[FFmpeg-trac] #6021(avcodec:closed): tx3g / mov_text subtitles are not encoded correctly in some specific cases

FFmpeg trac at avcodec.org
Sat Apr 6 01:28:36 EEST 2019


#6021: tx3g / mov_text subtitles are not encoded correctly in some specific cases
---------------------------------------+-----------------------------------
             Reporter:  erikbs         |                    Owner:
                 Type:  defect         |                   Status:  closed
             Priority:  normal         |                Component:  avcodec
              Version:  git-master     |               Resolution:  fixed
             Keywords:  utf8 mov_text  |               Blocked By:
             Blocking:                 |  Reproduced by developer:  0
Analyzed by developer:  0              |
---------------------------------------+-----------------------------------

Comment (by CoRoNe):

 > ''I discovered that MP4Box writes a subtitle box size while ffmpeg does
 not (or, more precisely, it apparently sets the box size to 0x0). Perhaps
 that is why the subtitles are not shown on the TV?
 Do you have a patch for this by any chance? I'm not a hero with C/C++.''
 \\
 > ''...demux and remux''
 This works. My tv renders the subtitles perfectly. So I'm guessing
 something is going wrong with the muxing process.
 Comparing the 2 mp4-files in MediaInfo I noticed the one created by FFmpeg
 mentioning "'''Muxing mode: sbtl'''" and "'''Alternate group: 3'''", where
 as the one created by MP4Box does not.

 And for everyone's info:
 {{{
 --- VPWON_1269922-t32_MP4Box.ttxt       2019-04-05 23:33:21.500000000
 +0200
 +++ VPWON_1269922-t32_FFmpeg_demux-MP4Box.ttxt  2019-04-05
 23:16:37.475016000 +0200
 @@ -1,12 +1,12 @@
  <?xml version="1.0" encoding="UTF-8" ?>
  <!-- GPAC 3GPP Text Stream -->
  <TextStream version="1.1">
 -<TextStreamHeader width="400" height="60" layer="0" translation_x="0"
 translation_y="0">
 +<TextStreamHeader width="0" height="0" layer="0" translation_x="0"
 translation_y="0">
  <TextSampleDescription horizontalJustification="center"
 verticalJustification="bottom" backColor="0
  0 0 0" verticalText="no" fillTextRegion="no" continuousKaraoke="no"
 scroll="None">
  <FontTable>
  <FontTableEntry fontName="Serif" fontID="1"/>
  </FontTable>
 -<TextBox top="0" left="0" bottom="60" right="400"/>
 +<TextBox top="0" left="0" bottom="0" right="0"/>
  <Style styles="Normal" fontID="1" fontSize="18" color="ff ff ff ff"/>
  </TextSampleDescription>
  </TextStreamHeader>
 }}}
 This appears to be the only output difference between MP4Box's and
 FFmpeg's tx3g encoder.
 '//VPWON_1269922-t32_MP4Box.ttxt//': `mp4box -ttxt
 "VPWON_1269922-t32.srt"`
 '//VPWON_1269922-t32_FFmpeg_demux-MP4Box.ttxt//': `mp4box -ttxt 3
 "VPWON_1269922(FFMpeg-mux_t32).mp4"`

--
Ticket URL: <https://trac.ffmpeg.org/ticket/6021#comment:19>
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