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:
xof 0303txt 0032
template Mesh
{
<3D82AB44-62DA-11CF-AB39-0020AF71E433>
DWORD nVertices;
array Vector vertices[nVertices];
DWORD nFaces;
array MeshFace faces[nFaces];
[]
}
template MeshFace
{
< 3D82AB5F-62DA-11cf-AB39-0020AF71E433 >
DWORD nFaceVertexIndices;
array DWORD faceVertexIndices[nFaceVertexIndices];
}
template MeshNormals
{
< F6F23F43-7686-11cf-8F52-0040333594A3 >
DWORD nNormals;
array Vector normals[nNormals];
DWORD nFaceNormals;
array MeshFace faceNormals[nFaceNormals];
}
template MeshTextureCoords
{
< F6F23F40-7686-11cf-8F52-0040333594A3 >
DWORD nTextureCoords;
array Coords2d textureCoords[nTextureCoords] ;
}
template Coords2d
{
< F6F23F44-7686-11cf-8F52-0040333594A3 >
float u;
float v;
}
template VertexDuplicationIndices {
<b8d65549-d7c9-4995-89cf-53a9a8b031e3>
DWORD nIndices;
DWORD nOriginalVertices;
array DWORD indices[nIndices];
}
template MeshMaterialList
{
< F6F23F42-7686-11CF-8F52-0040333594A3 >
DWORD nMaterials;
DWORD nFaceIndexes;
array DWORD faceIndexes[nFaceIndexes];
[Material <3D82AB4D-62DA-11CF-AB39-0020AF71E433>]
}
template Material
{
< 3D82AB4D-62DA-11CF-AB39-0020AF71E433 >
ColorRGBA faceColor;
FLOAT power;
ColorRGB specularColor;
ColorRGB emissiveColor;
[]
}
template ColorRGBA
{
< 35FF44E0-6C7C-11cf-8F52-0040333594A3 >
float red;
float green;
float blue;
float alpha;
}
template XSkinMeshHeader {
<3cf169ce-ff7c-44ab-93c0-f78f62d172e2>
WORD nMaxSkinWeightsPerVertex;
WORD nMaxSkinWeightsPerFace;
WORD nBones;
}
template SkinWeights {
<6f0d123b-bad2-4167-a0d0-80224f25fabb>
STRING transformNodeName;
DWORD nWeights;
array DWORD vertexIndices[nWeights];
array FLOAT weights[nWeights];
Matrix4x4 matrixOffset;
}
////////////////////////////////////////////////////////////////////////////////////////////////////
Frame Scene_Root {
FrameTransformMatrix {
1.000000,0.000000,0.000000,0.000000,
0.000000,1.000000,0.000000,0.000000,
0.000000,0.000000,1.000000,0.000000,
0.000000,0.000000,0.000000,1.000000;;
}
Frame body {
FrameTransformMatrix {
1.278853,0.000000,-0.000000,0.000000,
0.000000,0.000000,1.123165,0.000000,
0.000000,-1.470235,0.000000,0.000000,
0.135977,2.027985,133.967667,1.000000;;
}
Frame {
FrameTransformMatrix {
1.000000,-0.000000,-0.000000,0.000000,
-0.000000,1.000000,0.000000,0.000000,
-0.000000,0.000000,1.000000,0.000000,
-0.142114,0.000023,-49.556850,1.000000;;
}
Mesh {
4432; // nVertices: Number of vertices.
-34.720058;-12.484819;48.088928;, // vertices[nVertices]: Array of vertices
6841; // nFaces: Number of faces
3;61,0,4431;; // faces[nFaces]: Array of faces, each of type MeshFace
MeshNormals {
4432; // nNormals: Number of normals
-0.914875;-0.152402;-0.373869;; // normals[nNormals]: Array of normals
6841; // nFaceNormals: Number of face normals, equal to nFaces in Mesh.
3;61,0,4431;; // MeshFace faceNormals[nFaceNormals]: Array of mesh face normals
}
MeshTextureCoords {
4432; // nTextureCoords: Number of texture coordinates
0.551922;0.238188;; // Coords2d textureCoords[nTextureCoords]: Array of 2D texture coordinates
}
VertexDuplicationIndices {
4432; // nIndices: Number of vertex indices. This is the number of vertices in the mesh.
3420; // nOriginalVertices: Number of vertices in the mesh before any duplication occurs.
0, // The value indices[n] holds the vertex index that vertex[n] in the vertex array for the mesh
1, // would have had if no duplication had occurred. Indices in this array that are the same,
2, // therefore, indicate duplicate vertices.
3418,
3419,
1,
62,
11,
3419;
}
MeshMaterialList {
1; // nMaterials: A DWORD. The number of materials
6841; // nFaceIndexes: A DWORD. The number of indices.
0, // faceIndexes[nFaceIndexes]: An arrray of DWORDs containing the face indices
0;
Material {
1.000000;1.000000;1.000000;1.000000;; // faceColor: Face color. A ColorRGBA template.
0.000000; // power: Material specular color exponent.
1.000000;1.000000;1.000000;; // specularColor: Material specular color. A ColorRGB template.
0.000000;0.000000;0.000000;; // emissiveColor: Material emissive color. A ColorRGB template.
TextureFilename {
"Tiny_skin.bmp";
}
}
}
XSkinMeshHeader {
2; // nMaxSkinWeightsPerVertex: Maximum number of transforms that affect a vertex in the mesh
4; // nMaxSkinWeightsPerFace: Maximum number of unique transforms that affect the three vertices of any face
35; // nBones: Number of bones that affect vertices in this mesh
}
SkinWeights {
"Bip01_R_UpperArm"; // transformNodeName
156; // nWeights: the number of vertices affected by this bone
0, // vertexIndices[nWeights]: the vertices influenced by this bone
3449,
1738;
0.605239, // weights[nWeights]: the weights for each of the vertices influenced by this bone
0.979129;
// matrixOffset: The matrix matrixOffset transforms the mesh vertices to the space of the bone.
// When concatenated to the bone's transform, this provides the world space coordinates of the mesh
// as affected by the bone.
-0.941743,-0.646748,0.574719,0.000000,
-0.283133,-0.461979,-0.983825,0.000000,
0.923060,-1.114919,0.257891,0.000000,
-65.499557,30.497688,12.852692,1.000000;;
}
SkinWeights {
"Bip01_Head"; // transformNodeName
1955; // nWeights: the number of vertices affected by this bone
1746, // vertexIndices[nWeights]: the vertices influenced by this bone
3417;
1.000000, // weights[nWeights]: the weights for each of the vertices influenced by this bone
1.000000;
// matrixOffset: The matrix matrixOffset transforms the mesh vertices to the space of the bone.
// When concatenated to the bone's transform, this provides the world space coordinates of the mesh
// as affected by the bone.
0.000000,-0.000002,1.278853,0.000000,
1.112235,-0.156313,-0.000000,0.000000,
0.204616,1.455927,0.000002,0.000000,
-61.950306,-62.105236,-0.142288,1.000000;;
}
} // Mesh
} // frame
} // Body
Frame Box01 {
FrameTransformMatrix {
-1.000000,0.000000,-0.000000,0.000000,
-0.000000,0.000000,1.000000,0.000000,
0.000000,1.000000,0.000000,0.000000,
-88.696747,-246.341751,858.815247,1.000000;;
}
Frame Bip01 {
FrameTransformMatrix {
0.186552,-0.974653,0.123489,0.000000,
0.982171,0.187991,0.000000,0.000000,
-0.023215,0.121288,0.992346,0.000000,
-88.977890,-857.346008,247.541595,1.000000;;
}
}
}
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.