Deformation Algorithm - Technical question doubt

It is a basic mesh skinning method.

For each vertex of the mesh we calculate a local coordinate system rooted on 4 (or 3 if we can't find all 4 nodes) of the physics skeleton's nodes. Vertex Lattice works in a similar way (by using local coordinate systems).

 

The "rooted" means that the local coordinate system is overlaid on the 4 nodes. One node is the local (0,0,0) and the other three are the X, Y, Z axes.

There are multiple different strategies for finding good nodes like these. The basic constraints (i don't want to write a paper here), are that good nodes should be relatively close to the vertex (you don't want nodes in the back of the car to affect a hood vertex), and they should NOT be degenerate (all in the same line and so on).