title: ffmpeg how to create a effect video like: flower fall?
body: <p>I try to create an effect video like: flower fall. I have a main video (main.mp4) and a effect video (flower_fall.mp4). I want to make a output.mp4 video that merge 2 this videos and the result look like main video is a bottom layer and effect video is a top layer(opacity about 0.5 or something like that).  What is the ffmpeg command for solve my target? </p>