dx=0.5*(x3+x4-x1-x2)
dx=dx*
& (tmp_u(i,j,bi,bj)*dx+
& sqrt(dx*dx+dy*dy)
dx=0.5*(x2+x4-x1-x3)
_RL fac, x1, x2, x3, x4, y1, y2, y3, y4, dx, dy