Creating a transparent video out of an input video

Question

I'm trying to create a video with a transparent background out of an input video (with the same dimensions, etc.). I tried several strategies (using the chromakey filter and others) and none worked... I got this far:

ffmpeg -i input.mp4 -filter_complex "color=color=black@0.0,format=yuva420p[bg],[bg][0:v]scale2ref[bg][0v],[bg]drawbox=x=10:y=10:w=100:h=100:color=pink@0.5;[0v]nullsink" -c:v qtrle -an "alpha.mov"

• creating a video stream with transparency, and setting a transparent background (black@0.0)
• scaling the transparent video according to the input video
• [for testing purposes: drawing a box over the transparent video. I omitted the part in which I embed subtitles onto the video, for simplicity's sake]
• discarding the input video stream
• removing the audio stream

It basically works, but the processing never ends - I can't manage to limit the output video length so it'll match the input video's length... I've tried using the -shortest option anywhere in the command - with no success.

Answer 1

Ok, so I managed to solve it with a small trick using overlay filter (specifically - its shortest parameter), with its x and y set to some coordinates outside the relevant bounds, so we're left with the effect of limiting the video's length appropriately:

ffmpeg -i input.mp4 -filter_complex "color=color=black@0.0,format=yuva420p[bg],[bg][0:v]scale2ref[bg][0v],[bg]drawbox=x=10:y=10:w=100:h=100:color=pink@0.5[out];[out][0v]overlay=x=100000:y=100000:shortest=1" -c:v qtrle -an "alpha.mov"