biped here
Example of biped rig (3DS Max)

Rigging is a technique common in 3D character animation. It involves connecting a 3D character model to a skeleton framework or "rig." When the rig is animated the model character moves with it.

The process involves modelling the character and then developing the rig to suit the model. 3D modelling and animation packages generally have a basic biped and quadruped rigs which can be adapted. If these are not suitable then a rig can be constructed out of armatures and joints. In either case the rig is constructed with joints that match the range of movement of the body parts.

Rigging for facial movements, hand movements etc. can be technically difficult and rigging has become a specialist area in animation. Rigging is generally done with Bones but can be done with the Morpher Modifier

Once rigged the character can be animated by animating the rig controlling the model. Generally this is done using motion capture files to easily animate natural body movements. Once rigged a character can be efficiently animated, but weakness is that it does not accurately represent the movement of muscles under the skin as the character moves.

t pose here
Character in the "T" pose (iClone5)

A common default pose to rig a character up is the "T" pose (see left). This pose is used as it is easy to rig a character with all the limbs well differentiated. It is harder to line the rig up with the model if the arms and legs are bent and/or close to the body.

Animating with a rigged character is usually done with Forward Kinematics. This is a system where every adjustment in postion and rotation of a bone is also applied to every bone down a hierarchy. For example rotating a characters arm at the shoulder affects their whole arm.

The opposite of Forward Kinematics is Reverse Kinematics. This is use for animating motion, such as where you would rote and translate a persons whole body around their ankle when they take a step.

Placing the Skeleton: Placement of a skeleton is perhaps the easiest part of the rigging process. For the most part, joints should be placed exactly where
they would be in a real world skeleton, with one or two exceptions.

Joint Hierarchy: In order for a rig to work properly, the bones and joints must follow a logical hierarchy. When setting up a character's skeleton,
the first joint you place is called the root joint. Every subsequent joint will be connected to the root either directly, or indirectly through another joint.
Forward Kinematics: Forward kinematics (FK) is one of two basic ways to calculate the joint movement of a fully rigged character. When using FK rigging,
a any given joint can only affect parts of the skeleton that fall below it on the joint hierarchy.
For example, rotating a character's shoulder changes the position of the elbow, wrist, and hand. When animating with forward kinematics,
the artist typically needs to set the rotation and position of each joint individually—to achieve a desired pose the animator would work through the
joint hierarchy sequentially: root → spine → shoulder → elbow → etc. The final position of a terminating joint (like a knuckle) is calculated as a function
of the joint angles of every joint above it in the hierarchy.

This tutorial covers the rigging of a biped using Autodesk Maya. Defining a rig and then animating the character.

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