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• Is it possible to get exactly same output from Gstreamer and FFMpeg ? [closed]

  23 septembre 2024, par beda pišl

  I found out that dumping the same video file (link : https://drive.google.com/file/d/1KaVXUMzLBKF8vOwy-QR1VCjRujEOHhFi/view?usp=sharing) using Gstreamer :

  


  gst-launch-1.0 filesrc location=video.avi ! decodebin ! videoconvert | pngenc ! multifilesink location=gstreamer_frames/"frame_%d.png"


  


  gives slightly different images compared to using equivalent ffmpeg command

  


  ffmpeg -i video.avi ffmpeg_frames/frame_%d.png


  


  The differences can be checked for example with Python :

  


  >>> import cv2
>>> ffmpeg_image = cv2.imread("ffmpeg_frames/frame_280.png")
>>> gstreamer_image = cv2.imread("gstreamer_frames/frame_280.png")
>>> ffmpeg_image == gstreamer_image
>>> gstreamer_image[-1, -5:, 0]
array([12,  6,  4,  0,  0], dtype=uint8)
>>> ffmpeg_image[-1, -5:, 0]
array([13,  8,  5,  2,  2], dtype=uint8)


  


  Is it possible to get exactly same output with Gstreamer and with FFMPeg ?

  


• How can I extract PGS/SUP subtitles from mkv container via libav

  24 août 2024, par Elia

  Trying to write a C code that does the equivalent of the following ffmpeg command :

  


  ffmpeg -i video.mkv -map 0:"$STREAM_INDEX" -c:s copy subtitles.sup


  


  I have managed to read the media container, locate the subtitle streams, and filter the relevant one based on stream metadata. However I only succeeded in getting the bitmap data, without the full PGS header and sections.

  


  Tried dumping packet->data to a file hoping it includes full sections, but the data doesn't start with the expected PGS magic header 0x5047.
  
Also using avcodec_decode_subtitle2 doesn't seem to be useful as it only gives bitmap data without the full PGS header and sections.

  


  What steps are supposed to happen after finding the relevant subtitle stream that allows extracting it as raw data that exactly matches the output of the ffmpeg command above ?

  


  Some sample code :

  


      while(av_read_frame(container_ctx, packet) == 0) {
      if(packet->stream_index != i) {
        continue;
      }

      // Try 1 - dump packet data to file
      fwrite(packet->data, 1, packet->size, fout);

      // Try 2 - decode subtitles
      int bytes_read = 0, success = 0;
      AVSubtitle sub;

      if ((bytes_read = avcodec_decode_subtitle2(codec_context, &sub, &success, packet)) < 0) {
        fprintf(stderr, "Failed to decode subtitles %s", av_err2str(bytes_read));
        goto end;
      }
      fprintf(stdout, "Success! Status:%d - BytesRead:%d NumRects:%d\n", success, bytes_read, sub.num_rects);

      if (success) {
        // Only managed to extract bitmap data here via sub.rects
      }

      av_packet_unref(packet);
    }


  


• FFMPEG convert RTSP to MPEG-TS with constant size packets [closed]

  15 août 2024, par Zimri Leisher

  I have an RTSP stream which I want to convert, via FFMPEG, to an MPEG-TS stream with constant size UDP packets.
I have played around with many FFMPEG options and the closest one I've found so far is this :
ffmpeg -i rtsp://localhost:123 -f rtp_mpegts udp://localhost:456
This produces a stream of packets of size 1328, which is equivalent to 7 MPEG-TS packets (188 bytes each) plus a 12-byte RTP header. However, I don't want to have an RTP header. I could remove this programmatically but I'd rather figure out how to get FFMPEG to do it properly.

  


  I haven't been able to find a combination of options which results in constant-size packets with the -f mpegts option. How do I tell the muxer how many MPEG-TS packets I want in each UDP packet ?

  


  ffmpeg -i rtsp://localhost:123 -f rtp_mpegts udp://localhost:456
-> packets with Len=1328
ffmpeg -i rtsp://localhost:123 -f mpegts udp://localhost:456?pkt_size=1316
-> packets with lengths from 220-1348 (1-7 MPEG-TS packets)