           float ver = ( p11.r * 1.0 + p21.r * 2.0 + p31.r * 1.0 ) + ( p13.r * -1.0 + p23.r * -2.0 + p33.r * -1.0 );

                // map gradients to [0, 1] range
                // hor = (hor + 4.0) / 8.0;
                // ver = (ver + 4.0) / 8.0;

                float Ixx = hor * hor;
                float Iyy = ver * ver;
                float Ixy = hor * ver;

                out_color = vec4(Ixx, Iyy, Ixy, 1);
            }
            else
            {
                // vec4 I = ( PIXEL_AT(-3