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• FFMPEG decode from RTP dump file into mp3 file

  5 février 2021, par pingvincible

  I'm trying to save RTP stream into mp3 file. I use this command :

  


  ffmpeg -loglevel debug -protocol_whitelist file -f rtp -i microphone.rtpdump -f mp3 microphone.mp3


  


  I get this result :

  


  user@pc:~/$ ffmpeg-amrnb -loglevel debug -protocol_whitelist file -f rtp -i microphone.rtpdump -f mp3 microphone.mp3
ffmpeg version N-100958-g4f3d8cb554 Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 9 (Ubuntu 9.3.0-17ubuntu1~20.04)
  configuration: --enable-gpl --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-librtmp --enable-libtheora --enable-libvorbis --enable-nonfree --enable-version3
  libavutil      56. 64.100 / 56. 64.100
  libavcodec     58.120.100 / 58.120.100
  libavformat    58. 65.101 / 58. 65.101
  libavdevice    58. 11.103 / 58. 11.103
  libavfilter     7.102.100 /  7.102.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Splitting the commandline.
Reading option '-loglevel' ... matched as option 'loglevel' (set logging level) with argument 'debug'.
Reading option '-protocol_whitelist' ... matched as AVOption 'protocol_whitelist' with argument 'file'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'rtp'.
Reading option '-i' ... matched as input url with argument 'microphone.rtpdump'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'mp3'.
Reading option 'microphone.mp3' ... matched as output url.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option loglevel (set logging level) with argument debug.
Successfully parsed a group of options.
Parsing a group of options: input url microphone.rtpdump.
Applying option f (force format) with argument rtp.
Successfully parsed a group of options.
Opening an input file: microphone.rtpdump.
[rtp @ 0x556947200580] Unable to receive RTP payload type 97 without an SDP file describing it
microphone.rtpdump: Invalid data found when processing input


  


  It looks like that microphone.rtpdump file format is correct as ffmpeg can find RTP payload type 97. The problem is that I don't understand how to use SDP file in this situation.

  


  I have an SDP file for this payload type which I use, when I send data over network. It looks like this :

  


  v=0
o=- 0 0 IN IP4 127.0.0.1
s=No Name
c=IN IP4 127.0.0.1
t=0 0
a=tool:libavformat 58.65.101
m=audio 1234 RTP/AVP 97
b=AS:12
a=rtpmap:97 AMR/8000/1
a=fmtp:97 octet-align=1


  


  And now I want to decode RTP stream from file, not by receiving it from network.

  


  How to adapt my SDP file to read RTP stream from file ?

  


  UPDATE : My rtpdump file is not a real rtpdump file format. It is just payloads from UDP packets written into file without any additional headers.

  


• output stream video produced by ffmpeg gets divided into sections on screen upon streaming

  22 avril 2024, par Kaustubh Sonawane

  I'm trying to combine cv2 frames with numpy audio array to produce a hls playlist. I'm giving input using pipe. The problem is that while streaming the output hls playlist produced by ffmpeg, the video screen is divided into 4 parts and the sizes of the 4 parts keep getting changed slowly.
Also the audio is extremely distorted.
  
Attaching images for reference :
at timestamp 00:00
at timestamp 00:03

  


  Here is the script I have written until now :

  


  import cv2
import subprocess
import numpy as np
import librosa

def create_hls_playlist(width, height, frames, audio_data, output_playlist, fps=30, sample_rate=44100):
    print(fps)
    print(height, width)
    print(audio_np.dtype)
    video_args = [
        '-y', 
        "-fflags", "+discardcorrupt",
        '-f', 'rawvideo',
        '-vcodec', 'rawvideo',
        '-s', '{}x{}'.format(width, height),
        '-pix_fmt', 'bgr24',
        '-r', str(fps),
        '-i', 'pipe:', 
    ]

    audio_args = [
        '-f', 's16le',  
        '-ar', str(sample_rate),  
        '-ac', '1',  
        '-i', 'pipe:', 
    ]


    output_args = [
        "-vf", "scale=1920x1080,setdar=16/9",
        '-an',
        '-c:v', 'libx264',
        '-preset', 'ultrafast',
        '-f', 'hls',
        '-hls_time', '2',
        '-hls_list_size', '0',
        #'-loglevel', 'debug',
        output_playlist,
    ]

    ffmpeg_command = ['ffmpeg'] + video_args + output_args

    ffmpeg_process = subprocess.Popen(ffmpeg_command, stdin=subprocess.PIPE)

    try:
        for frame in frames[:10]:
            print(f"Frame shape: {frame.shape}, Frame data type: {frame.dtype}")
            # Check for any NaN or infinite values in the frame
            if np.isnan(frame).any() or np.isinf(frame).any():
                print("Frame contains NaN or infinite values!")
            ffmpeg_process.stdin.write(frame.to_string())

    except Exception as e:
        print(f"Error occurred while writing frames: {e}")
    
