3D Game Animation: Creating Realistic Skin
In 3D game animation, the character model that is applied to a bone structure is called a skin. The skin should be one combined object with no overlapping parts. You can think of the skin as a glove that is placed over a hand. The glove doesn’t move when it is off the hand, but when the fingers are moved underneath, the glove moves and bends along with the fingers. If there are any holes in the glove, they get stretched open as the fingers move. The same works for a skin over a bone structure.
Skinning a character
Character models typically are created one part at a time, but to make a model into a skin, you need to combine all parts into a single object. If the model consists of any groups, you should dissolve these groups. Objects still can be attached to the skin, but all parts of the skin that move with the bones need to be able to bend and flex.
After a skin is attached to a skeleton, the second part of skinning is to make sure that the skin deforms well when the bones are moved. Each bone has an influence volume that defines the portions of the skin that moves with the bone. If these influence volumes are incorrect, then the skin can deform unrealistically when the bone is moved.
For example, if the influence volume for the upper arm includes part of the side of the character, then rotating the upper arm pulls the character’s side out as the arm is raised, causing a funny-looking bump. To control these deformations, you can precisely control the influence volume. Attaching a skin to a skeleton
After a bone structure is created, it can be attached to the character skin that has already been created.
Some software programs make this a separate step, and others simply require that the bones be positioned within the skin.
Follow these steps to attach a skin to a bone structure in Maya:
1. Choose File –> Open Scene, and open the IK enabled.mb file.
This file is the same file that was saved at the end of the preceding example. Be sure to move the bones back into place before attaching the skin.
2. Select the character model, press and hold the Shift key, and select the pelvis bone. Then choose the Skin –> Bind Skin –> Smooth Bind menu command.
The skin is now attached to the bone structure, so moving any of the bones moves the character model with the bone.
3. Select the ankle joint with the IK chain attached, and move the ankle forward in the Front view. Then select and rotate one of the arms about its upper arm joint.
With the skin binding in place, the character model moves along with the bone movements.
4. Select the File –> Save Scene As menu command, and save the file. Close the file when you are finished.
Setting a bone’s influence
Surrounding each bone is an influence volume (called an envelope in several animation packages) that you can control. The skin that is within this volume moves along with the underlying bone, and the skin that is included in overlapping influence volumes are the areas of the skin where the bending is most extreme, such as at the elbow.
The influence volume is initially set to surround each bone, and the closer the bone matches the skin for each body part, the more accurate the influence volumes are. If any portion of the skin isn’t included within any influence area, then that portion of the skin is left behind when its bones are moved.
Influence volumes can be altered using controls to increase their radius on either end of the bone.
Another way to control which skin parts deform with the underlying bone is to paint the skin vertices using a paint brush. Select a bone and then paint the weight. Different colors depict the skin vertices’ influence. It is important to have a gradual change between skin influences so that the skin moves realistically.
Adding deformations to create muscle bulges
Another aspect of working with the skin object is that you can define deformations based on bone positions. This provides a way to make muscles bulge. For example, deforming the bicep muscle to bulge when the angle between upper and lower arm bones is shortened is a realistic deformation that you’d expect from a realistic character.