wishlist vtt: support styling when converting to ass

[Yi-Jyun Pan]

Summary of the bug

When I convert a vtt file to ass, it appears like styles are removed.

How to reproduce

Logs

$ ffmpeg -i AIAIAI.zh-TW.vtt -f ass out.ass
ffmpeg version n4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9.2.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvdec --enable-nvenc --enable-omx --enable-shared --enable-version3
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
[webvtt @ 0x5648bc272780] Dropping 2 duplicated subtitle events
Input #0, webvtt, from 'AIAIAI.zh-TW.vtt':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Subtitle: webvtt
Output #0, ass, to 'out.ass':
  Metadata:
    encoder         : Lavf58.29.100
    Stream #0:0: Subtitle: ass (ssa)
    Metadata:
      encoder         : Lavc58.54.100 ssa
Stream mapping:
  Stream #0:0 -> #0:0 (webvtt (native) -> ass (ssa))
Press [q] to stop, [?] for help
size=      96kB time=00:03:03.99 bitrate=   4.3kbits/s speed=2.72e+03x    
video:0kB audio:0kB subtitle:56kB other streams:0kB global headers:1kB muxing overhead: 69.999657%

Platform

• System: Arch Linux (testing / kde-unstable) x86-64
• System Language: zh_TW.UTF-8
• Desktop Environment: Plasma 5.17.3 / KDE Frameworks 5.64.0 / Qt 5.14
• Font: Noto Sans CJK TC
• youtube-dl installed

Expected Behavior

When playing this video with zh_TW subtitle on YouTube Web. (0:14)

Actual behavior

When playing this video with the converted ass on mpv. (0:14)

(For developers) some reference of styling

Text Color: ​https://www.w3.org/TR/webvtt1/#default-text-color
Cue settings, like alignment, position and etc.: ​https://www.w3.org/TR/webvtt1/#cue-settings

Sample File

• ​The zh_TW subtitle of that video

Referenced Issues

• ​GitHub - mpv-player/mpv: #7214 wishlist: vtt: support styling

[Carl Eugen Hoyos]

See also #9609