title: How to overlay two videos with blend filter in ffmpeg
body: <p>I need to do a lot of videos with the next specifications:</p>
  
  <ul>
  <li>A background video (bg.mp4)</li>
  <li>Overlay a sequence of png images img1.png to img300.png (img%d.png) with a rate of 30 fps</li>
  <li>Overlay a video with dust effects using a blend-lighten filter (dust.mp4)</li>
  <li>Scale all the inputs to 1600x900 and if the input have not the aspect ratio, then crop them.</li>
  <li>Specify the duration of the output-video to 10 sec (is the duration of image sequence at 30fps).</li>
  </ul>
  
  <p>I've being doing a lot of test with different commands but always shows error.</p>