 a circle, or
an ellipse representing the location and shape of an image
feature. Frame types are closed under certain classes of
transformations of the plane (for example circles are closed under
similarity transformations) and can be used in corresponding
co-variant feature detectors.</p>

<h1 id="id-579">Types of frames</h1>

<p>VLFeat uses five types of frames:</p>

<ul>
<li><em>points</em> defined by their center $(x,y)$;</li>
<li><em>circles</em> defined by their center $(x,y)$ and radius
$\sigma$;</l