Skinning a character

6 min read

(For more resources related to this topic, see here.)

Our world in 5000 AD is incomplete without our mutated human being Mr. Green. Our Mr. Green is a rigged model, exported from Blender. All famous 3D games from Counter Strike to World of Warcraft use skinned models to give the most impressive real world model animations and kinematics. Hence, our learning has to now evolve to load Mr. Green and add the same quality of animation in our game.

We will start our study of character animation by discussing the skeleton, which is the base of the character animation, upon which a body and its motion is built. Then, we will learn about skinning, how the bones of the skeleton are attached to the vertices, and then understand its animations. In this article, we will cover basics of a character’s skeleton, basics of skinning, and some aspects of Loading a rigged JSON model.

Understanding the basics of a character’s skeleton

A character’s skeleton is a posable framework of bones. These bones are connected by articulated joints, arranged in a hierarchical data structure. The skeleton is generally rendered and is used as an invisible armature to position and orient a character’s skin.

The joints are used for relative movement within the skeleton. They are represented by a 4 x 4 linear transformation matrices (combination of rotation, translation, and scale). The character skeleton is set up using only simple rotational joints as they are sufficient to model the joints of real animals.

Every joint has limited degrees of freedom (DOFs). DOFs are the possible ranges of motion of an object. For instance, an elbow joint has one rotational DOF and a shoulder joint has three DOFs, as the shoulder can rotate along three perpendicular axes. Individual joints usually have one to six DOFs. Refer to the link to understand different degrees of freedom.

A joint local matrix is constructed for each joint. This matrix defines the position and orientation of each joint and is relative to the joint above it in the hierarchy. The local matrices are used to compute the world space matrices of the joint, using the process of forward kinematics. The world space matrix is used to render the attached geometry and is also used for collision detection.

The digital character skeleton is analogous to the real-world skeleton of vertebrates. However, the bones of our digital human character do have to correspond to the actual bones. It will depend on the level of detail of the character you require. For example, you may or may not require cheek bones to animate facial expressions.

Skeletons are not just used to animate vertebrates but also mechanical parts such as doors or wheels.

Comprehending the joint hierarchy

The topology of a skeleton is a tree or an open-directed graph. The joints are connected up in a hierarchical fashion to the selected root joint. The root joint has no parent of itself and is presented in the model JSON file with the parent value of -1. All skeletons are kept as open trees without any closed loops. This restriction though does not prevent kinematic loops.

Each node of the tree represents a joint, also called bones. We use both terms interchangeably. For example, the shoulder is a joint, and the upper arm is a bone, but the transformation matrix of both objects is same. So mathematically, we would represent it as a single component with three DOFs. The amount of rotation of the shoulder joint will be reflected by the upper arm’s bone.

The following figure shows simple robotic bone hierarchy:

Understanding forward kinematics

Kinematics is a mathematical description of a motion without the underlying physical forces. Kinematics describes the position, velocity, and acceleration of an object. We use kinematics to calculate the position of an individual bone of the skeleton structure (skeleton pose). Hence, we will limit our study to position and orientation. The skeleton is purely a kinematic structure. Forward kinematics is used to compute the world space matrix of each bone from its DOF value. Inverse kinematics is used to calculate the DOF values from the position of the bone in the world.

Let’s dive a little deeper into forward kinematics and study a simple case of bone hierarchy that starts from the shoulder, moves to the elbow, finally to the wrist. Each bone/joint has a local transformation matrix, this.modelMatrix. This local matrix is calculated from the bone’s position and rotation. Let’s say the model matrices of the wrist, elbow, and shoulder are this.modelMatrixwrist, this.modelMatrixelbow, and this.modelMatrixshoulder respectively. The world matrix is the transformation matrix that will be used by shaders as the model matrix, as it denotes the position and rotation in world space.

The world matrix for a wrist will be:

this.worldMatrixwrist = this.worldMatrixelbow * this.modelMatrixwrist

The world matrix for an elbow will be:

this.worldMatrixelbow = this.worldMatrixshoulder * this.modelMatrixelbow

If you look at the preceding equations, you will realize that to calculate the exact location of a wrist in the world space, we need to calculate the position of the elbow in the world space first. To calculate the position of the elbow, we first need to calculate the position of shoulder. We need to calculate the world space coordinate of the parent first in order to calculate that of its children. Hence, we use depth-first tree traversal to traverse the complete skeleton tree starting from its root node.

A depth-first traversal begins by calculating modelMatrix of the root node and traverses down through each of its children. A child node is visited and subsequently all of its children are traversed. After all the children are visited, the control is transferred to the parent of modelMatrix. We calculate the world matrix by concatenating the joint parent’s world matrix and its local matrix. The computation of calculating a local matrix from DOF and then its world matrix from the parent’s world matrix is defined as forward kinematics.

Let’s now define some important terms that we will often use:

  • Joint DOFs: A movable joint movement can generally be described by six DOFs (three for position and rotation each). DOF is a general term:

    this.position = vec3.fromValues(x, y, z); this.quaternion = quat.fromValues(x, y, z, w); this.scale = vec3.fromValues(1, 1, 1);

    We use quaternion rotations to store rotational transformations to avoid issues such as gimbal lock. The quaternion holds the DOF values for rotation around the x, y, and z values.

  • Joint offset: Joints have a fixed offset position in the parent node’s space. When we skin a joint, we change the position of each joint to match the mesh. This new fixed position acts as a pivot point for the joint movement. The pivot point of an elbow is at a fixed location relative to the shoulder joint. This position is denoted by a vector position in the joint local matrix and is stored in m31, m32, and m33 indices of the matrix. The offset matrix also holds initial rotational values.

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