipation of it’s coming crest, squashes as its weight shifts from an upward motion to a downward motion, stretches again in anticipation of landing, and then squashes when it hits the ground.

Animation 5

Image 5

Animation 5 and Image 5 show the pawn utilizing the steps listed above. For this particular animation, I’ve added little hic-up in the middle of the hop where the squash at the crest of the hop is accentuated to give the character a very rubbery effect, as though it’s base it heavier that it’s top. Notice also, the added anticipation leaning that occurs at the beginning of the hop sequence and the rebound after the hop is done. Anticipation and followthru are always essential to any motion.

Now we have an animation that is quite a bit better at portraying life. There are definite similarities and parallels between the Muybridge photographs and the animations produced. Now there is one more important aspect to making a believable hop.....timing.

IV. Timing

One aspect that is often overlooked, but vital to effective motion is timing. A sure sign of amateur hopping are "one speed hops;" that is, a hop that is the same speed when it takes off, as when it crests, as when it lands. In actuality, an object’s speed changes fairly dramatically over the course of almost all motion; and the hop is no exception.

When a person jumps (as the two figures in Images 2 and 3 do), there is an initial burst of speed as they push off the ground with enough force to cause them to be airborne. As their "flight" reaches it’s crest, their speed slows as their upward momentum tapers off. When they reach the crest of their jump, their speed in an up-down direction slows to a near stall, as the momentum begins to shift downward. The further they have to fall, the faster they move, until they are moving quite fast when they hit the ground on the other side of their jump.

Animation 6

Image 6

Animation 6 shows the pawn making a well-timed hop. Image 6 breaks down the fast launch, the slowing as it reaches its peak, its near stop at the crest, and the fast decent.

V. Conclusion

These are still a multitude of other character-establishing touches that could be added to this particular animation. Objects can actually hop will all sorts of attitudes that express power, diminuitiveness, attitude and so forth. However, good hops and motion in general is based upon some important principles that can be learned from real world examination. Even if the animate object is far from real world, a viewer will only buy into the motion presented if he can relate it (even on a subconscious level) to what he knows in reality.

These concepts presented here aren’t new or revolutionary. They’ve been around as long as motion animation has been on the screen. From dancing hippos to running coyotes, the ideas of stretch-squash and weight are present in all good animation. Watch for it next time you are viewing a well animated cartoon.

-Adam Watkins