if and only if within the window varies along the direction but is constant along . This condition of eigenvalues is also called linear symmetry condition because then the iso-curves of consist in parallel lines, i.e there exists a one dimensional function which can generate the two dimensional function as for some constant vector and the coordinates .

If , on the other hand, the gradient in the window has no predominant direction; which happens, for instance, when the image has rotational symmetry