    # Write audio data to ffmpeg process
    audio_bytes = audio_data
    ffmpeg_process.stdin.write(audio_bytes)

    for frame in frames[10:]:
      ffmpeg_process.stdin.write(frame)
    
    ffmpeg_process.stdin.flush()

    ffmpeg_process.stdin.close()

    ffmpeg_process.wait()

    ffmpeg_process.stdin.close()


def capture_frames(video_path):
    frames = []
    cap = cv2.VideoCapture(video_path)
    width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
    height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))

    while cap.isOpened():
        ret, frame = cap.read()
        if not ret:
            break
        frames.append(frame)

    cap.release()
    return frames, width, height

def load_audio(file_path, duration=None, sr=44100):
    audio_data, _ = librosa.load(file_path, sr=sr, duration=duration)
    print("finished audio-----------------------------------")
    print(audio_data)
    audio_duration = librosa.get_duration(y=audio_data, sr=sr)

    if duration is not None and audio_duration > duration:
        start_sample = 0
        end_sample = int(duration * sr)
        audio_data = audio_data[start_sample:end_sample]
    if np.isnan(audio_data).any() or np.isinf(audio_data).any():
        raise ValueError("Audio data contains NaN or infinite values")

    # Normalize audio data
    # audio_data /= np.max(np.abs(audio_data))
    audio_data = (audio_data * 32767).astype(np.int16)
    print(f"Audio data shape: {audio_data.shape}")
    print(f"Audio data type: {audio_data.dtype}")
    return audio_data

def get_video_frame_rate(video_path):
    cap = cv2.VideoCapture(video_path)
    if not cap.isOpened():
        raise ValueError(f"Unable to open the video file: {video_path}")

    frame_rate = cap.get(cv2.CAP_PROP_FPS)
    print(frame_rate)
    cap.release()
    return frame_rate

video_path = 'sample.mp4'

output_playlist = 'output.m3u8'
mp3_file_path = 'audio.mp3'
duration = 11  # seconds
sample_rate = 44100  # Hz


if __name__ == "__main__":
    frames, width, height = capture_frames(video_path)
    audio_np = load_audio(mp3_file_path, duration=duration, sr=sample_rate)
    create_hls_playlist(width, height, frames, audio_np, output_playlist)


  


  And the output :

  


