title: Should I initialize a cv::Mat
body: <p>I have this code:</p>
  
  <pre><code>mapx.create(image.size(), CV_32FC1);
  mapy.create(image.size(), CV_32FC1);
  </code></pre>
  
  <p>what is the values in the mapx and mapy after this? Are all data is zero?</p>
  
  <p>what about this type of initialization:</p>
  
  <pre><code>  cv::Mat  mapx(image.size(), CV_32FC1);
  </code></pre>
  
  <p>Do I need explicitly set the value of each element to zero?</p>
  
  <p>How can I set the value of each element to say -1?</p>