Binding to Geometry
An integral part of rigging a character or an object with a skeleton is binding, also known as skinning. Binding is another way to attach geometry to a skeletal system. With the block man you directly attached the whole pieces of geometry to the bones through parenting, whereas binding involves attaching clusters, or groups of vertices or CVs, of the geometry to the skeleton to allow it to deform the model. This is typically how skeletons are used in character animation work. (For more on grouping and parenting, refer to the Solar System exercise in Chapter 3, "Your First Maya Animation.")
The basic technique of binding a character is easy. However, Maya gives you tremendous control over how your geometry deforms.
Binding Overview
Binding is, in theory, identical to the lattice deformer you saw in Chapter 6, "Further Modeling Topics: Deformers and Subdivision Surfaces." A lattice attached to an object exerts influence on parts of the model according to the sections of the lattice. Each section affects a NURBS surface's CVs or a polygon surface's vertices within its borders, and as a section of the lattice moves, it takes those points of the model with it.
Skeletal binding does much the same thing. It attaches the model's points to the bones, and as the bones pivot around their joints, the section of the model that is attached follows.
By attaching vertices or CVs (depending on your geometry) to a skeleton, you can bend or distort the geometry. When a bone moves or rotates about its joint, it pulls with it the points that are attached to it. The geometry then deforms to fit the new configuration of the bones bound to it.
You can directly bind geometry to a skeleton in two ways: using Smooth Bind and using Rigid Bind. You can indirectly deform geometry using deformers and lattices attached to skeletons, but here you'll use the direct methods. Figure 9.20 shows a rigid bind, and Figure 9.21 shows a smooth bind.
Create a tall NURBS cylinder, with a span of 16 or more. The more spans you have in the deformable model, the better it will bend. Duplicate the cylinder, and move it over in your window. Now in the front view, create a four-bone (five-joint) skeleton that starts at the bottom of the first cylinder and goes straight up the middle, ending at the tip. Duplicate the skeleton, and move it to the center of the second cylinder.
CREATING A RIGID BIND
A rigid bind is the simpler of the two, because only one surface point (vertex or CV) is affected by a joint at a time. A rigid bind groups the CVs of a NURBS or the vertices of a polygon into joint clusters that are then attached to the bones. No one surface point is influenced by more than one joint.
Figure 9.21 Smooth bind of the cylinder shown in Figure 9.20. The crease is smoother, yet less defined.
Figure 9.21 Smooth bind of the cylinder shown in Figure 9.20. The crease is smoother, yet less defined.
Therefore, bending a model about a joint with a rigid bind yields a more articulate crease than a smooth bind, which allows more than one joint to affect the CV or vertex, resulting in a more rounded and smooth bend.
To create a rigid bind, select the first skeleton and Shift+click its cylinder. In the Animation menu, choose Skin * Bind Skin * Rigid Bind □.
In the option box, you'll find that almost everything you need is already set to the default. The Bind To parameter lets you rigid bind the entire skeleton to the geometry or just the joints selected. Using Selected Joints gives you the option of using just part of a skeleton system to rigid bind, which also gives you flexibility in how your rig affects the model. You'll leave that option on Complete Skeleton to attach the whole thing.
Click the Color Joints check box to set a different color for each joint in the bind, which can make for an easier workflow. The Bind Method parameter deals with how the points in the model will be attached. The default, Closest Point, organizes the points into skin point sets according the joint to which they are closest. They are then assigned to be influenced by that joint only.
The Partition Set option lets you define your own points before you bind and select which points are set to which joints. If you define a partition set for each joint you have, Maya assigns each set to the nearest joint. For example, you can define some points at the top of the surface to be a part of a set controlled by a joint in the bottom part. Closest Point is the best option for most work.
Using the defaults and turning on Color Joints, click the Bind Skin button in the option box. The root of the skeleton is now selected and the cylinder turns magenta, signifying it has input connections (such as history).
CREATING A SMOOTH BIND
A smooth bind allows a joint to influence more than one skin point on the model. This allows for areas of the model farther from the joint to bend when that joint rotates. Joints influence points to varying degrees between 0 and 1 across the surface, decreasing in influence the farther the point is from the joint. The multiple influences on a point all need to add up to 1 across all the joints that influence it. Maya automatically generates the proper influence amounts upon binding, although the animator can change these values later.
To create a smooth bind, select the second skeleton and its cylinder, and choose Skin * Bind Skin * Smooth Bind □.
In the option box, you'll find the familiar Bind To parameter. You will also find, under the Bind Method drop-down menu, the options Closest In Hierarchy and Closest Distance. Choosing Closest In Hierarchy assigns the skin points to the nearest joint in the hierarchy. This is most commonly used for character work, because it pays attention to the way the skeleton is laid out.
For example, a surface point on the right leg would not be affected by the thigh joint on the left leg simply because it is near it on the model. Closest Distance, on the other hand, disregards a joint's position in the hierarchy of the skeleton and simply assigns influences according to how far the point is from the joint.
Max Influences sets a limit on how many joints can affect a single point. Dropoff Rate determines how a joint's influence diminishes on points farther from it. For example, with Smooth Bind, one shoulder joint can influence, to varying degrees, points stretching down the arm and into the chest and belly. By limiting these two parameters, you can control how much of your model is pulled along by a particular joint.
Using all the defaults is typically best. So click Bind Skin in the option box to smooth bind your second cylinder to the bones.
Bend both cylinders to get a feel for how each creases at the bending joints. Figure 9.22 shows the difference.
- Figure 9.22 Rigid and smooth bound cylinders. The smaller cylinder is rigid bound, and the larger is smooth bound.
DETACHING A SKELETON
If you want to do away with your binding, select the skeleton and its geometry and choose Skin * Detach Skin. The model will snap to the shape it had before the bind was applied and the joints were rotated. It's common to bind and detach skeletons several times on the same model as you try to figure out the exact configuration that works best for you and your animation.
If you need to go back to the initial position of the skeleton at the point of binding it to the model, you can automatically set the skeleton back to the bind pose after any rotations have been applied to any of its joints. Simply select the skeleton and choose Skin * Go To Bind Pose to snap the skeleton and model into the position they were when you bound them together. It is also best to set your skeleton to the bind pose whenever you edit your binding weights.
A MODELING TRICK USING A SKELETON
An easy way to create bends and creases in a model is to create the surface without the bend and use a skeleton to deform it the way you want. You can then detach the skin and bake the history so that the surface retains its deformation but loses its connection to the skeleton. Bind your geometry to the skeleton chain using Smooth Bind or Rigid Bind. Bend the skeleton to deform the geometry, and then choose Skin * Detach Skin □. In the option box, set the History parameter to Bake History and click Detach. The model will retain its deformed state but will lose all connections to the skeleton. This is just like using a nonlinear or lattice deformer on an object and then deleting the object's history to rid it of the deformer. With a detached skin, however, you won't lose any other history already applied to that object as you would if you deleted history through the Edit menu.
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