Muxing with gstreamer

Last week I tried to build a little GNOME application to convert movies and videos to formats supported by our PS3 and PSP consoles. I started by playing with the command line. Sounded simple enough. The only problem I has was subtitle support but the nice guys at #gstreamer answered all my questions and I was ready to go. The first thing I tried was this:

gst-launch filesrc location=test.txt ! \
subparse subtitle-encoding=cp1250 ! subs. \
filesrc location=test.avi ! \
decodebin name=demuxer \
demuxer. ! queue ! ffmpegcolorspace ! \
textoverlay name=subs font-desc="Sans Bold 20" ! \
ffmpegcolorspace ! x264enc ! queue ! muxer. \
demuxer. ! queue ! audioconvert ! faac ! queue ! muxer. \
ffmux_mp4 name=muxer ! \
filesink location=output.mp4

It loos complicated but it’s not. It basically works like this:

  1. filesrc opens test.txt
  2. subparse decodes the file as one of the supported subtitle formats and sets fallbac encoding to cp1250 (unfortunately one of the most popular encodings used for subtitles in Poland)
  3. another filesrc opens test.avi
  4. decodebin detects the container format used, demuxes it and decodes the audio and video streams inside
  5. the video stream gets passed to textoverlay which takes the subtitle stream decoded earlier and overlays it in the Sans Bold 20 font
  6. x264enc takes the resulting video stream from and encodes it into the H264 format used by PS3
  7. faac takes the audio stream and encodes it to the AAC format used by PS3
  8. ffmux_mp4 takes both audio and video from the earlier elements and muxes it into the mp4 format
  9. filesink writes the results back to disk

Unfortunately it turns out the mp4 muxer does not work as intended. Either that or all the sound encoders are broken. The resulting file plays fine in mplayer but any attempt to use it with gstreamer gives you smooth video with broken audio (one sample per half a second). Playing it bac on the PS3 is impossible as it claims the file is simply corrupt.

Then I thought well, I can still use xvid, right? Wrong. xvidenc + lame + avimux = broken file. Again, mplayer plays the video without any sound, gstreamer just hangs on the first frame. This time I’m pretty sure it’s my fault but I have no idea what params to pass to the encoders and the muxer. Even the avimux example from the gstreamer docs results in an unplayable file on my machine.

Simple idea: how about creating a encodebin element to mimick the decodebin behavior? It could work like this:

audiotestsrc ! encoder. \
videotestsrc ! encoder. \
encodebin name=encoder profile=ogg ! \
filesink location=output.ogm

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Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Poland
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Poland.