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• How to add arbitrary or custom metadata in MP4 ?

  13 avril 2021, par 大大大大萝卜凉

  The MP4 muxer in ffmpeg only allows certain metadata by default. I would like to add :

  


  com.android.model: Mi 10 Pro
xyz: +22.9835+113.3621/
com.android.version: 1
com.android.manufacturer: Xiaomi


  


  How can I add this with ffmpeg ?

  


• How would I assign multiple MMAP's from single file descriptor ?

  9 juin 2011, par Alex Stevens

  So, for my final year project, I'm using Video4Linux2 to pull YUV420 images from a camera, parse them through to x264 (which uses these images natively), and then send the encoded stream via Live555 to an RTP/RTCP compliant video player on a client over a wireless network. All of this I'm trying to do in real-time, so there'll be a control algorithm, but that's not the scope of this question. All of this - except Live555 - is being written in C. Currently, I'm near the end of encoding the video, but want to improve performance.

  To say the least, I've hit a snag... I'm trying to avoid User Space Pointers for V4L2 and use mmap(). I'm encoding video, but since it's YUV420, I've been malloc'ing new memory to hold the Y', U and V planes in three different variables for x264 to read upon. I would like to keep these variables as pointers to an mmap'ed piece of memory.

  However, the V4L2 device has one single file descriptor for the buffered stream, and I need to split the stream into three mmap'ed variables adhering to the YUV420 standard, like so...

  buffers[n_buffers].y_plane = mmap(NULL, (2 * width * height) / 3,
  
                                      PROT_READ | PROT_WRITE, MAP_SHARED,
  
                                      fd, buf.m.offset);
  
  buffers[n_buffers].u_plane = mmap(NULL, width * height / 6,
  
                                      PROT_READ | PROT_WRITE, MAP_SHARED,
  
                                      fd, buf.m.offset +
  
                                      ((2 * width * height) / 3 + 1) /
  
                                      sysconf(_SC_PAGE_SIZE));
  
  buffers[n_buffers].v_plane = mmap(NULL, width * height / 6,
  
                                      PROT_READ | PROT_WRITE, MAP_SHARED,
  
                                      fd, buf.m.offset +
  
                                      ((2 * width * height) / 3 + 
  
                                      width * height / 6 + 1) / 
  
                                      sysconf(_SC_PAGE_SIZE));

  Where "width" and "height" is the resolution of the video (eg. 640x480).

  From what I understand... MMAP seeks through a file, kind of like this (pseudoish-code) :

  fd = v4l2_open(...);
  
  lseek(fd, buf.m.offset + (2 * width * height) / 3);
  
  read(fd, buffers[n_buffers].u_plane, width * height / 6);

  My code is located in a Launchpad Repo here (for more background) :
  http://bazaar.launchpad.net/ alex-stevens/+junk/spyPanda/files (Revision 11)

  And the YUV420 format can be seen clearly from this Wiki illustration : http://en.wikipedia.org/wiki/File:Yuv420.svg (I essentially want to split up the Y, U, and V bytes into each mmap'ed memory)

  Anyone care to explain a way to mmap three variables to memory from the one file descriptor, or why I went wrong ? Or even hint at a better idea to parse the YUV420 buffer to x264 ? :P

  Cheers ! ^^

• How to write mp4 video file with H264 codec ?

  21 mai 2017, par kramer65

  On OSX I can record from my webcam and write a video file with the following simple script :

  import cv2
  
  camera = cv2.VideoCapture(0)
  
  
  
  # Define the codec and create VideoWriter object to save the video
  
  fourcc = cv2.VideoWriter_fourcc(*'XVID')
  
  video_writer = cv2.VideoWriter('output.avi', fourcc, 25.0, (640, 480))
  
  
  
  while True:
  
      try:
  
          (grabbed, frame) = camera.read()  # grab the current frame
  
          frame = cv2.resize(frame, (640, 480))  # resize the frame
  
          video_writer.write(frame)  # Write the video to the file system
  
  
  
      except KeyboardInterrupt:
  
          camera.release()
  
          break

  The resulting avi file is quite big though. I want a smaller file, preferably an mp4. So I changed the filename to output.mp4 and the fourcc codec to H264. That writes a video file which works, but gives me the following error :

  $ python write_video_file.py 
  
  OpenCV: FFMPEG: tag 0x34363248/'H264' is not supported with codec id 28 and format 'mp4 / MP4 (MPEG-4 Part 14)'
  
  OpenCV: FFMPEG: fallback to use tag 0x00000021/'!???'

  Since I thought I’m missing the H264 codec in ffmpeg I decided to uninstall ffmpeg and opencv and reinstall them again with H264 support. For this I used the following commands :

  # First ffmpeg
  
  brew install ffmpeg --with-fdk-aac --with-libvidstab --with-openh264 \
  
        --with-openjpeg  --with-openssl --with-tools --with-webp --with-x265 --with-zeromq
  
  # then opencv3
  
  brew tap homebrew/science
  
  brew install opencv3 --with-contrib --with-ffmpeg --with-tbb

  After this I ran the script again, using the following combinations :

  • output.mp4 with H264
  • output.mp4 with X264

  Unfortunately I still get the OpenCV warnings/errors. The file is readable, but it still annoys me that I get these errors. Does anybody have any idea how I can make OpenCV write mp4 video file with the H264 codec ?

  All tips are welcome !