Barycentric[XYZ m]

Here you define the position that should be recalculated as a barycentric vector, relative to the triangle defined by the positions A, B and C.

A[XYZ m]

B[XYZ m]

C[XYZ m]

These three vectors describe to positions of the three vertices of a triangle. The relative location of the position to this triangle is output as a barycentric vector.