[theora] Theora for talking heads and presentations
gmaxwell at gmail.com
Sun Sep 20 14:18:06 PDT 2009
Greetings, I've been trying some things with Theora to address a
popular niche use case: providing decent quality streaming video for
presentations for very low bandwidth users.
The idea I'm working with here is that it's important to have high
visual quality so the audience can read the slides, but it's also
important to have strict limits on the peak and average bitrate
because it's annoying to have the video stall during the playback.
These two can be met if the video is still enough, as is typical for
'presentation' and meeting video, but if there is a period of high
motion such as someone panning the quality will suffer tremendously.
It occurred to me that we have another degree of freedom: We can
reduce the effective frame rate during high motion scenes in order to
control the bitrate peaks without hurting quality too badly. For
presentation material I think this is superior to allowing the quality
to go down, but I'd like to hear some other people's opinions. This
can be accomplished in 1.1 SVN by using both -V and -v to set both
bitrate constraint and a minimum quality, although it doesn't
currently seem to work right with ffmpeg2theora.
While playing with this I found some behaviour in the rate control
with respect to the minimum quality knob which I think is undesirable,
so I've prepared a test encode with that behaviour changed.
I'm interested in opinions on these three clips:
If limitmod and limit are looking too similar to you, an example of
how they differ is at about 35 seconds in around the words "basic
outline of the talk".
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