title: (FFMPEG) Make areas transparent before overlaying a vid with perspective
body: <p>I'm trying to add perspective to a small video then overlay it on top of an other.</p>
  
  <p>So far I can make the video small, add perspective to it and overlay it to an other video. but after applying the perspective filter, the excess areas don't turn transparent, they just stretch out the pixels.</p>
  
  <p>the perspective filter doesn't have much documentation but as far as i could find out there was no way of setting the extra pixels to transparent.</p>
  
  <p>So I'm guessing ill need to apply some kind of alpha mask, to the desired area?</p>
  
  <p>this is the command I'm using for perspective:</p>
  
  <pre><code>ffmpeg -i /synced_folder/testvid.mp4 -vf perspective=0:0:W:H/4:0:H:W:3*H/4:0:1:0 /synced_folder/output5.mp4
  </code></pre>
  
  <p>Don't think it matters but I'm running ffmpeg on a VM with centOS.</p>
  
  <p><a href="http://i.stack.imgur.com/PM2Zw.png" rel="nofollow">This is a SS of how the result looks like:</a></p>