  brain@brainywire:~/k/ffmpeg-e$ /bin/python3 /home//ffmpeg-e/stream_f.py
finished audio-----------------------------------
[1.7182100e-06 1.8538897e-06 1.9645518e-06 ... 5.4211184e-02 5.2549735e-02
 5.0875921e-02]
Audio data shape: (485100,)
Audio data type: int16
30
720 1280
int16
Frame shape: (720, 1280, 3), Frame data type: uint8
Error occurred while writing frames: 'numpy.ndarray' object has no attribute 'to_string'
ffmpeg version 4.4.2-0ubuntu0.22.04.1 Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 11 (Ubuntu 11.2.0-19ubuntu1)
  configuration: --prefix=/usr --extra-version=0ubuntu0.22.04.1 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librabbitmq --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-pocketsphinx --enable-librsvg --enable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 70.100 / 56. 70.100
  libavcodec     58.134.100 / 58.134.100
  libavformat    58. 76.100 / 58. 76.100
  libavdevice    58. 13.100 / 58. 13.100
  libavfilter     7.110.100 /  7.110.100
  libswscale      5.  9.100 /  5.  9.100
  libswresample   3.  9.100 /  3.  9.100
  libpostproc    55.  9.100 / 55.  9.100
Input #0, rawvideo, from 'pipe:':
  Duration: N/A, start: 0.000000, bitrate: 663552 kb/s
  Stream #0:0: Video: rawvideo (BGR[24] / 0x18524742), bgr24, 1280x720, 663552 kb/s, 30 tbr, 30 tbn, 30 tbc
Guessed Channel Layout for Input Stream #1.0 : mono
Input #1, s16le, from 'pipe:':
  Duration: N/A, bitrate: 705 kb/s
  Stream #1:0: Audio: pcm_s16le, 44100 Hz, mono, s16, 705 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (rawvideo (native) -> h264 (libx264))
  Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))
[libx264 @ 0x5557bf4369c0] using SAR=1/1
[libx264 @ 0x5557bf4369c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x5557bf4369c0] profile High 4:4:4 Predictive, level 4.0, 4:4:4, 8-bit
[libx264 @ 0x5557bf4369c0] 264 - core 163 r3060 5db6aa6 - H.264/MPEG-4 AVC codec - Copyleft 2003-2021 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=6 threads=18 lookahead_threads=3 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=250 keyint_min=25 scenecut=0 intra_refresh=0 rc=crf mbtree=0 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=0
[s16le @ 0x5557bf3fcf80] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8)
Output #0, hls, to 'output.m3u8':
  Metadata:
    encoder         : Lavf58.76.100
  Stream #0:0: Video: h264, yuv444p(tv, progressive), 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 30 fps, 90k tbn
    Metadata:
      encoder         : Lavc58.134.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
  Stream #0:1: Audio: aac (LC), 44100 Hz, mono, fltp, 69 kb/s
    Metadata:
      encoder         : Lavc58.134.100 aac
[rawvideo @ 0x5557bf3e8640] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8)
[hls @ 0x5557bf4335c0] Opening 'output0.ts' for writingtrate=N/A speed=5.06x    
[hls @ 0x5557bf4335c0] Opening 'output.m3u8.tmp' for writing
[rawvideo @ 0x5557bf3e8640] Packet corrupt (stream = 0, dts = 345), dropping it.
[hls @ 0x5557bf4335c0] Opening 'output1.ts' for writingtrate=N/A speed=5.31x    
[hls @ 0x5557bf4335c0] Opening 'output.m3u8.tmp' for writing
frame=  345 fps=166 q=-1.0 Lsize=N/A time=00:00:11.50 bitrate=N/A speed=5.52x    
video:21389kB audio:95kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
[libx264 @ 0x5557bf4369c0] frame I:2     Avg QP:21.00  size:666528
[libx264 @ 0x5557bf4369c0] frame P:343   Avg QP:24.78  size: 59968
[libx264 @ 0x5557bf4369c0] mb I  I16..4: 100.0%  0.0%  0.0%
[libx264 @ 0x5557bf4369c0] mb P  I16..4: 22.8%  0.0%  0.0%  P16..4: 51.1%  0.0%  0.0%  0.0%  0.0%    skip:26.1%
[libx264 @ 0x5557bf4369c0] coded y,u,v intra: 24.0% 3.7% 3.3% inter: 30.6% 4.9% 4.8%
[libx264 @ 0x5557bf4369c0] i16 v,h,dc,p: 43% 38% 10%  9%
[libx264 @ 0x5557bf4369c0] kb/s:15236.21
[aac @ 0x5557bf438400] Qavg: 118.713


  


  I noticed this packet corrupt error and after some digging on stackoverflow found a flag to drop corrupt packets. But that didn't solve the problem.

  


  If I do not drop the corrupted packets, I get this error, if it helps to give any more info :

  


  [rawvideo @ 0x61a72571ab80] PACKET SIZE: 2764800, STRIDE: 3840
[libx264 @ 0x61a725754c40] frame= 323 QP=24.00 NAL=2 Slice:P Poc:146 I:2340 P:4474 SKIP:1346 size=62313 bytes
[rawvideo @ 0x61a72571ab80] PACKET SIZE: 2764800, STRIDE: 3840
[libx264 @ 0x61a725754c40] frame= 324 QP=24.00 NAL=2 Slice:P Poc:148 I:1538 P:4449 SKIP:2173 size=56970 bytes
[rawvideo @ 0x61a72571ab80] PACKET SIZE: 2764800, STRIDE: 3840
[libx264 @ 0x61a725754c40] frame= 325 QP=24.00 NAL=2 Slice:P Poc:150 I:2553 P:4310 SKIP:1297 size=64782 bytes
pipe:: corrupt input packet in stream 0
[rawvideo @ 0x61a72571ab80] PACKET SIZE: 2751960, STRIDE: 3822
[rawvideo @ 0x61a72571ab80] Invalid buffer size, packet size 2751960 < expected frame_size 2764800
Error while decoding stream #0:0: Invalid argument
[out_0_0 @ 0x61a72575f400] EOF on sink link out_0_0:default.
[out_0_1 @ 0x61a726577f80] EOF on sink link out_0_1:default.
No more output streams to write to, finishing.
[libx264 @ 0x61a725754c40] frame= 326 QP=24.00 NAL=2 Slice:P Poc:152 I:1076 P:4002 SKIP:3082 size=44541 bytes
[libx264 @ 0x61a725754c40] frame= 327 QP=24.00 NAL=2 Slice:P Poc:154 I:2493 P:4545 SKIP:1122 size=62617 bytes
[libx264 @ 0x61a725754c40] frame= 328 QP=24.00 NAL=2 Slice:P Poc:156 I:1383 P:4328 SKIP:2449 size=50782 bytes
[libx264 @ 0x61a725754c40] frame= 329 QP=24.00 NAL=2 Slice:P Poc:158 I:2179 P:4376 SKIP:1605 size=57972 bytes
[libx264 @ 0x61a725754c40] frame= 330 QP=24.00 NAL=2 Slice:P Poc:160 I:925  P:3772 SKIP:3463 size=37181 bytes


