[FFmpeg-user] Image extraction broken with ffmpeg, but not with mencoder. How can I fix this?

[James Board]

>> My question is: what does -vsync do?  And, why does it work (so

>> I can avoid this problem in the future)?
>>  
>
>See http://ffmpeg.org/ffmpeg.html and search for vsync. All the answers 
>to your present and past problems are there.

Already did that.  This is what it says about -vsync 0:

    ‘0, passthrough’
    Each frame is passed with its timestamp from the demuxer to the muxer. 
So why does ffmpeg duplicate frames when I don't use '-vsync 0'.  And
why does it not duplicate frames when I use '-vsync 0'.  Why does it matter
what timestamps is passed to the muxer?  I'm generating individual files as
output so why does the timestamp matter?