Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at https://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: https://gpac.io/bug-reporting/

----

I am testing muxing text subtitles to .mp4. It seems that forced subtitles work only in tx3g format (and correctly display in MPC-HC, VLC). I use the following commands:

"Timed Text (tx3g)" 3 .srt files
```
mp4box.exe -add "video.264#video:name=" -add "audio.aac#audio:group=1:name=" -add "srt/SUB1.srt:hdlr=text:txtflags=0xC0000000:group=2:lang=eng:name=" -add "srt/SUB2.srt:hdlr=text:group=2:disable:lang=ger:name=" -add "srt/SUB3.srt:hdlr=text:group=2:disable:lang=fre:name=" -itags reset -tmp tmp -new tx3g_from_3_srt.mp4
```

"WebVTT (wvtt)" 3 .vtt files
```
mp4box.exe -add "video.264#video:name=" -add "audio.aac#audio:group=1:name=" -add "vtt/SUB1.vtt:hdlr=text:txtflags=0xC0000000:group=2:lang=eng:name=" -add "vtt/SUB2.vtt:hdlr=text:group=2:disable:lang=ger:name=" -add "vtt/SUB3.vtt:hdlr=text:group=2:disable:lang=fre:name=" -itags reset -tmp tmp -new wvtt_from_3_vtt.mp4
```

MediaInfo:

```
Complete name               : tx3g_from_3_srt.mp4
...
Text #1
ID                          : 3
Format                      : Timed Text
Codec ID                    : tx3g
Duration                    : 1 min 29 s
Bit rate mode               : Variable
Bit rate                    : 30 b/s
Frame rate                  : 0.212 FPS
Stream size                 : 333 Bytes (0%)
Language                    : English
Default                     : Yes
Forced                      : Yes
Alternate group             : 2
```

```
Complete name               : wvtt_from_3_vtt.mp4
...
Text #1
ID                          : 3
Format                      : wvtt
Codec ID                    : wvtt
Duration                    : 1 min 29 s
Bit rate mode               : Variable
Bit rate                    : 55 b/s
Frame rate                  : 0.201 FPS
Stream size                 : 617 Bytes (0%)
Language                    : English
Default                     : Yes
Forced                      : No
Alternate group             : 2
```

I have posted the [detailed steps and test files here](https://github.com/zablockil/srt_vtt_mp4). `mp4/wvtt_from_3_vtt.mp4` file.

tested:
```
MP4Box - GPAC version 2.3-DEV-rev942-g11c9f998-master
MP4Box - GPAC version 2.3-DEV-rev911-g13a18c3f-master
```



Is it possible to set forced subtitles WebVTT (wvtt) in .mp4?
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Is it possible to set forced subtitles WebVTT (wvtt) in .mp4?
Issue date:

--- start body ---
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at https://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: https://gpac.io/bug-reporting/

----

I am testing muxing text subtitles to .mp4. It seems that forced subtitles work only in tx3g format (and correctly display in MPC-HC, VLC). I use the following commands:

"Timed Text (tx3g)" 3 .srt files
```
mp4box.exe -add "video.264#video:name=" -add "audio.aac#audio:group=1:name=" -add "srt/SUB1.srt:hdlr=text:txtflags=0xC0000000:group=2:lang=eng:name=" -add "srt/SUB2.srt:hdlr=text:group=2:disable:lang=ger:name=" -add "srt/SUB3.srt:hdlr=text:group=2:disable:lang=fre:name=" -itags reset -tmp tmp -new tx3g_from_3_srt.mp4
```

"WebVTT (wvtt)" 3 .vtt files
```
mp4box.exe -add "video.264#video:name=" -add "audio.aac#audio:group=1:name=" -add "vtt/SUB1.vtt:hdlr=text:txtflags=0xC0000000:group=2:lang=eng:name=" -add "vtt/SUB2.vtt:hdlr=text:group=2:disable:lang=ger:name=" -add "vtt/SUB3.vtt:hdlr=text:group=2:disable:lang=fre:name=" -itags reset -tmp tmp -new wvtt_from_3_vtt.mp4
```

MediaInfo:

```
Complete name               : tx3g_from_3_srt.mp4
...
Text #1
ID                          : 3
Format                      : Timed Text
Codec ID                    : tx3g
Duration                    : 1 min 29 s
Bit rate mode               : Variable
Bit rate                    : 30 b/s
Frame rate                  : 0.212 FPS
Stream size                 : 333 Bytes (0%)
Language                    : English
Default                     : Yes
Forced                      : Yes
Alternate group             : 2
```

```
Complete name               : wvtt_from_3_vtt.mp4
...
Text #1
ID                          : 3
Format                      : wvtt
Codec ID                    : wvtt
Duration                    : 1 min 29 s
Bit rate mode               : Variable
Bit rate                    : 55 b/s
Frame rate                  : 0.201 FPS
Stream size                 : 617 Bytes (0%)
Language                    : English
Default                     : Yes
Forced                      : No
Alternate group             : 2
```

I have posted the [detailed steps and test files here](https://github.com/zablockil/srt_vtt_mp4). `mp4/wvtt_from_3_vtt.mp4` file.

tested:
```
MP4Box - GPAC version 2.3-DEV-rev942-g11c9f998-master
MP4Box - GPAC version 2.3-DEV-rev911-g13a18c3f-master
```



--- end body ---

None