In ffmpeg, how do I process subtitle streams when using filter_complex with trim and atrim?

user_839646 0
Viewed 361

I'm using ffmpeg filtergraphs to extract and concatenate chunks of videos. As a simple example, consider this, which takes an input file with a video stream and an audio stream and produces a 20-second output that includes timestamps 00:10-00:20 and 00:30-00:40 of the input:

ffmpeg -i in.mkv \
    -filter_complex "\
        [0:0]trim=start=10:end=20,setpts=PTS-STARTPTS[v1]; \
        [0:1]atrim=start=10:end=20,asetpts=PTS-STARTPTS[a1]; \
        [0:0]trim=start=30:end=40,setpts=PTS-STARTPTS[v2]; \
        [0:1]atrim=start=30:end=40,asetpts=PTS-STARTPTS[a2]; \
        [v1][a1][v2][a2]concat=n=2:v=1:a=1[v][a]\
    " -map [v] -map [a] \
    -c:0 libx264 -preset:0 ultrafast \
    -c:1 ac3 -b:1 128k -ac:1 2 out.mkv

Is there a way to process subtitle streams in a similar way so that the subtitles will match the other, trimmed streams? I'm looking for something that'll work from the command line to be part of a non-interactive batch process.

Update - solution

With feedback from @kesh, we arrived at using the concat demuxer to process the subtitles and combine that with reading the audio and video streams into a complex filtergraph. First, you need a concat input file like:

file 'in.mkv'
inpoint 10
outpoint 20

file 'in.mkv'
inpoint 30
outpoint 40

Then, if subtitles are stream 5, for example, just:

ffmpeg -i in.mkv \
    -f concat -i concat-file \
    -filter_complex "\
        [0:0]trim=start=10:end=20,setpts=PTS-STARTPTS[v1]; \
        [0:1]atrim=start=10:end=20,asetpts=PTS-STARTPTS[a1]; \
        [0:0]trim=start=30:end=40,setpts=PTS-STARTPTS[v2]; \
        [0:1]atrim=start=30:end=40,asetpts=PTS-STARTPTS[a2]; \
        [v1][a1][v2][a2]concat=n=2:v=1:a=1[v][a]\
    " -map [v] -map [a] -map 1:5 \
    -c:0 libx264 -preset:0 ultrafast \
    -c:1 ac3 -b:1 128k -ac:1 2 \
    -c:2 copy out.mkv
ffmpeg
1 Answers

No, you cannot. FFmpeg filtergraph can only add subtitle text to video (e.g., subtitle and ass filters). It has no way of manipulating subtitle streams.

Your best bet is the concat demuxer. You can list the same file multiple times with different start and end times. In your batch file, you can create the concat list in memory and pipe it into FFMpeg.

[edit]

Assuming that in.mkv has it all: video, audio, and subtitle streams. Then you can prepare a concat demuxer file like:

listing.txt

ffconcat version 1.0

file in.mkv
inpoint 10
outpoint 20

file in.mkv
inpoint 30
outpoint 40

Basically, list the input file multiple times with different start and end timestamps.

Then,

ffmpeg -f concat -i listing.txt -map [v] -map [a] -map [s] -c copy out.mkv

would copy all 3 streams to out.mkv.

Disclaimer: I have not verified it myself, but it should work on paper.

user_45160270
Related