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Animation principles are the laws of motion as they apply to animation. In other words, Newton's laws translated for animators. Now, all of these were developed in the 1920s and 1930s, mostly at Disney, but they apply to just about any type of animation you want to do. Over the next few movies, we're going to take a look at these animation principles. Now, these are slow in and slow out, which concerns the acceleration and deceleration of objects.
Arcs, which is the paths that objects move through space, Overlap & follow-through, which is how systems of objects start and stop their motion, Squash & stretch, which is something that's familiar to those who like cartoons, but it also helps to convey weight and volume. Then Weight itself, which is the relative weights of objects in the scene and how they move. Finally, we also have Anticipation, which is getting objects moving and the motions required to gain momentum.
Now all of these we'll go through, but let's go ahead and start with the first one, which is slow in and slow out. Slow in and slow out is basically a restatement of Newton's second law of motion. That means that objects accelerate and decelerate as they move. Now again, it's Newton's second law, which is Force = Mass x Acceleration. Now, in an animation context, this means that slow in and slow out will smooth out or soften your action, so things will come toward abrupt halt and jar the eye.
They'll slow out, things will be easy. Anyone who has used animation curves or Bezier curves or ease curves will be familiar with slow in and slow out. But let me go ahead and show you how it works in an animation context. Let's take our standard situation of a ball that has a force applied to it. Now, as the force is applied, the ball accelerates and when a counter force is applied, the ball decelerates.
In animation lingo, that would be it slows in and it slows out. Now, if we continue adding that force, it will slow in again and get moving in the opposite direction and then slow out. So if we play this, you would see basically slow in and slow out. Now, if we didn't do slow in and slow out, it would look as though the ball had hit a brick wall at the very end of this. It wouldn't slow to a stop and then get going again. It would just hit that brick wall and then just move in the opposite direction, which isn't really a very natural way of motion.
Now, there is a situation where something will hit essentially a brick wall and get moving in the opposite direction, and that is when a ball is bouncing or an object is basically recoiling off of another object. In this case, the issue is that it's actually under the control of one force. So when I have a bouncing ball, what happens is the force of gravity starts pulling down the ball and it accelerates. Then when it hits the ground, it recoils or the momentum of that object changes, and then it moves upward, but the force of gravity is still pulling down on it, so it again slows out.
So, we have a slow in and a slow out, but we also have a bounce. Let me play this one more time, slow in, bounce, slow out, and so on. Okay, now playing with spheres and balls is great, but let's go ahead and put this into an animation context. So here I have a part of a character, an arm. So what exactly is moving this arm? Well, we have two muscles. We have a biceps and a triceps.
The contraction of those two muscles is what is moving the arm in either direction. So when the arm is moving up, it's the biceps contracting, and when the arm is moving in the opposite direction, it's the triceps contracting. So what the muscles do is they actually generate force. So as the arm starts to bend, the biceps contracts. By contracting, it actually pulls on the forearm just above the elbow to accelerate it.
You can see by this graph that it actually is accelerating. Then as the force weakens, it decelerates. And again, in the opposite direction, we have the force of the triceps activating and it accelerates the arm in the opposite direction. Again, as that force fades, the arm slows to a stop. So when we play this, you can see that we've got an acceleration, and then another acceleration, or in animation lingo, we have slow in, slow out, and then again, slow in and slow out.
Now the important thing about this is that any motion you have with your character is going to be based in the muscles of that character. What causes a character to move is that character's muscles. The muscles provide force, and by providing force, they will accelerate, decelerate, or slow in and slow out all motions. So as you animate your character, keep that in mind, and use slow in and slow out for all motions of that character.
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