Webm progress bar issue, analysis and a fix

Intro

When we generate a webm with getUserMedia, MediaRecorder, and similar APIs, the resulting webm cannot seek. Unless you convert it with FFmpeg to another format, or wait until the webm finishes playing, then you can drag the progress bar.

Analysis

After a few hours of digging, it was not a misuse of MediaRecorder. Other demos on the web produce the same broken webm.

I first focused on the progress bar, found nothing online, tried all kinds of keywords, still nothing.

Then I thought of analyzing the file with FFmpeg. I ran ffprobe rebirth-demo.webm:

$ ffprobe rebirth-demo.webm
ffprobe version 4.1.3 Copyright (c) 2007-2019 the FFmpeg developers
  built with Apple LLVM version 10.0.1 (clang-1001.0.46.4)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1.3_1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-videotoolbox --disable-libjack --disable-indev=jack --enable-libaom --enable-libsoxr
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, matroska,webm, from 'rebirth-demo.webm':
  Metadata:
    encoder         : Chrome
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Stream #0:1(eng): Video: vp8, yuv420p(progressive), 1920x1080, SAR 1:1 DAR 16:9, 60 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      alpha_mode      : 1

Here is the key: Duration and bitrate are both N/A, which is wrong. I searched webm duration and found plenty of write-ups.

The gist is that getUserMedia and MediaRecorder do not write Duration and bitrate into the webm, which causes this.

Solutions

1. Compute duration and assign it to the `blob`

The idea is: record a start time on start, subtract it from now on stop, and assign that duration to the blob. See: fix-webm-duration (opens in a new tab)

2. Give the audio element a large duration

While playing the webm, you can dynamically give audio a huge duration. This only works in chrome right now. See: How can I add predefined length to audio recorded from MediaRecorder in Chrome? (opens in a new tab)

3. Seek to the end, then back to the start

As above, once the video has played through, seeking works. So you can just seek with JS. See: hello-its-me (opens in a new tab)

4. Fix it with ffmpeg

The first command is: ffmpeg -i rebirth-demo.webm xixi.webm, but it is slow, not recommended. A 30 second video takes about 3 minutes.

The second command is: ffmpeg -i rebirth-demo.webm -vcodec copy -acodec copy new_rebirth-demo.webm. This is fast, because it copies instead of converting:

ffmpeg -i rebirth-demo.webm  -vcodec copy -acodec copy new_rebirth-demo.webm
ffmpeg version 4.1.3 Copyright (c) 2000-2019 the FFmpeg developers
  built with Apple LLVM version 10.0.1 (clang-1001.0.46.4)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1.3_1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-videotoolbox --disable-libjack --disable-indev=jack --enable-libaom --enable-libsoxr
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, matroska,webm, from 'rebirth-demo.webm':
  Metadata:
    encoder         : Chrome
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Stream #0:1(eng): Video: vp8, yuv420p(progressive), 1920x1080, SAR 1:1 DAR 16:9, 60 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      alpha_mode      : 1
Output #0, webm, to 'new_rebirth-demo.webm':
  Metadata:
    encoder         : Lavf58.20.100
    Stream #0:0(eng): Video: vp8, yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 60 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      alpha_mode      : 1
    Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
Stream mapping:
  Stream #0:1 -> #0:0 (copy)
  Stream #0:0 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 3589 fps=0.0 q=-1.0 Lsize=    2107kB time=00:01:59.92 bitrate= 143.9kbits/s speed=4.75e+03x
video:2053kB audio:16kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.849351%

$ ffprobe new_rebirth-demo.webm
ffprobe version 4.1.3 Copyright (c) 2007-2019 the FFmpeg developers
  built with Apple LLVM version 10.0.1 (clang-1001.0.46.4)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/4.1.3_1 --enable-shared --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags='-I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include -I/Library/Java/JavaVirtualMachines/adoptopenjdk-11.0.2.jdk/Contents/Home/include/darwin' --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libmp3lame --enable-libopus --enable-librubberband --enable-libsnappy --enable-libtesseract --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-videotoolbox --disable-libjack --disable-indev=jack --enable-libaom --enable-libsoxr
  libavutil      56. 22.100 / 56. 22.100
  libavcodec     58. 35.100 / 58. 35.100
  libavformat    58. 20.100 / 58. 20.100
  libavdevice    58.  5.100 / 58.  5.100
  libavfilter     7. 40.101 /  7. 40.101
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  3.100 /  5.  3.100
  libswresample   3.  3.100 /  3.  3.100
  libpostproc    55.  3.100 / 55.  3.100
Input #0, matroska,webm, from 'new_rebirth-demo.webm':
  Metadata:
    ENCODER         : Lavf58.20.100
  Duration: 00:01:59.96, start: 0.000000, bitrate: 143 kb/s
    Stream #0:0(eng): Video: vp8, yuv420p(progressive), 1920x1080, SAR 1:1 DAR 16:9, 60 tbr, 1k tbn, 1k tbc (default)
    Metadata:
      ALPHA_MODE      : 1
      DURATION        : 00:01:59.928000000
    Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:01:59.955000000

No longer broken.

Wrap-up

I prefer the last one, because the earlier methods do not actually fix it.

This is a Chrome Bug. The community is discussing it, but there is still no fix.

Discussion: https://bugs.chromium.org/p/chromium/issues/detail?id=642012 (opens in a new tab)