ify a similarity transformation (e.g. a circle leaves the rotation
undetermined).</p>

<p>Since frames specify <em>transformations of the image domain</em>,
i.e.  coordinate changes, they are surrogates of geometric reference
frames. In particular, the mapping from a standard frame to one
measured by a local feature detector is often undone
to <em>normalize</em> the local image appearance, a key process in the
computation of invariant feature descriptors.</p>

<h1 id="id-604">Oriented frames</h1>

<p>While 