Sur d’autres sites (10258)

• 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 ?

  


• 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)

• subprocess.run output is empty (python 3.8)

  26 juillet 2022, par Marino Linaje

  I'm trying to capture the output of a command with the following code :

  


  lines = subprocess.run(['ffmpeg', '-hide_banner', '-nostats', '-i', in_filename, '-vn', '-af', 'silencedetect=n={}:d={}'.format(silence_threshold, silence_duration), '-f', 'null', '-'], capture_output=True, text=True, shell=True, check=True, encoding='utf-8').stdout 
print (lines)


  


  But lines is an empty string and nothing is printed.
When capture_output=True is removed, the correct output is showed (without printing it).

  


  I tested many combinations, including removing all the subprocess.run parameters and only include capture_output=True with the same result.

  


  I also tested with few argument for a minimal example : subprocess.run(['ffmpeg', '-version'], capture_output=True, text=True, shell=True, check=True, encoding='utf-8').stdout

  


  Also tested stdout=subprocess.PIPE and stderr=subprocess.PIPE as subprocess.run arguments instead of capture_output=True

  


  I can't figure out what is happening. Thanks in advance !