这些层可以被动态的增加或删除，你也可以对这些层进行改动，因为每层都有一个轨道，你可以在轨道中的特定时间点加入关键帧到达改变动画的目的。
These layers and be added or removed dynamically and you can change each layer as it has a track that lets you change it as a function of time by inserting keys at certain frames in the track.
从而为一般的关键帧动画理论在三维交互环境中的具体应用提供了一个现实的实践模型。
By all these, it has provided an implemental model for the general Key-Frame Animation Theory.
记住你可能在动画中要修改定点的位置，所以在你的关键帧结构体中，除了角度和长度值外，你可能还要向修改坐标值。
Also remember that you may want to change vertexes positions during the animation, so in your Keyframe structure, in addition to Angle and length fields, you may want to modify the coords values.
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