Before a model can start to be animated it must be bound to a system of joints and control handles. A character rig is essentially a digital skeleton bound to a 3D mesh, the model. Like an actual skeleton the rig acts as joints and bones that can be altered to produce a certain pose. To begin the rigging process the first process is placing the skeleton. Possibly the easiest part of the rigging process as for most of these points, it will be logical where they go basing it on a real skeleton. The next thing you need is a joint hierarchy as for in order for the rig to work properly they must follow the point before them. Much like the hand follows the wrist, wrist follows the elbow, and the elbow follows the shoulder in a real skeleton. The first point is called the root joint and every other joint will be either connected to this point or through another joint to this point.
Kinematics
Once the model and the skeleton is set up we can begin to look at the two types of kinematics that are used to operate the movement when the character is doing something. The two types are:
- Forward Kinematics
- Inverse Kinematics
Forward kinematics is the process in which can move joints normally and how they would if the character was real. This is the process of the joints using the hierarchy to move. For example, rotating the characters shoulder will also alter the position of the elbow, wrist and hand, so when using this technique the animator will have to set the rotation of each part as well, following where the shoulder is moving to, to make the movement seem fluid and natural. However, inverse kinematics is slightly different in which its used we move say the hand, and the software will automatically move the joints above it to where it thinks they should go. For example if the character was walking up a ladder, the hands and feet are the first to move as they can be placed on the rungs directly we can let the software do the the rest. But this can often involve a big cleanup process to get the movement looking correct.
Pros and Cons
There are a few different benefits and drawbacks to take into consideration when thinking about rigging as it can be a difficult process and very time consuming.
Pros
- Animator control fewer characteristics of the model
- Animator can focus on the large scale motion
- Bones are independently moveable
- Animator can focus on the movement of the bone instead of the polygon mesh
Cons
- Does not provide realistic muscle movement or skin motion
- To achieve this muscle controllers must be separately attached the the bones
- Needs physiology experts to help create the muscle realism under the skin
Considerations
When rigging there are a few more things that we have to consider, like the degree of freedom and constraints of a character. When modelling a human for example, we do not have joints that can rotate a full 360 degrees, so we need to take that into account by only bending or moving the joints around one axis to ensure that there are constraints set into place. Another thing that is needed to be thought about during the rigging process is whether the character will have realistic motion or animated and use the squash and stretch technique. This is a technique used in animation to created an over-exaggerated cartoon look so it is important to constraint the joints to suit the needs of the character. The last thing to consider is does the model need to have facial rigging? Facial rigging is usually seperate from the main motion controls as it is a lot more intricate and precise. It is usually very difficult to create a satisfactory facial rig, using the joint/bone structure like the rest of the body, so different solutions to this have now been brought into place.
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