title: What is Vec4b, MatObj.ptr in opencv ?
body: <p>My question is based on the following code.</p>
  
  <pre><code>Vec4b *rv = mMat.ptr&lt;Vec4b&gt; (50);
  </code></pre>
  
  <p>I don't understand what Vec4b means. I know about Vec4i which means line segment coordinates. So similarly I tried to find what it contains.
  The below code</p>
  
  <pre><code>std::cout&lt;&lt;rv[1]&lt;&lt;std::endl;
  </code></pre>
  
  <p>gave an output:</p>
  
  <blockquote>
    <p>[8, 7, 10, 10]</p>
  </blockquote>
  
  <p>I dont know what those parameters mean. Surprisingly it showed outputs for parameters greater than four. Eg., rv[4],rv[5] and so on.</p>
  
  <p>So <strong>I really dont't get what Vec4b does. Also the mMat.ptr.</strong> I Could not find good online sources about Vec4b and Mat.ptr.</p>
  
  <p>Any clarification about what the first code does would really be helpful in enlightening my mind.</p>