I found this interesting method to solve body turn after I learn Selgin's way to solve this problem in this post,and maybe it will be a new way to making animation.

and these are some Renderings:


Now, I will explain how it works step by step,and hope someone can help me improve this technique.

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First of all , we can understand "follow path" better from the picture below.And I think some points should be figured out:
- follow path layer can be rotate in 3D environment.
- 3D-rotating "follow path" can make the action layers works awesome ,even the layer sort problem can be automatically solved with "sort layers by depth" option.
- with “rotate to follow path” option the action layers will works like 3D objects.

and the anme file
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Secondly , After we understand the principle of 3D follow path,we can do more with it .
Organized our thoughts,we can conclude that follow path can be regard as 3D line which will be a simpler one for us to operate it than 3D object.And the layers binded to follow path can be regard as 2D paper,which means 2D papers will move to the right position when 3D line rotating or moving.
So , we can regard the 3D line as bone or auxiliary line ,which means we can apply the charactor's action to the 3D line, and the 2D papers will move and rotate right automatically. and I make a similar one as the example in Selgin's tutorial:


and the anme file
As we can see that with the "sort layers by depth " option selected , the depth problem can be solved automatically,isn't it awsome?
tips:the black lines above are 3D lines in "follow path" group.

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Thirdly , we can make a more complex one ,and what we will know is that the follow path can be reused so easy,thought it is complex.That means when we complete an action like walking , dancing , fighting or others, just import the follow path group and bind the new chacractor , and then ,it works fine.awesome? isn't it.
So , I make a prototype one:

and the anime file
As we see , the left one is "bones",after we finish a new bone action ,we can apply it to the other one easily.And an interesting thing is , because the bones are 3D line ,we can rotate the whole on the walk action ,yes ,we can use the 3D skill on the 3D lines:

Tip: only the follow path group has animation , all the others' animation are generated . it's awesome.
here I am so exciting about the seperation of action and character set, As we know ,it is difficult to do the two parts the same time,but with 3D tool ,we can make action so easy,even more,the Video tracking can be use to generate the action , and if someone write a script to translated the universal action file like .bvh to the AS one,the world will became so bright.

Just imaging that an animation task come , the storyboard,the action library and character set can be work the same time. woo~

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fourthly , we can find out that some detail still imperfect in the prototype above, what i want to say is that using the 3D line to handle the overall , and the 2D paper to handle the detail just like the tradition animation draw.
but,some deeper thought somes out ,when facing the detail problem. All we know is character set complete a character's different angle apperance,so two solution comes out:
- use the blend morphs to generate the 3D line animation and the 2D paper animation.
- use an script to bind blend morphs and 3D line layer 3D rotate,which means the 2D paper will be applied blend morphs automatically when 3D line is rotated .
and here is my ugly script for the second solution, hope someone can help me finish it .
and this post still modifing,and please give me hands to improve this technique.

one more thing,in my opinion, combining 3D and 2D is a fantasy way for us to mak animation, acquiring 3D's and 2D's strong points, for example, 3D can handle perspective easily,with 2D techniques you can just draw pictures, and it just mixed the 3D and 2D ,that is what i regard as the moho's spirit. hah~