[FFmpeg-user] Set Timed Text Stream To "Forced"

[Oli Jeal]

Hi,

I was wondering if there is a way to set a timed text stream as forced in an
mp4 container. I notice that the quick time spec has a flag "All Samples Are
Forced". I've tested converting a file with a Forced and Unforced subtitle
stream into an mp4 file. The subtitles are placed into the container as
expected and I can select and view them both. However when I look inside the
mp4 file at the following box

MovieBox -> TrackBox -> MediaBox -> MediaInformationBox -> SampleTableBox ->
SampleDescriptionBox -> SampleEntry(tx3g)

the 32bit unsigned int denoting display flags is always 0.

Is there a way to manually set this flag or a switch that will tell ffmpeg
the stream is forced and thus set the flag automatically?

Many Thanks