me of each of these types can then be represented by 2, 3, 4,
5, or 6 numbers respectively, packed into a vector <code/>frame</code>
using the conventions detailed in <code/>vl_plotframe</code>.</p>

<h1 id="id-595">Features frames as geometric frames</h1>

<p>The purpose of a frame is twofold. First, it specifies a local
image region. Second, and perhaps more importantly, it specifies an
image transformation. A frame instance can in fact be thought as a
transformed variant of a <em>canonical</em> or <em>st