propto \begin{bmatrix}1\\ 0\end{bmatrix}.
\]

<p>This effect can be better understood by starting from some oriented
frames, removing the orientation, and then
using <code/>vl_frame2oell</code> to generate oriented ellipses back:
in the process, orientation information is lost and replaced by a
conventional orientation:</p>

<div class="figure">
 <img src="../demo/frame_frame2oell.jpg"></img>
 <div class="caption">
  <span class="content">
    Top: randomly sampled oriented ellipses. Middle: the same ellips