title: cv::Exception at memory location
body: <p>I'm new in openCV and I have many problems using it.
  I have to work with a pointer on a cv::Mat and I can't even initialize it:</p>
  
  <pre><code>cv::Mat* _image_3D = new cv::Mat();
  *_image_3D = cv::Mat::zeros(height,width,CV_32FC3);
  Point3f vt = (Point3f)_image_3D-&gt;at&lt;uchar&gt;(0,0);
  std::cout &lt;&lt; vt.x &lt;&lt; " " &lt;&lt; vt.y &lt;&lt; " " &lt;&lt; vt.z&lt;&lt; " ";
  </code></pre>
  
  <p>I thought that <code>zeros</code> would initialize my Matrix, so normally it's possible to access the first point of the matrix, but when I execute the code I get this error:</p>
  
  <p><code>Unhandled exception at 0x761cb727 in glwidget.exe: Microsoft C++ exception: cv::Exception at memory location 0x002eeda0.</code></p>
  
  <p><code>glwidget</code> is my application</p>
  
  <p>I tried to do this to a Matrix that I loaded and  it worked:</p>
  
  <pre><code> Mat mat = imread( "..XXX.jpg",0);
   Point3f vt = (Point3f)mat.at&lt;uchar&gt;(0,0);
   std::cout &lt;&lt; vt.x &lt;&lt; " " &lt;&lt; vt.y &lt;&lt; " " &lt;&lt; vt.z &lt;&lt; " ";
  </code></pre>
  
  <p>I need actually to access/edit the pixels of my matric !</p>
  
  <p>Thanks!</p>