[Libav-user] how to predict the size of destination clips
steinbach at scionics.de
Mon Jun 6 09:27:05 CEST 2016
> How would that work? (in theory)
Pretty simple from what I understand. You simply assume the worst case
scenario: (speaking in h264/hevc tongue) that would mean every frame is
an reference frame (IIRC the terminology) and multiply by the number of
frames you expect plus the overall encoding overhead. LZW-type encoders
do it this way:
> Either you did set an (average) bitrate when starting the
> encoder, then you know which value you chose and if you
> also know the length of the input, you can calculate the
> size of the output file.
In my use case, this is not possible as I am after the highest encoding
bitrate possible and on top, I do know the number of frames to come.
> FFmpeg generally cannot know the length of the input.
Well, because the API is built around a frame/packet concept rather then
a complete input buffer. If I demux/convert a video, I also know the the
length/size of the incoming video. It's only if you stream from a
recording device that you don't know the length of the recording to come.
> If you did not set a bitrate (but constant quality) then
> it is impossible to calculate the output file size.
I doubt that. But feel free to prove me wrong. Of course, it will be
hard to predict the exact size of the encoded buffer. But that is not
what I am after. I would love to have an upper bound with some degree of
More information about the Libav-user