[FFmpeg-user] cannot seek into png 'video'

[Thorsten von Eicken]

I'm trying to put an overlay over a video and I'm having difficulties 
seeking into the png in order to time-shift the video and overlay 
relative to one another. In short, using `-ss` on the png results in an 
error like "overlay.png: could not seek to position 10.000". Is there a 
work-around?

What I'm doing is I'm generating a png 'video' for the overlay, i.e., a 
file with png images one after the other at a frame rate of 1 fps. I can 
do the overlay with something like:

ffmpeg -i 20190928_155656_stab.mp4 -framerate 1 -c:v png -i overlay.png \
     -filter_complex "[0:v]setpts=PTS-STARTPTS[top]; 
[1:v]setpts=PTS-STARTPTS[bottom]; 
[top][bottom]overlay=x=(W-w)/2:eof_action=pass" \
     -acodec aac -vcodec libx264 final.mp4

If I'd now like to skip the first 10 seconds of the overlay in order to 
line up the timing better I try to use:

ffmpeg -i 20190928_155656_stab.mp4 -framerate 1 -ss 10 -c:v png -i 
overlay.png ...

But that results in the error. Using `-ss` on the mp4 input works great 
to time-shit in the other direction.

Can anyone recommend a work-around?