i>
<li><em>ellipses</em> defined by their center $T = (x,y)$, and a
positive semidefinte matrix $\Sigma$ such that the ellipse is the set
of points $\{\bx \in \real^2: (\bx-T)^\top\Sigma^{-1}(\bx-T)=1\}$;</li>
<li><em>oriented circles</em> defined by their center $(x,y)$, their
radius $\sigma$, and rotation $\theta$;</li>
<li>and <em>oriented ellipses</em> defined by an affine transformation
$(A,T)$, where $A\in\real^{2\times2}$ is the linear component and
$T\in\real^2$ the translation.</li>
</ul>

<p>A fra