The tangents of the mesh.
w
used to flip the binormal if needed.Unity calculates the other surface vector (binormal) by taking a cross product between normal
and tangent, and multiplying result by tangent.w. Thus w
should always be 1 or -1.You should calculate tangents yourself if you plan to use bump-mapped shaders on the mesh.
Assign tangents after assigning normals or using RecalculateNormals.