title: What are opencv_ffmpeg and opencv_world for?
body: <p>I am a novice in OpenCV and start learning from now on.
  From the zipped file downloaded from the official site, we have </p>
  
  <pre><code>opencv_ffmpeg341_64.dll
  opencv_world341.dll
  opencv_world341d.dll
  </code></pre>
  
  <p>in <code>c:\opencv\build\x64\vc15\bin\</code></p>
  
  <p>However in <code>c:\opencv\build\bin\</code>, there are only</p>
  
  <pre><code>opencv_ffmpeg341.dll
  opencv_ffmpeg341_64.dll
  </code></pre>
  
  <p>As far as I know, the application only needs either one of these files in <code>c:\opencv\build\bin\</code>.</p>
  
  <h1>Question</h1>
  
  <p>What are the following files for?</p>
  
  <pre><code>opencv_world341.dll
  opencv_world341d.dll
  </code></pre>
  
  <p>I also knew that <code>d</code> means debug.</p>