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• 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 ! ^^

• FFmpeg metadata tags for YouTube 3D side-by-side movies

  17 février 2021, par Some1Else

  YouTube requires specific metadata tags for 3D Side-By-Side format movies. If the tag is not there then YouTube does not give the anaglyph playback options for the end user.

  



  Their "help" is here
https://support.google.com/youtube/answer/7278886?hl=en-GB

  



  I am trying to get FFmpeg to add the MP4 tag when building my movie.

  



  Code that works now for a series of PNG files into a MP4.

  



  "ffmpeg.exe" -framerate 60 -i "VID%05d.PNG" -c:v libx264 -preset:v veryslow -profile:v high -crf 15 -s 1280x720 -pix_fmt yuvj420p -an -y "OUTPUT.MP4"


  



  But when I add the tag from the YouTube help page

  



  "ffmpeg.exe" -framerate 60 -i "VID%05d.PNG" -c:v libx264 -preset:v veryslow -profile:v high -crf 15 -s 1280x720 -pix_fmt yuvj420p -an -y -x264opts "frame-packing=3:frame-packing-interpret=1:frame-packing-quincunx=0:frame-packing-grid=0,0,0,0" "OUTPUT.MP4"


  



  I get this error from FFmpeg

  



  [libx264 @ 0000000002a8c3a0] bad option 'frame-packing-interpret': '1'


  



  This page (ignore the proselytising)

  



  http://www.pantherdynamics.yolasite.com/panther-dynamics-blog/uploading-3dsbs-content-to-youtube

  



  says you only need to use this tag

  



  -x264opts frame-packing=3


  



  Using that does make FFmpeg finish correctly, but the SBS is still ignored by YouTube

  



  The YouTube engineer blog post here

  



  https://youtube-eng.googleblog.com/2011/09/getting-3d-content-on-youtube_8.html

  



  links to an older supposed 3D movie that also does not give 3D options any more.

  



  https://www.youtube.com/watch?v=ubRHSg5daMs

  



  So maybe even YouTube's own 3D movies are crippled ?

  



  What am I doing wrong ? Any FFmpeg gurus able to help ?

  


• How to use ffmpeg to encode isolated motion vectors (using codecview)

  22 juillet 2021, par Temmeh

  I have been successfully using this command to superimpose MPEG motion vectors onto a video :

  


  ffmpeg -v error -flags2 +export_mvs -i input.mp4 -vf codecview=mv=pf+bf+bb -y output.mp4

  


  Now I would like to be able to export an h264 mp4 of just the motion vector arrows, like in this example, without the original video layered underneath. I checked the codecview documentation and and did not find a flag or option to do this.

  


  Any help is appreciated !