title: Convert FLV video with alpha channel to PNGs with transparency
body: <p>I have some FLV videos with alpha channels, and I want to convert each of them to PNG images using ffmpeg but keep the transparency.</p>
  
  <p>So far, I've tried this:</p>
  
  <pre><code>ffmpeg -i input.flv -an -y %d.png
  </code></pre>
  
  <p>But this outputs the PNG files with black background.</p>
  
  <p>Is there any way to do this? </p>
  
  <p><em>Alternate acceptable solution</em>: If I can output the images and give the alpha channel a certain color of my choice. I can then remove it later via imagemagick and convert that color to transparency.</p>