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Asked: 2019-06-03 00:11:47 +0800 CST

Star Trek Continues: ffmeg dvdsub: overlapping subtitles

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So I've downloaded the free fan-created Star Trek Continues Blue-Ray ISO for Episodes 1-3 and I'm trying to convert the FHD 1920x1080 to SD 960x540 to be able to watch on my phone:

ffmpeg -canvas_size 960x540 \
-i /media/STC_VOL1_BR/BDMV/STREAM/00000.m2ts \
-map_metadata -1 \
-c:v libx264 -map 0:0 -strict -2 -r 24 -s 960x540 \
-c:a libmp3lame -map 0:1 -b:a 384K \
-metadata:s:s:0 language=EN -c:s dvdsub -map 0:2 \
/media/Videos/Star\ Trek\ Continues/S01E01\ Pilgrim\ Of\ Eternity.mkv

but after a few seconds of watching the converted mkv, the subtitles become:

  • unreadable due to overlapping
  • are slightly big (no issue)
  • shifted to the right and bottom of the screen

VLC Screenshot

Full ffmpeg output here

What am I doing wrong?

Note 1: I've only ever dealt with srt subtitles and I know that dvdsubs are images, so that's why I thought the canvas_size parameter might be of help, but it makes no difference.
Note 2: Yeah, it's not really a pure Ubuntu OS question, but over on video.se not getting any attention, even with a bounty so trying here if anyone does any video conversion and has run into the same situation.

ffmpeg video-conversion

1 Answers

  1. Best Answer

    Technically the cleanest look with these subtitles would be to:

    1. Extract the subtitles
    2. Turn them to text using your favourite OCR application
    3. Carefully review and edit the text
    4. Burn them back into the media stream in the size, colour, orientation and format of your choice

    This is a large and time consuming job which I have personally undertaken in the past but is probably overkill in this case.

    The better option is to simply add in the FFmpeg option -fix_sub_duration (placed before the input file) and this will fix the doubled up subtitles as I illustrate below, and make the subtitles easily readable:

    enter image description here

    The details of the -fix_sub_duration option can be found in the FFmpeg-all man pages but I have taken the liberty of quoting these details here:

    -fix_sub_duration

    Fix subtitles durations. For each subtitle, wait for the next packet in the same stream and adjust the duration of the first to avoid overlap. This is necessary with some subtitles codecs, especially DVB subtitles, because the duration in the original packet is only a rough estimate and the end is actually marked by an empty subtitle frame. Failing to use this option when necessary can result in exaggerated durations or muxing failures due to non-monotonic timestamps.

    Note that this option will delay the output of all data until the next subtitle packet is decoded: it may increase memory consumption and latency a lot.

    You will note that this is the first of the Advanced Subtitle options, the second being the -canvas_size that you have already used...

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