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• FFmpeg/libav Storing AVPacket data in a file and decoding them again in a different stream - How to prepare and send custom packets in the decoder ?

  21 avril 2022, par Hitokage

  I want to be able to open a stream with a video file and send in the decoder my own packets with data that I previously stored from a different stream with the same codec. So like forging my own packets in the decoder.

  


  My approach is that I encode frames into packets using H.265 and store the data in a file like this :

  


  AVPacket *packet;
std::vector data;
...encoding...
data->insert(data->end(), &packet->data[0], &packet->data[packet->size]);
...storing the buffer in a file...


  


  I also have one mkv video with H.265 stream of the same parameters. Now I want to get the stored packet data in the file, create a new packet, and send it into the decoding process of the mkv file. To make it easier I just copy the parameters of the first packet in the mkv stream into a new packet where I insert my data and send it to the decoder. Is this a right approach ?

  


  ...open decoder with mkv file...
auto packet = av_packet_alloc();
av_read_frame(formatContext, packet);
//here is the packet I will use later with my data
av_packet_copy_props(decodingPacket, packet);
decodingPacket->flags = 0;
//sending the packet from the mkv
avcodec_send_packet(codecContext, packet);
//the data contains the previously stored data
av_packet_from_data(decodingPacket, data.data(), data.size());
avcodec_send_packet(codecContext, decodingPacket);
...retrieving the frame...


  


  However, I am getting this error when I try to send the forget packet :

  


  Assertion buf_size >= 0 failed at libavcodec/bytestream.h:141


  


  I have tried to manually change the decodingPacket->buf->size but that is probably not the problem. I believe that I need to set up some other parameters in the forged packet but what exactly ? Maybe I can also somehow store not only the packet data but the whole structure ? Please let me know, if there is a better way to store and reload a packet and force it into an unrelated decoder with the same parameters.
Thanks !

  


  EDIT : Seems like the buf_size problem was related to my data which were wrongly retrieved. I can confirm that this works now but I am getting a new error :

  


  [hevc @ 0x559b931606c0] Invalid NAL unit size (0 > 414).
[hevc @ 0x559b931606c0] Error splitting the input into NAL units.


  


• FFmpeg and Blu-ray subtitles [closed]

  14 octobre 2020, par Paul DeRocco

  I have some Blu-ray rips with PGS (image-based) subtitles. My goal is to transcode the video to lower-rate H265, to save space and lower the bitrate, but I need the subtitles. I don't really care what the container is—it can remain .mt2s, or be anything else, but I've been unable to get FFmpeg to do anything with this kind of video that doesn't discard the subtitles, or convert them into "PES packets containing private data", as VLC Player describes it.

  



  For instance, how can I do something as simple as making another .mt2s file with the first minute of an existing .mt2s file, copying all the streams verbatim, just truncating them all ? I can understand that perhaps you can't put PGS subtitle streams into some containers, but if the output is the same kind of container as the input, why would that be a problem ?

  



  ---

  



  If I use FFmpeg to list the streams in my original file, it says :

  



  Stream #0:0[0x1011]: Video: h264 (High) (HDMV / 0x564D4448), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 90k tbn, 47.95 tbc
Stream #0:1[0x1100]: Audio: ac3 (AC-3 / 0x332D4341), 48000 Hz, mono, fltp, 192 kb/s
Stream #0:2[0x1200]: Subtitle: hdmv_pgs_subtitle ([144][0][0][0] / 0x0090), 1920x1080


  



  If I do

  



  ffmpeg -i foobar.m2ts -c copy -t 1:00 test.m2ts


  



  I end up with the following streams in test.m2ts :

  



  Metadata:
  service_name    : Service01
  service_provider: FFmpeg
Stream #0:0[0x100]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(progressive), 1920x1080 [SAR 1:1 DAR 16:9], 23.98 fps, 23.98 tbr, 90k tbn, 47.95 tbc
Stream #0:1[0x101]: Audio: ac3 ([129][0][0][0] / 0x0081), 48000 Hz, mono, fltp, 192 kb/s
Stream #0:2[0x102]: Data: bin_data ([6][0][0][0] / 0x0006)


  



  I was trying to create a really short file so that I could play with different options for doing what I really want to do, which is to compress the video more, without losing the subtitles, and I don't want to wait for it to convert a whole movie before I discover that it didn't work right.

  


• ffmpeg subtitles background issue when multiple lines (using .srt format)

  3 mai 2022, par Dr. House

  While using ffmpeg to burn .srt subtitles to mp4 files I'm having an issue with multiple text lines - background is overlaying each other.

  


  Command I'm using :

  


  ffmpeg -i source_video_path.mp4 -vf "subtitles=srt_source.srt:force_style='OutlineColour=&H80000000,BorderStyle=3,Outline=1,Shadow=0,MarginV=25,Fontname=Arial,Fontsize=10,Alignment=2'" video_destination.mp4


  


  

  


  Question is - is it possible to overcome the overlay while still having a transparent background while using .srt format or I need to use .ass format as a fix ?