Animation Alley - Balance

Article by Sergio Rosa [nemirc] - Featured Columnist

Hello everybody. Before we discussed posing. This time we will go over the fine points of balance. Actually balance is related to posing, however, as posing is such a broad subject, we will take each element separately, this week's lesson revolves around balance.

Balance is very important when it comes to posing characters, because it makes the poses appear natural and believable. A drawing doesn't have any weight, so we have to simulate that weight. In a drawing we have the freedom to break any rule that we want, for example; you can make your character stand on a 45 degree angle, and it will never fall, because gravity doesn't affect it. However, everybody will see that, in reality, that pose would be impossible to achieve.

Let’s start with the center of mass, and the center of gravity. Center of mass is the place where all the body mass could be gathered and gravity would still affect the body in the same way. In a body of constant density, the center of mass would be located in the center of the object.

Center of gravity is the spot where you could apply all of the gravitational force and yet the body would react in the same way. On rigid bodies, with constant density, the center of mass and center of gravity are located on the same spot. However, we are talking about characters, so the position center of gravity changes depending on the pose.



Above is a character in a "hero pose." The arrow on his abdomen would be the approximate location of the center of gravity. We really don't care about the center of mass, because in the end what affects the body is the gravity. A figure has a good balance if both feet are on the sides of the center of gravity. That means that the character will not fall.

Let's see another example:



In the above case, the contact point and the center of gravity are not in equilibrium (more about this later). This means that the character would actually fall forward. To explain this, we need to understand "momentum" or "torque." Torque consists on a force applied on some point of the body other than the pivot, and the distance between that point and the pivot. This force causes a rotational effect that would make the character fall forward in this particular case. Try to stand in the same position as the figure in the above image, and see what happens. You'll be lucky if you don't break your nose trying.

Notice, in this case, the center of gravity was not located within the abdomen, like the other case. For artists like us, finding the location of the center of gravity is rather subjective. You can obviously try to find it using mathematical formulas, but that can be very complicated, especially on characters. So, you really have to use your common sense for this; imagine the weight of every body part — then ponder those weights.

If, for example, the character on that picture had a big head, the center of gravity would be located somewhere around the neck.

Another thing to consider is when the character is carrying something. Every object of your scene has its own center of gravity. When a character carries something, both centers of gravity make a new center of gravity, which is located somewhere between the first two called "average center of gravity." You can see an example in the next image.



Are we allowed to break the balance? Absolutely. However, we have to know when we can do it and why. We can make static poses as well as dynamic poses, where we can portray: characters walking, running, falling, and dancing. An animated character is in a constant "balance — off balance," during the flow of the action. For example, let's take a look at a walk:



When the feet are on the ground, we have a good balance. When we are on the highest position (like the character featured in the above image), we are off balance, and we don't fall because the front foot catches our body. So, you can make off balance poses as well, but you have to know why and when. If you want to further give the impression of movement you can add motion blur … but that's a different subject.

It's been a good thing to come back to these little articles. I hope you found this useful.
Happy posing, and keep on animating!
nemirc


Animation Alley
is a regular featured column
with Renderosity Staff Writer
Sergio Rosa [nemirc].

Sept. 12, 2005