This work is part of a many years project ‘full life cycle automation of computer animation generation’. Automatic animation generation requires automatic plot planning. Among various animation planning tasks, it is crucial to plan the moving traces of avatars. The last author of this paper has implemented the animation generation software《Swan》 which includes a path planning module. However, that module generates only a 2D plan path. In this paper, we introduce a recent work of 3D path planning that finds the best 3D path given a 3D environment. Considering the possibility that inadequate 3D environments may request the avatars to undertake special posture while traversing the environment, we combine path planning with posture planning in automatic 3D traverse. The result can be shown in form of animation supported by motion graph techniques. Other contributions of this paper include: development a 2D mathematical model for 3D real environment and an algorithm transforming the latter to the former; a new principle of constraint based avatar sensitive path determination; design a calculus for calculating constraint oriented cost calculation based on spatial movement complexity of motion graph clips.