title: FFmpeg: Add alpha channel to a video using a PNG mask
body: <p>I saw that FFmpeg can merge/blend two videos with alpha channel and can encode video with alpha channel using the <code>qtrle</code> codec, but, It is possible to apply a PNG file mask, either a black and white or a PNG with an alpha channel to a video in order to have a video with the alpha channel of the PNG applied?</p>
  
  <p>Here is the concept in images</p>
  
  <p>Original video:
  <a href="https://i.stack.imgur.com/iysoJ.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/iysoJ.jpg" alt="Original video"></a></p>
  
  <p>PNG file with alpha channel, the "mask":
  <a href="https://i.stack.imgur.com/qXS2o.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/qXS2o.jpg" alt="PNG file with alpha channel"></a></p>
  
  <p>And the result:
  <a href="https://i.stack.imgur.com/xmEsM.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/xmEsM.jpg" alt="Result"></a></p>
  
  <p>Or even a black and white image as a mask for alpha channel:
  <a href="https://i.stack.imgur.com/ABKn1.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/ABKn1.jpg" alt="bnw image"></a></p>
  
  <p>Any help would be appreciated, thank you!</p>