  


  I tried to give bufsize flags too, but even that wouldnt help.

  


  I'm a total beginner to ffmpeg. I tried to give the correct resolution, tried all pix_fmt from the documentation, and also experimented with different values of fps. But the problem with split screen and audio distortion continues.

  


• FFmpeg stream extraction modifies subtitles [closed]

  21 mai 2024, par user18812922

  I have a video with the following ffprobe output :

  


  Input #0, matroska,webm, from 'video.mkv':
  Metadata:
    title           : Video - 01
    creation_time   : 2021-07-14T02:49:59.000000Z
    ENCODER         : Lavf58.29.100
  Duration: 00:22:57.28, start: 0.000000, bitrate: 392 kb/s
  Chapters:
    Chapter #0:0: start 0.000000, end 86.169000
      Metadata:
        title           : Opening
    Chapter #0:1: start 86.169000, end 641.266000
      Metadata:
        title           : Part A
    Chapter #0:2: start 641.266000, end 651.359000
      Metadata:
        title           : Eyecatch
    Chapter #0:3: start 651.359000, end 1286.160000
      Metadata:
        title           : Part B
    Chapter #0:4: start 1286.160000, end 1356.355000
      Metadata:
        title           : Ending
    Chapter #0:5: start 1356.355000, end 1376.876000
      Metadata:
        title           : Preview
  Stream #0:0: Video: hevc (Main 10), yuv420p10le(tv, bt709), 854x480 [SAR 1280:1281 DAR 16:9], 23.98 fps, 23.98 tbr, 1k tbn (default)
      Metadata:
        DURATION        : 00:22:56.959000000
  Stream #0:1(eng): Audio: vorbis, 48000 Hz, stereo, fltp (default)
      Metadata:
        title           : English [FLAC 2.0]
        DURATION        : 00:22:57.278000000
  Stream #0:2(jpn): Audio: vorbis, 48000 Hz, stereo, fltp
      Metadata:
        title           : Japanese [FLAC 2.0]
        DURATION        : 00:22:57.276000000
  Stream #0:3(eng): Subtitle: ass (ssa)
      Metadata:
        title           : Signs and Songs [FMA1394/Redc4t]
        DURATION        : 00:22:51.090000000
  Stream #0:4(eng): Subtitle: ass (ssa)
      Metadata:
        title           : English [FMA1394/Redc4t]
        DURATION        : 00:22:51.090000000
  Stream #0:5(eng): Subtitle: hdmv_pgs_subtitle (pgssub), 1920x1080
      Metadata:
        title           : Full English Retail
        DURATION        : 00:22:51.120000000
  Stream #0:6: Attachment: ttf
      Metadata:
        filename        : 8bitoperator.ttf
        mimetype        : application/x-truetype-font
  Stream #0:7: Attachment: ttf
      Metadata:
        filename        : Cabin-Bold.ttf
        mimetype        : application/x-truetype-font
  Stream #0:8: Attachment: ttf
      Metadata:
        filename        : calibrib.ttf
        mimetype        : application/x-truetype-font
  Stream #0:9: Attachment: ttf
      Metadata:
        filename        : daniel_0.ttf
        mimetype        : application/x-truetype-font
  Stream #0:10: Attachment: ttf
      Metadata:
        filename        : DEATH_FONT.TTF
        mimetype        : application/x-truetype-font
  Stream #0:11: Attachment: ttf
      Metadata:
        filename        : Dominican.ttf
        mimetype        : application/x-truetype-font
  Stream #0:12: Attachment: ttf
      Metadata:
        filename        : gishabd.ttf
        mimetype        : application/x-truetype-font
  Stream #0:13: Attachment: ttf
      Metadata:
        filename        : PATRICK_0.TTF
        mimetype        : application/x-truetype-font
  Stream #0:14: Attachment: ttf
      Metadata:
        filename        : Qlassik-Medium.ttf
        mimetype        : application/x-truetype-font
Unsupported codec with id 98304 for input stream 6
Unsupported codec with id 98304 for input stream 7
Unsupported codec with id 98304 for input stream 8
Unsupported codec with id 98304 for input stream 9
Unsupported codec with id 98304 for input stream 10
Unsupported codec with id 98304 for input stream 11
Unsupported codec with id 98304 for input stream 12
Unsupported codec with id 98304 for input stream 13
Unsupported codec with id 98304 for input stream 14


