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Taking on Skeletal Animation

Skeletal animation−two words that bring to mind thoughts of B−rate horror movies in which the dead have risen from the grave to stalk the living. However, those two words mean something entirely different to programmers. If you're like me, this topic gives you more tingles down your spine than any cheesy horror movie ever could.

Skeletal animation is quickly becoming the animation technique of choice for programmers because it is quick to process and it produces incredible results. You can animate every detail of a character using skeletal animation. It gives you control of every aspect of the character's body, from the wrinkles in his skin to the bulges in his muscles. You can use every joint, bone, and muscle to deform the shape of your character's meshes.

Think of skeletal animation like this: Your body (or at least your skin) is a mesh, complete with an underlying set of bones. As your muscles push, pull, and twist your bones, your body changes shape to match. Instead of thinking of the muscles changing the shape of your body, think of the bones altering the rotation of each body part.

If you lift your arm your shoulder rotates, which in turn causes your entire arm to rotate and your skin to change shape. Your body (the mesh) changes shape to accommodate the changes in the bones. Skeletal animation works the same way. As the underlying skeletal structure changes orientation from the rotating of the joints, the overlaid mesh (appropriately called a skinned mesh) changes form to match.

As you can see, there are two separate entities to deal with when you are working with skeletal animation−the skeletal structure and the skinned mesh. Take a closer look at each entity in more detail to see how they work in unison, starting with the skeletal structure.

 

Using Skeletal Structures and Bone Hierarchies

The skeletal structure, as you can imagine, is a series of connected bones that form a hierarchy (a bone hierarchy, to be exact). One bone, called the root bone, forms the pivotal point for the entire skeletal structure. All other bones are attached to the root bone, either as child or sibling bones.

The word "bone" refers to a frame−of−reference object (a frame object, which is represented in DirectX by the D3DXFRAME structure or a Frame template inside .X files). If you were to examine the D3DXFRAME structure, you would indeed find the linked list pointers (D3DXFRAME::pFrameSibling and D3DXFRAME::pFrameFirstChild) that form the hierarchy. The pFrameSibling pointer links one bone to another on the same level in the hierarchy, whereas the pFrameFirstChild pointer links one bone to another as a child bone, which is one level lower in the hierarchy.

Generally, you would use a 3D−modeling package to create these skeletal structures for your projects. Exporting the bone hierarchy in the form of an .X file is a perfect example. Microsoft has released exporters for 3D Studio Max and Maya that allow you to export skeletal and animation data into .X files, and many modeling programs have the same exporting capabilities. I'll assume you have a program that will export these hierarchies to an .X file for you.

You'll find a number of things inside an .X file that contains skeletal animation data. First (and most important at this point), you'll find a hierarchy of Frame templates, which is your bone hierarchy in disguise.

Now let me show you some contents from .x file named with tiniy.x:

 

You should find a standard Mesh data object embedded in the Frame data object hierarchy. The Mesh data object contains information about your skeletal animation object and the bones used in your skeletal structure. That's right−the Frame data object and the Mesh object both contain information about your skeletal structure! Whereas the Frame objects define the actual hierarchy, the Mesh object defines which frames represent the bones.

For now, however, the importance of the bone data is irrelevant. Because the bones depend on the frame hierarchy, it's important to concentrate solely on the frames at this point. You simply need to load the hierarchy (from an .X file, for example) and set it up for later use. Read on to see how to load hierarchies from .X.