// get vertex indices for facea = pIndices[ j+0 ];b = pIndices[ j+1 ];c = pIndices[ j+2 ];// get edge 1 of polyedge1[0] = v[b].x  - v[a].x;	// xedge1[1] = v[b].y  - v[a].y;	// yedge1[2] = v[b].z  - v[a].z;	// zedge1[3] = v[b].tu - v[a].tu;	// s// get edge 2 of polyedge2[0] = v[c].x  - v[a].x;	// xedge2[1] = v[c].y  - v[a].y;	// yedge2[2] = v[c].z  - v[a].z;	// zedge2[3] = v[c].tu - v[a].tu;	// s// cross product of edges is d_du vectord_du.x = edge1[3]*edge2[0] - edge1[0]*edge2[3];d_du.y = edge1[3]*edge2[1] - edge1[1]*edge2[3];d_du.z = edge1[3]*edge2[2] - edge1[2]*edge2[3];d_du = Normalize( d_du );// texture space z is same as smoothed face normaltz.x = v[a].nx + v[b].nx + v[c].nx;tz.y = v[a].ny + v[b].ny + v[c].ny;tz.z = v[a].nz + v[b].nz + v[c].nz;tz = Normalize( tz );// use dot projection to make a perpendicular to tzt = DotProduct( d_du, tz );tx.x = d_du.x - (t * tz.x);tx.y = d_du.y - (t * tz.y);tx.z = d_du.z - (t * tz.z);// tx x tz gets final perpendicular vector tyty = CrossProduct( tx, tz );