title: What is meant by img.at<uchar>(i,j)?
body: <p>Its kind a basic question.  I don't know what this structure.</p>
  
  <pre><code>img.at&lt;uchar&gt;(i,j) 
  </code></pre>
  
  <p>is meant to be? I try to find the definition of <strong><code>at</code></strong>, but can't understand it. and what this  syntax means?  </p>
  
  <pre><code>&lt;uchar&gt; 
  </code></pre>
  
  <p>Similarly, what is meant by </p>
  
  <pre><code>img.at&lt;cv::Vec3b&gt;(row,col)[channel] 
  </code></pre>
  
  <p>Yes, <code>Mat</code> is matrix class to manipulate matrix data. I understand the behavior of those above lines but can't understand theory behind them? </p>
  
  <p>What does this syntax mean?</p>
  
  <pre><code>img.at&lt;cv::Vec3b&gt;
  </code></pre>