FFmpeg encoding part of the video with subtitles

Question

encode part of a file and at the same time add the subtitles for that part. I am using:

ffmpeg -ss 00:02:00 -t 00:00:30 -i "path/to/input" -c:v libvpx -crf 5 -b:v 3M -c:a libvorbis -vf "ass=path/ass" "output.webm"

it shows that it loads the subtitles but in the output they don't exist (subtitle:0kB). As far as I could tell the subtitles part doesn't work with -ss ? (Or it ignores -ss and tries to add the whole subs beginning with 00:00:00 of the .ass on 00:02:00 of the video stream).

Only way I could do it was to re-encode the whole file and then use:

ffmpeg -ss 00:02:00 -i "output.webm" -t 00:00:30 -c:v copy -c:a copy "new_output.webm"

to keep what I wanted. Am I missing something or it's just not possible to do what i want all in one?

Hope what I wrote made some sense and thx.

---

Output #0, webm, to 'target/test.webm':
  Metadata:
    encoder         : Lavf55.37.101
    Stream #0:0: Video: vp8 (libvpx), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=-1--1, 2500 kb/s, 1k tbn, 23.81 tbc (default) (forced)
    Stream #0:1: Audio: vorbis (libvorbis), 44100 Hz, stereo, fltp (default) (forced)
Stream mapping:
  Stream #0:0 -> #0:0 (h264 -> libvpx)
  Stream #0:1 -> #0:1 (aac -> libvorbis)
Press [q] to stop, [?] for help
frame=  953 fps= 22 q=0.0 Lsize=    6990kB time=00:00:40.02 bitrate=1430.6kbits/s dup=0 drop=5
video:6499kB audio:464kB subtitle:0kB other streams:0kB global headers:4kB muxing overhead: 0.376779%

Answer 1

If you put -ss after -i, ffmpeg will first encode the entire file up to the point you've specified, and only then will it write to the output.

This means that the subtitles will start at the correct point.

Also, you should consider putting -t after -i because it's used as an output option here, meaning, output only 30 seconds.

ffmpeg -i "path/to/input" -ss 00:02:00 -t 30 -c:v libvpx -crf 5 -b:v 3M -c:a libvorbis -vf "ass=path/ass" "output.webm"