title: Access element of float cv::Mat
body: <p>I don't understand why I can't get this code to work:</p>
  
  <pre><code>cv::Mat M(2, 3, CV_32FC1);
  cv::Point2f center(20, 20);
  M = cv::getRotationMatrix2D(center, 20, 1.0);
  float test;
  test = M.at&lt;float&gt;(1, 0);
  test = M.at&lt;float&gt;(0, 1);
  test = M.at&lt;float&gt;(1, 1);
  </code></pre>
  
  <p>The code fails when accessing the elements with M.at. The following assertion comes up:</p>
  
  <pre><code>OpenCV Error: Assertion failed (dims &lt;= 2 &amp;&amp; data &amp;&amp; (unsigned)i0 &lt; (unsigned)si
  ze.p[0] &amp;&amp; (unsigned)(i1*DataType&lt;_Tp&gt;::channels) &lt; (unsigned)(size.p[1]*channel
  s()) &amp;&amp; ((((sizeof(size_t)&lt;&lt;28)|0x8442211) &gt;&gt; ((DataType&lt;_Tp&gt;::depth) &amp; ((1 &lt;&lt; 3
  ) - 1))*4) &amp; 15) == elemSize1()) in unknown function, file C:\OpenCV2.2\include\
  opencv2/core/mat.hpp, line 517
  </code></pre>