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• Create a mkv file with colored background and containing a given audio and subtitle stream

  25 mai 2023, par rdrg109

  Table of contents

  


  

  • The context

  


  • Minimal working example

  


  • What I've tried

    

    • Create a mkv file with colored background and an audio stream

    


    • Create a mkv file with colored background, an audio stream and a subtitles stream

    


    


  


  • The question

  


  


  


  The context

  


  I have a *.flac file and a *.srt file. I want to merge those files in a MKV file, but at the same time, I want to add a video stream. I want the video stream to show a green background the entire time.

  


  


  Minimal working example

  


  For our experimentation, let's create two sample files : one *.flac file and one *.srt file.

  


  The following command creates a *.flac file that lasts 60 seconds and contains a sine wave.

  


  $ ffmpeg -y -f lavfi -i "sine=f=1000:d=60" input.flac


  


  The following command creates a *.srt file. Note that our last subtitle lasts until the sixth second, this is intended.

  


  $ cat << EOF > input.srt
1
00:00:00,000 --> 00:00:03,000
This is the first subtitle in a
SRT file.

2
00:00:03,000 --> 00:00:06,000
This is the second subtitle in a
SRT file.
EOF


  


  


  What I've tried

  


  


  Create a mkv file with colored background and an audio stream

  


  I know how to create a MKV file containing a given audio stream and a colored background as the video stream.

  


  The following command creates a MKV file containing input.flac as the audio stream and green background as the video stream. The MKV file have the same duration as input.flac.

  


  $ ffmpeg \
  -y \
  -f lavfi \
  -i color=c=green:s=2x2 \
  -i input.flac \
  -c:v libx264 \
  -c:a copy \
  -shortest \
  output.mkv


  


  The following command shows the duration of the streams in the resulting file.

  


  $ ffprobe -v error -print_format json -show_entries stream=codec_type:stream_tags=duration output.mkv | jq -r ''


  


  {
  "programs": [],
  "streams": [
    {
      "codec_type": "video",
      "tags": {
        "DURATION": "00:00:58.200000000"
      }
    },
    {
      "codec_type": "audio",
      "tags": {
        "DURATION": "00:01:00.000000000"
      }
    }
  ]
}


  


  


  Create a mkv file with colored background, an audio stream and a subtitles stream

  


  To add a subtitles stream, I just need to specify the *.srt file. However, when I do this, the duration of the video is set to the time of the last subtitle in the *.srt file. This is expected because I have used -shortest. I would get the result I'm looking for if it were possible to specify the stream that -shortest gives top priority to. I haven't found this information on the Internet.

  


  $ ffmpeg \
  -y \
  -f lavfi \
  -i color=c=green:s=2x2 \
  -i input.flac \
  -i input.srt \
  -c:v libx264 \
  -c:a copy \
  -shortest \
  output.mkv


  


  The following command shows the duration of the streams in the resulting file. Note that the maximum duration of the resulting file is 6 seconds, while in the resulting file from the previous section it was 1 minute.

  


  $ ffprobe -v error -print_format json -show_entries stream=codec_type:stream_tags=duration output.mkv | jq -r ''


  


  {
  "programs": [],
  "streams": [
    {
      "codec_type": "video",
      "tags": {
        "DURATION": "00:00:01.160000000"
      }
    },
    {
      "codec_type": "audio",
      "tags": {
        "DURATION": "00:00:03.134000000"
      }
    },
    {
      "codec_type": "subtitle",
      "tags": {
        "DURATION": "00:00:06.000000000"
      }
    }
  ]
}


  


  


  The question

  


  Given a *.flac file and a *.srt file. How to merge them in a *.mkv file so that it has the *.flac file as the audio stream, the *.srt file as the subtitles stream and a green background as the video stream ?

  


• How to sum audio from two streams in ffmpeg

  9 février 2021, par user3188445

  I have a video file with two audio streams, representing two people talking at different times. The two people never talk at the same time, so there is no danger of clipping by summing the audio. I would like to sum the audio into one stream without reducing the volume. The ffmpeg amix filter has an option that would seem to do what I want, but the option does not seem to work. Here are two minimal non-working examples (the audio tracks are [0:2] and [0:3]) :

  


  ffmpeg -i input.mkv -map 0:0 -c:v copy \
       -filter_complex '[0:2][0:3]amix' \
       output.m4v

ffmpeg -i input.mkv -map 0:0 -c:v copy \
       -filter_complex '[0:2][0:3]amix=sum=sum' \
       output.m4v


  


  The first example diminishes the audio volume. The second example is a syntax error. I tried other variants like amix=sum and amix=sum=1, but despite the documentation I don't think the sum option exists any more. ffmpeg -h filter=amix does not mention the sum option (ffmpeg version n4.3.1).

  


  My questions :

  


  


  1. Can I sum two audio tracks with ffmpeg, without losing resolution. (I'd rather not cut the volume in half and scale it up, but if there's no other way I guess I'd accept and answer that sacrifices a bit.)

     


  


  2. Is there an easy way to adjust the relative delay of one of the tracks by a few milliseconds ?

     


  


  


• sws_scale() does not convert image simply copying it from source to target

  14 mars 2020, par Slav

  Trying to read arbitrary video as plain RGB24 pixels so convert frame with sws_scale() this way :

      //...
  
      AVFrame* pic_out = av_frame_alloc();
  
      pic_out->format = AV_PIX_FMT_RGB24;
  
      pic_out->width  = 1920;
  
      pic_out->height = 1080;
  
      av_frame_get_buffer( pic_out, 32 );
  
  
  
      struct SwsContext * img_convert_ctx = sws_getContext(
  
          1920, 1080, AV_PIX_FMT_YUV420P,
  
          1920, 1080, AV_PIX_FMT_RGB24,
  
          SWS_BICUBIC,
  
          NULL, NULL, NULL
  
      );
  
      //...
  
      sws_scale(
  
          img_convert_ctx,
  
          pic_src->data,     //pic_src is from avcodec_receive_frame()
  
          pic_src->linesize,
  
          0,
  
          1080,
  
          pic_out->data,
  
          pic_out->linesize
  
      );

  Everything goes without any errors, but pic_out ends up having the same data as pic_src.
  What could be the problem ?

  Full minimal example is here (supposed to be RGB24 image is there as 2.bmp which looks like actually being YUV-something)