[FFmpeg-user] ffmpeg bitrate flag

Francois Visagie francois.visagie at gmail.com
Wed Sep 3 12:52:43 CEST 2014

> -----Original Message-----
> From: ffmpeg-user-bounces at ffmpeg.org [mailto:ffmpeg-user-
> bounces at ffmpeg.org] On Behalf Of A. Semprimožnik
> Sent: 03 September 2014 11:47
> To: ffmpeg-user at ffmpeg.org
> Subject: [FFmpeg-user] ffmpeg bitrate flag
> Hi,
> We are using ffmpeg to transcode videos to various video formats. Large
> number of videos is transcoded to MPEG1 and OGG. Those videos are then
> played in javascript video player on web mobile. We noticed that especially
> on OGG, there is a big difference in video quality when changing bitrate.
> My question is, are there any flag in ffmpeg, that sets bitrate dynamically
> based on video content at transcoding time. What I mean by that, for
> example, if a video is static, we don’t need high bitrate for new video when
> decoding (static scene, where just one object is moving a little for example).
> When video is full of action for example, so it is very dynamic, we need
> higher bitrate to achieve a good quality of a video.
> Is there any flag that would determine how dynamic video is? Or
> selects/returns/sets the optimal bitrate for transcoding, based on video’s
> content?

If you specify a maximum (reasonable) bitrate and use two-pass encoding, the encoder will achieve what you want automatically. It will allocate as much bitrate as possible to high-motion or -detail sequences, and as little as possible to the opposite. Two-pass encoding may not be ideal during live streaming; in that case consider "average bitrate encoding". 

Although I'm talking mostly from recent x264 experience, I know ffmpeg's MPEG encoder supports two-pass encoding, and I believe it also supports "average bitrate encoding". See https://trac.ffmpeg.org/wiki/Encode/H.264. 

If OGG supports constant rate factor as x264 does, you could also consider that for live streaming.

> Thank you,
> BR, A. Semprimoznik
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