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• FFMPEG / MJPEG Encoding / How to add DQT table in packet data ?

  20 mai 2020, par GoodSimon

  I write my own transcoder in C++ to convert any image to jpeg based on the examples in ffmpeg sources. I use the MJPEG codec to create a jpeg image. By sending AVFrame to the MJPEG encoder I get AVPacket. Then I just take the data from the packet->data structure with the size avpacket->size and save it to a file with the jpg extension.
The problem is that the finished JPEG image has only one DQT (Define Quantization Table). As far as I understand, there should be two tables in the file.

  



  What do I need to do to have 2 quantization tables in avpacket->data ?

  


• Downsampling / Filtering Data Stream with FFMPEG

  15 septembre 2021, par sds

  We have a .ts input file that contains (among other streams) a video stream and MISB 0604-compliant KLV data stream. The output of ffprobe for these stream are :

  


  Stream #0:0[0x111]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p(progressive), 1280x720 [SAR 1:1 DAR 16:9], 29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
...
Stream #0:2[0x1001]: Data: klv (KLVA / 0x41564C4B)


  


  We are hoping to extract every Nth frame of the video as a .tiff. We also need to associate each of these frames with a corresponding KLV packet from the MISB 0604-compliant data stream.

  


  The following command that select filters and adjusts the original FPS by the corresponding ratio does result in the expected frames being saved out as TIFF (in this case the original video has 1187 frames, and I expect to get 12 frames from the select filter).

  


  ffmpeg -y -i 2205Z.ts -map 0:0 -vf "select='not(mod(n,100))'",fps=30000/1001/100 -compression_algo raw -pix_fmt rgb24 %05d.tif


  


  However I can't seem to get any filters working on the data stream. For example using filter:d does not throw an error, but also doesn't seem to actually filter. My question is whether ffmpeg can be used to save out a "downsampled" data stream corresponding to the downsampling operations performed on the video stream above ?

  


• How to force avcodec to use unaligned frame data planes ?

  26 février 2015, par user3244284

  I have been searching high and low for an option to force avcodec to use unaligned memory for its AVFrame data.

  Depending on the pixel format, the horizontal planes of an AVFrame->data may be padded with extra data to be aligned to memory for performance.

  eg : a 1920 * 1080 video with 4 bytes per pixel will have 1920 * 4 = 7680 bytes per plane.

  With avcodec if you are decoding this video it will create 7808 bytes per plane.

  This adds 7808 - 7680 = 128 bytes of extra padding.

  For my purposes I would like to force avcodec to use unaligned data so I can copy an entire continuous chunk of frame data instead of copying and formatting smaller pieces one at a time to a continuous chunk.

  The following flag found in the headers :

  /* encoding support
  
     These flags can be passed in AVCodecContext.flags before initialization.
  
     Note: Not everything is supported yet.
  
  */
  
  
  
  /**
  
   * Allow decoders to produce frames with data planes that are not aligned
  
   * to CPU requirements (e.g. due to cropping).
  
   */
  
  #define CODEC_FLAG_UNALIGNED 0x0001

  Setting this AVCodecContext.flags to be CODEC_FLAG_UNALIGNED, the assumption is that the AVFrame->data is now unaligned, this is not the case.

  I’m not sure if I am looking at the right place or using this flag correctly.

  Regards,

  Curious George