Theora

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Theora is the preferred video codec for the OLPC.

Contents

Useful Encoding software

You can encode Theora files using the command line application ffmpeg2theora.

Recommended settings

[This file] plays back well on a B4 laptop, running build 623. Ogg multiplexed audio/video file, Theora/Vorbis, length 1m14s, 320×240 pixels, 148kbps overall.

These settings created a file which played back from the local disk successfully:

ffmpeg2theora -x 320 -y 256 -V 148 -A 32 "inputfilename.ogg"

3 June 08--For the latest version of ffmpeg2theora(version 0.21), I needed to use the following command to convert an AVI formatted movie: ffmpeg2theora-0.21.exe -x 320 -y 256 -o output.ogg --no-skeleton -A 32 -V 148 input.avi

The output video produced by this was some2what blurry, but file size was small.

I used the following to increase video quality and it worked pretty well, but filesize was about 10 times larger: ffmpeg2theora-0.21.exe -x 320 -y 256 -o output.ogg --no-skeleton -A 32 input.avi

After creating the .ogg file, I loaded it into the OLPC (Build 656) using a USB flash drive. I loaded Watch and Listen (v.10) on the OLPC and played the file from Journal selecting the Browse activity.

Testing

I have done a few simple libtheora tests needed for the theora encoder ticket (http://dev.laptop.org/attachment/ticket/5988/) using theora-svn.

I have used 2 simple raw yuv file (ftp://ftp.ldv.e-technik.tu-muenchen.de/dist/test_sequences/601) as input, encoded with ffmpeg and converted the video to yuv4mjpeg format (mplayer input.avi -vo yuv4mpeg) needed by thetheoraencoder. I have used the encoder from theora-tools-svn (should be the same encoder as the one in the thero svn examples/ folder)

theoraenc stream.yuv -o output.ogg

I compared 3 theora configurations with the unpatched encoder and with the patched encoder which enables super fast encoding (patch: http://dev.laptop.org/attachment/ticket/5988/speed_example.diff)

The theora builds/configurations I used are the following:

default: standard theora library on my G1G1 OLPC (/usr/lib/libtheora.so.0.2.0)

gcc4.3-geode: CFLAGS= -O3--force-addr -fomit-frame-pointer -finline-functions -funroll-loops -march=geode -mtune=geode

gcc4.3-geode-nommx: configure theora with --disable-asm (most of the mmx specific code is hand assembly, and the mno-mmx. mno-3dnow} do not make a difference) and CFLAGS = -O3--force-addr -fomit-frame-pointer -finline-functions -funroll-loops -mno-mmx -mno-3dnow

Numbers below are in seconds:

video input 1 (576i25_mobcal_ter.yuv)

 library            | patched | unpatched 
 default            | 79.5    | 119.5 
 gcc4.3geode        | 84.3    | 118.5 
 gcc4.3geodenommx   | 137     | 347    


video input 2 (576i25_parkrun_ter.yuv)


 library            | patched | unpatched 
 default            | 89      | 197       
 gcc4.3 geode       | 92.4    | 190 
 gcc4.3 geode nommx | 147     | 892 


- the patch for super fastmode seems to give an impressive speedup. Depending on video input, encoding time is reduced by 33%-55% for the default org cc4.3-geode library.

- the gcc4.3 geode-aware configuration/compilation does not seem to provide performance gains against the default libtheora. In fact the default libtheora seems to be slightly faster for the superfast encoder mode - do we know the configuration/compiler flags used? Many performance critical routines seem to be hand-coded assembly. Are there important routines that the compiler affects? Any other compiler flag suggestions are welcome.

- disabling mmx has a big performance hit as expected.

- Do we have specific encoding timing restraints for videoconferencing applications? - Can anyone suggest suitable inputs that combine both video and audio?

I have not used the gnashdev gcc4.3 build (http://wiki.gnashdev.org/wiki/index.php/Building_OLPC_Tools), but my own build. Let me know if something seems to be missing from the configuration

configured with: /home/vliaskov/compilers/gcc-geode/gcc/configure --prefix=/opt/gcc4-3-gode --enable-shared --enable-threads-posix --with-systen-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-languages=c,c++ --disable-dssi --with-cpu=geode Thread model: posix gcc version 4.3.0 20080116 (experimental) (GCC)

[Vasilis Liaskovitis]

See also

External Links

  • [VLC], the free media player software can also encode Theora files.
  • Colingo's wiki contains an entry on how to encode your video in Theora and in Theora/Speex. Translation and walkthrough for this space to come.
  • [[1]] Manual page for ffmpeg2theora.
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