CAP 201a - Computer Animation I

Lab 2 - Animation Concepts


Wyatt discusses several concepts that our basic animation chapter asks you to use. A follow up assignment will be made from this material. Objectives important to this lesson:

  1. Stretch and squash
  2. Timing and weight
  3. Anticipation

On pages 68 and 69, Mr. Wyatt introduces the stretch and squash techniques that you use with the bouncing ball exercises in the Derakhshani book. He actually goes into a little detail that should be helpful in working out an animation for a new object or character.

The basic idea behind stretching an object in motion and squashing an object that is interacting with a barrier can be overdone, but sometimes it is intentionally done to meet a purpose. Consider the bouncing ball illustrations on pages 68 and 69. Mr. Wyatt remarks that in illustration A the ball looks dead. It just moves from point to point at a constant velocity. (We will return to this example in the next section.) He tells us that illustration B looks more alive because the ball is moving just the same, but it is deforming by stretching toward and away from the floor, and squashing when it hits the floor. This is the kind of animation that would be familiar if you had studied the work of Preston Blair, among others.

A point to consider here it the artificial nature of the ball stretching toward the floor. It is unnatural and it is unrealistic. If I drop a ball from any given height, will it deform as it approaches the floor? No. It will deform as it hits the floor (squash), and it will recover from that deformation as it bounces away, perhaps deforming in the opposite way (stretch) momentarily, before regaining its initial shape. Mr. Wyatt offers that the amount of squash and stretch should be affected by three aspects of the object being animated: the material it is made of, its weight (and mass), and its speed.

So why do animators sometimes animate a ball this way? One answer lies in Mr. Wyatt's third example, illustration C. What if the bouncing object is not just a ball, but a ball shaped character? Then the stretch action might be perceived as a choice the character is making. Could the ball still be a character without the arms, legs, and head that were applied in illustration C? Okay, why not. And if this is true, then the animation of the stretch on approach to the floor might be perceived as an indication of the ball being more alive.

Watch the animation carefully in this classic piece from Why Man Creates, A Parable. What gives the impression that a ball is alive? Some squash and stretch are used, but the sense of life is given more by the sound track and the viewer's reaction to the piece. Is there a point at which you believe one ball is alive? Is there another point where you believe more balls are alive? What works in this piece that you could use in a work of your own? What else can you do with our software?

Another aspect to animation is addressed on pages 70 and 71: timing and weight. Timing is easier to grasp when you see an example that does not have it. Consider illustration A, back on page 68. The ball falls to the floor, hits the floor, and bounces back up at a constant velocity. Simple, easy, and wrong. Even if you have never studied physics, you should have a gut feeling for the fact that objects fall faster the longer they fall ignoring aerodynamic factors like wings and parachutes. (This is because a falling object is under constant acceleration from gravity. If that makes your head hurt, just trust me, but consider a different profession.)

The images of the ball represent key frames, and the horizontal axis represents time (as it does in 3DS Max), then the images that are closer to the floor should appear at smaller and smaller time intervals because the ball will be moving faster and faster the longer it falls. When the ball is bouncing back up, the opposite is true: the ball is moving fast at first, but will slow down more and more the longer it fights gravity on the bounce. Is the squash and stretch technique trying to simulate that effect? Not really. To account for timing, you have to account for how far the object moves per unit of time, and realize that the rate changes depending on what the ball is doing.

Weight and mass are not quite the same thing. An object in space is "weightless" because it is in free fall, but it still has mass. The more mass it has, the more effort/force it takes to move it. On Earth, and other places like it, weight and mass are directly proportional. Why do you care? Newton's first and second laws. Objects in motion or at rest tend to stay in motion or at rest until acted upon by an outside force. The relationship between an object's mass, its acceleration, and the force acting on it is F=m*a. This means that for more mass, it takes more force to attain the same acceleration. Example: I can tap a ping pong ball and it will move away quickly. I have to hit a bowling ball much harder to move it away at the same speed.

The science above (less intimidating than "physics"?) leads to two examples in Mr. Wyatt's discussion. Consider the illustration at the bottom of page 70. This is harder, because it is not laid out like a regular timeline. Reading the picture from left to right, the horizontal scale is labeled for key frame numbers, not time. For time, assume about a half second per inch. In this case, the ball is slowing down. The picture shows constant time intervals, but less and less distance traveled per time interval. Why? The ball is slowly approaching a stop, as it would if it were heavy (more mass). If the ball were light (less mass), it could stop more quickly. An object that is harder to stop is said to have more momentum or inertia. We will review the relevant classic animation principle videos on this in class.

For the second example, consider the characters interacting with props on page 71. The characters seem to have a harder time moving, carrying, or lifting some items than other items. What impression does this give the viewer? It's a cartoon. You don't really believe that one circle is heavier than another circle do you? No, you shouldn't, but you should believe that in context, in the world we are seeing, objects can be heavier or lighter, and characters should interact with them accordingly.

On pages 74 and 75, Mr. Wyatt discusses anticipation. As is typical, Newton's third law is quoted here. Mr. Derakhshani says the same thing. I will be a heretic and suggest that the third law has nothing to do with the effect they are trying to show us. Mr. Wyatt explains that a pitcher throwing a ball will wind up first, preparing for the throw by making an anticipatory opposite movement. Sorry, kids, that is not an example of action and reaction. That is an example of leverage and application of force. The pitcher winds up first and takes a step during the throw to apply force to the ball more effectively and for a longer time before letting go of the ball. It is still anticipation, but it is not Newton's third law.

Now that the myth is disposed of, what is the value of this technique? It makes the animation more true to life and more believable. That's how people move when they are doing something seriously. It is also useful for emphasis.

Consider the three images of a double take on page 75.

  • The character is starting to look away in frame 1.
  • She turns back, looks down a bit, shuts her eyes, and maybe compresses her lips a bit in frame 2.
  • In frame 3, she looks up, changes to an alert posture, opens her eyes wide, and forms a classic open-mouthed "Oh" with her lips.

Could this movement be done without frame 2? Yes, if you were in a hurry. However, the pose and action in frame 2 emphasize the differences between it and frame 3, making the action broader, more exaggerated, and harder for the audience to miss.

This leads to a different question: how much is enough? You must judge that based on the level of realism you are trying to portray. Do you recall our discussion in CAP 101 about the levels of character realism? Animation itself requires the same kind of decision process. Do you want to have Three Stooges slapstick in Hamlet or Macbeth? Probably not. Let the acting (puppetry) reflect the seriousness of the scene or the production.