Bear in mind the explanation why tangent house is used. It is to supply an orientation for the traditional map to know whether or not the traditional must be adjusted within the u or v course.
For that you want to know which manner the u and v axis level on the fragment you might be computing and it must be constant throughout a number of triangles.
Additionally you could have 6 methods to select the sting pairs of a triangle (3 for those who repair winding order), how would you realize within the fragment shader which manner was picked.
