title: How to set zeros values in a part of cv::Mat
body: <p>I have a <code>cv::Mat</code> of a RGB image as</p>
  <pre><code>cv::Mat cv_img 
  </code></pre>
  <p>I want to set zeros value for cv_img at some positions. For example from the bottom to the half location of the image will be filled by zero values. How can I do it in c++ and opencv? Thanks all.</p>
  <p>I have searched a setTo function and <code>mask</code> may be a candidate solution, but how to define a binary mask is difficult for me.</p>
  <pre><code>cv_img.setTo(Scalar(0,0,0), mask);
  </code></pre>