title: Find pixel color out of cv::Mat on specific position
body: <p>well my problem is, I need to find the sub matrix of a cv::Mat image which includes all white pixels. Therefore I want to iterate through all pixels, check if they are white and build a cv::Rect with that information.<br>
  I figured out how to iterate through all the pixels but I don't know how to get the pixels color out of it. The cv::Mat was previously converted to greyscale with <code>CV_GRAY2BGR</code></p>
  
  <pre><code>for(int y = 0; y &lt; outputFrame.rows; y++)
  {
      for(int x = 0; x &lt; outputFrame.cols; x++)
      {
          // I don't know which datatype I should use
          if (outputFrame.at&lt;INSERT_DATATYPE_HERE&gt;(x,y) == 255)
             //define area
      }
  }
  </code></pre>
  
  <p>My final question is, which datatype should I insert in the code on the position INSERT_DATATYPE_HERE and is 255 the right value to compare with than?</p>
  
  <p>Thanks alot</p>