ures
computed by <code/><a href=../matlab/vl_covdet.html>vl_covdet</a></code> are <em>oriented ellipses</em> and
are defined by a translation $T$ and linear map $A$ (a $2\times 2$)
which can be extracted as follows:</p>

<pre>
 T = frame(1:2) ;
 A = reshape(frame(3:6),2,2)) ;
</pre>

<p>The map $(A,T)$ moves pixels from the feature frame (also called
normalised patch domain) to the image frame. The feature is
represented as a circle of unit radius centered at the origin in the
feature reference frame, and t