Posing a Flour Sack
One easy character to use to begin studying posing is a simple flour sack. This little character has become a classic tool for the study of animation. The flour sack is simple to construct, but it has volume and weight and can be easily posed (see Figure 2).
Figure 2 A simple flour sack is a good character to pose.
How a flour sack relates to a human body may be a bit confusing to the newcomer. The easiest way to conceptualize this connection is that each corner of the sack represents the arms and legs of the character. Of course, these are very stubby arms and legs (see Figures 3 and 4). Another way to think of the character is that the corners of the sack are the hips and shoulders of the character. This is a bit more accurate and leads you to a better understanding of how the body works. Almost all motion in the body starts at the hips and then translates to the limbs. This is also how you should pose characters--from the center of the body outward.
Figure 3 If the corners of the sack are stretched, you can see how they can be thought of as arms and legs.
Figure 4 If separate arms and legs are added, you can see how the corners of the sack can also be thought of as hips and shoulders.
If you pose the hips, spine, and shoulders properly, the arms and legs will be much easier to position naturally. This is because almost all motion begins at the hips and continues down the tree of your skeleton. The only time this changes is when a character is reacting from being hit or pulled with or by something. When the character is interacting with something else, this rule changes to lead with whatever part of the body is being hit or pulled.
Setting Up a Flour Sack
The flour sack can be deformed using any number of methods. Some people use a simple method, such as a rectangular lattice (see Figure 5). The points of the lattice are deformed, which reshapes the sack into the desired pose. A simple skeleton, without arms and legs, can also be used (see Figure 6). A skeleton is good, particularly for learning, because it lends itself to an understanding of the underlying anatomy.
Figure 5 A flour sack can be deformed using a simple lattice.
Figure 6 It can also be deformed using a simple skeleton and a mesh deformation tool.