  


  I am trying to extract the subtitles, edit them and reattach them to the video.
(I need my program to do that so I don't want to use other software)

  


  Command 1

  


  ffmpeg -i video.mkv -map 0:3 -c:s ssa subs.ass
ffmpeg -i video.mkv -i subs.ass -map 0 -map -0:s -map 1 -c copy out.mkv


  


  Command 2

  


  ffmpeg -i video.mkv -map 0:3 subs.ass
ffmpeg -i video.mkv -i subs.ass -map 0 -map -0:s -map 1 -c copy out.mkv


  


  Command 3

  


  ffmpeg -i video.mkv -map 0:3 subs.srt
ffmpeg -i video.mkv -i subs.srt -map 0 -map -0:s -map 1 -c copy out.mkv


  


  Command 4

  


  ffmpeg -i video.mkv -map 0:3 subs.srt
ffmpeg -i subs.srt subs.ass
ffmpeg -i video.mkv -i subs.ass -map 0 -map -0:s -map 1 -c copy out.mkv


  


  Command 5

  


  ffmpeg -i video.mkv -map 0:3 subs.ass
ffmpeg -i subs.ass subs.srt
ffmpeg -i video.mkv -i subs.srt -map 0 -map -0:s -map 1 -c copy out.mkv


  


  The problem

  


  After extraction the subtitles seem to be really quick, meaning they are displayed and disappear really quickly.

  


  For example the first subtitle is as follows in srt :

  


  1&#xA;00:00:03,100 --> 00:00:03,560&#xA;<font face="Dominican" size="77" color="#f7f7f7">Within the spreading darkness</font>&#xA;

  


  Now, in srt it also has wrong size but I assume that's because of the conversion from ass to srt.

  


  If I reattach the subtitle file in the video and open it, it is displayed and disappears way too fast and it doesn't match the original subtitles in the video.

  


  (ie, the original video subtitles are showing for at least a second)

  


  Expected behaviour

  


  The subtitles should be displayed for the same duration as the original subtitles.

  


  NOTE

  


  It's my first question for ffmpeg related issues so feel free to ask me for anything else you may need.

  


  UPDATE 1

  


  I realized that the subtitles were ok for the timings as they had the same line multiple times, so the problem for not playing is something else.

  


  Example of the file

  


  1&#xA;00:00:03,100 --> 00:00:03,560&#xA;<font face="Dominican" size="77" color="#f7f7f7">Within the spreading darkness</font>&#xA;&#xA;2&#xA;00:00:03,560 --> 00:00:04,650&#xA;<font face="Dominican" size="77" color="#f7f7f7">Within the spreading darkness</font>&#xA;&#xA;3&#xA;00:00:04,650 --> 00:00:05,100&#xA;<font face="Dominican" size="77" color="#f7f7f7">Within the spreading darkness</font>&#xA;

  


  So the problem is that VLC doesn't show more than the first subtitle.

  


  The strange thing is when I use the below command

  


  ffmpeg -i video.mkv -i subs.srt -map 0 -map -0:s -map 1 -c copy -c:s subrip out.mkv


  


  Then more lines of the subtitle (but not all) play.

  


  It stops at the 17th line.

  


  I believe that's an encoder's problem ? but I really don't know.

  


  Also what I noticed is that VLC stops the subtitles but Windows Media Player (Windows 11 version) display the subtitles correctly even after the 17th line.

  


  BUT, if I add subtitles from another video they are played correctly in both VLC and Windows Media Player.

  


  Update 2
As @Gyan said in his answer I should use the following command

  


  ffmpeg -i video.mkv -map 0:3 -c:s copy subs.ass


  


  But then if I attach the subs again with

  


  ffmpeg -i video.mkv -i subs.ass -map 0 -map -0:s -map 1 -c copy -c:s ass out.mkv


  


  The subtitles show up to 17th line in both VLC and Windows Media Player.

  


  or

  


  ffmpeg -i video.mkv -i .\subs.ass -map 0 -map -0:s -map 1 -c copy out.mkv


  


  The subtitles do not show up at all. (Not even in Windows Media Player)