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• Concatenate MOV files without re-encoding on iOS with ffmpeg libs

  2 juillet 2013, par Developer82

  I would like to concatenate MOV files without re-encoding. I want to do it on iOS (iPhone). All the MOV files are recorded with the same settings, no difference in dimensions or encoding profiles.

  I have succeeded to do it with the command line ffmpeg :
  ffmpeg -re -f concat -i files.txt -c copy ...
  But I have difficulties using the libraries.

  I think the demuxing part is ok, I have the h.264+AAC packets. After demuxing I shift the PTS and DTS info of each packet to have ascending values in the joined MOV file.
  The hard part is the muxing.

  I have built the ffmpeg libs with x264 lib, so it can be used if necessary, but I am not sure whether I need the x264 codec since I don't want to re-encode the MOV files, I just want to join them.

  Problems I have encountered :

  1. In this case I do not use x264 codec. At muxing I create the stream with NULL codec parameter. I have successful writing of header, packets and trailer. All the function calls return with zero error code. However, the output can be opened, but black "screen" is displayed during playback. FFprobe report is attached. I have also examined the output with MediaInfo tool. I have attached that report as well (MediaInfo report - without x264 codec.txt). As you can see there is no h.264 profile or pixel info found that might be a problem.

  2. In this case I use x264 codec with functions : avcodec_find_encoder, avformat_new_stream and avcodec_open2. Again : no decode-encode ! In this case I have much more metadata in the output file like h.264 profile and pixel info (YUV), but the av_interleaved_write_frame call simply does nothing but returns success code (0). No packet is written to the file. :( I don't know how this could happen. fwrite works, but results in un-openable file. I have also attached the MediaInfo report of this output (MediaInfo report - with x264 codec.txt).

  Questions :

  • How should I process the demuxed packets to feed the muxer ?
  • What format context and codec context setting should be done including AVOption settings ?
  • Should I use the x264 codec to do this ? I just vant to re-mux the chunks into a single joined file.
  • The chunks have their own header/trailer. Should I somehow filter the demuxed packets to skip them ?
  • The final goal is creating a network stream (RTP or RTMP) - also with re-muxing and without re-encoding. It works with command line ffmpeg :
    ffmpeg -re -f concat -i files.txt -vcodec copy -an -f rtp rtp://127.0.0.1:20000 -vn -acodec copy -f rtp rtp://127.0.0.1:30000

  Concatenating to MOV format is only an intermediate pilot. Is it recommended to work on the network format since it is so different task that there is no benefit of solving the MOV format muxing ?

  Any help, advice, suggestion is greatly appreciated.
  I can reveal code to make deeper investigation possible.

  Thanks !

• ffmpeg video translation with filter

  23 juin 2013, par user987058

  I have a problem doing translation of pixels in a frame of a video. What I want to do is to translate all pixels by certain amount (say 50 pixels) in a frame. I tried to modify the example filters to assign pixels to out array by 50 pixels. It does somehow shift the content but however, the color is shifted (or the color stays at the same place.) I'm very new to video processing and video format. Could anyone tell me what's wrong with my approach and what should I change it.

  Thank you,
  Yi

• How to use libavformat to concat 2 video files with same codec (re-muxing) ?

  27 septembre 2013, par Tarhan

  I have downloaded videos from CDN in flv format (video H264 and audio AAC) and remux to them to MP4 format. But videos are limited by length. So i've downloaded each video in several parts : started at start point, at point 1, at point 2 (by using seek parameter in url). Each point starts little earlier than ending of previous one.
  Using av_read_frame i scanned all parts and found that intersecting packets not only have same sizes and order but also their dts/pts shifted from each other by constant value. So to concat starting video with video started at point 1 I must do following :
  1. Create output header in output file
  2. Copy all non-intersecting packets from starting video.
  3. Copy all non-intersecting packets from video started at point 1 with changed dts values by shifted it by constant

  How to do all of this using libav (not ffmpeg) ? I read How can libavformat be used without using other libav libraries. But in libav it is not working since there not avformat_alloc_output_context2 in libav. Also source avconv.c source is too complex for newbie like me to isolate parts related to stream copy operations.
  Can someone provide me example to :
  
   open input_file1 and input_file2 (only needed if procedure differs from standard in generic tutorials)
  
   open and write header for output_file with same container format and same video and audio formats
  
   write packets from input_file1 to output_file up to packet with for example pos == XXX
  
   write packets from input_file2 to output_file changing their dts (or whatever needed) by constant value
  
   write correct trailer

  Calculating of time shift in dts i made before.