Re: [Assimp-discussions] Nodes and Bones

[Thomas S. <th...@dr...>]

Hi Carsten,

Thanks for your nice words. Concerning bone animations: I already had my  
share of explanation tries. My last and most exhaustive version can be  
found here:  
http://sourceforge.net/projects/assimp/forums/forum/817654/topic/3880745

Nodes: are local named coordinate systems that specify a transformation in  
relation to their parent. They're standard building blocks of a scene  
hierarchy. By default, they don't have anything to do with bones.

Bones: are math helper structures that describe how certain parts of a  
mesh deform with the movement of a scene node. The bone's name matches  
exactly one node's name - that's the connection between bones and nodes.  
The bones describes which mesh parts are affected, the node describes the  
current pose of the skeleton. The bone's offset matrix desribes the  
transformation from mesh space to local bone space. The node's current  
transformation in concatenation with its parents describes the other way  
around. Together they form the bone's transformation matrix which you can  
use to deform vertices, e.g. in a vertex shader.

>      - Why do they exist at all (there are aiNodes already)?
Bones contain vertex weights and the offset matrix, sometimes also called  
inverse bind matrix.

>      - Does a bone with name "x" unambiguously refer to a node with the  
> same name "x"?

Yes. This is also validated by Assimp if you specified the corresponding  
post processing step on import. You have to instruct your artists to  
comply, though. In my experience they tend to do this automatically, but  
copy&paste sometimes bites you in the back.

>      - If so, why do bones have another 4x4 offset matrix? (are bones a  
> kind of sub-node?)

The bone's offset matrix describes the transformation from mesh space to  
local bone space. The corresponding node and its parents describe the  
reverse direction.

>      - An aiAnimation instance animates aiNodes (never aiBones), right?
Yes.

>      - How do I compute the coordinate of an animated vertex?

See the link above. Good luck.

Bye, Thomas