Re: Video Encoding Slowness

On Thu, Dec 29, 2011 at 07:18:01PM +0000, Matt Burgess wrote:
> Hi all,
> 
> I'm looking into ripping my DVD collection into WebM format (VP8 video
> via libvpx, and vorbis audio via libvorbis).  The following command
> works, in that it gives me a suitable quality video with audio in sync:
> 
> ffmpeg -i input.vob -aspect 16:9 -vpre libvpx-720p -aq 5 -f webm -y \
>   output.webm
> 
> However, it takes a long time to encode.  It averages around 6fps.
> Bearing in mind that the source file is 24fps, this means that encoding
> takes 4 times as long than actually watching the DVD.
> 
> This may be usual in the video encoding world, but I've only ever had
> experience of encoding audio, which typically happens at a faster rate
> than it takes to listen to.
> 
> So, I guess my questions are: 1) am I expecting too much, 2) Does other
> people's experience match mine or 3) How do I speed this process up?
> 
> Thanks,
> 
> Matt.
> 
 Hi Matt,

 I've never extracted vob files (for one thing, my current disks are
too small - upgrading one machine is planned, but needs new hardware
[ old via_sata doesn't do even sata2 ] and there's a shortage of my
favourite cheap CPUs at the moment).  But, even converting my own
short files takes significant cpu cycles (ffmpeg2theora typically
takes a bit longer than the playing time, converting short video to
other formats can take significantly longer.

 Transcoding audio can be a lot simpler than transcoding video:
in audio, lossy codecs remove things that are supposedly inaudible.
In video, the aim is to reduce the bitrate without getting offensive
visual disturbance - a lot more data may need to be reviewed. That's
why there are the 'Advanced Video Options' listed in the ffmpeg
manpage.

 I'm on a single AMD processor - intel are reputedly better for
transcoding, and more CPUs always helps [ you might need to force the
desired number of threads, or ffmpeg might do an adequate job out of
the box ].

 When I was searching for ffmpeg options that might be useful, I
found quite a lot of suggestions.  As always, the accepted
command-line switches, and the defaults, differ across different
versions of ffmpeg.  Try searching for 'ffmpeg cheat sheet' if you
haven't already done that.

ĸen
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