The Cal3D library provides a powerful and flexible, yet easy-to-use animation control system through the so called 'Mixer'. This helper class handles the following things for a model instance:
The addition and removal of animations to the active animation set.
The update of the active animation states including fade in and out.
The weighted blending and prioritized overlay of the active animations according to their type.
The update of the current pose of the skeleton.
When triggering a new animation you can choose between different behaviors. Currently implemented are the cycle and the action types. Once started, a cycle animation will loop until you explicitly stop it. This is well-suited for motions such as walking or swimming. The action type is a one-time animation that is executed only once and is removed automatically from the active animation set thereafter. As you can imagine this is useful for motions such as fighting moves or emotes. There are plans to extend the number of animation types in future releases of the Cal3D library.
Each active animation has a weight associated with it. This makes it possible to fine tune the amount of influence each animation has on the model. Furthermore each animation type has a different priority in the blending process. In the current version of the Cal3D library, the action type has a higher priority than the cycle one. This is necessary to make actions completely overlay the ongoing cycles.
Example 4-3. The waving Limper
Unfortunately, our waving human-like walker from a previous example stumbled and is now limping with his left leg. In addition to the walk animation defined for all bones in the model skeleton, and the locally waving animation, we need an additional limping animation, which has tracks for all the bones in the lower part the body. With the help of the mixer, we can now easily assign a light limping motion to the model by blending the walking animation cycle with the limping animation cycle at a ratio of 40%:60% as example. The waving animation completely overlays the motion for the skeleton part it is defined on, because it was triggered as an action and is therefore of higher priority than the two cycles. When the little injury starts to heal, you can successively lower the weight of the limping animation until you fade it completely out at the end.
There are a few important things you have to take care of to make this blending system work as intended. Make sure that the animations that are used as actions only contain tracks for bones that should really be affected. Otherwise they will completely overlay all cycle animations, because of their higher priority. Cycles that will be blended together should also be synchronized to achieve a good-looking result. Note that this synchronization has to be done for the displacement of the motion only, not for the duration of the animation, as this adjustment is handled in the blending process inside the mixer.