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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 ! ^^

• 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 ?

  


• What are the correct conversion steps to generate a video from PDF with ffmpeg and mp4box

  25 juin 2020, par Michael Rall

  I need to convert a PDF-Document to a DASH-compatible stream. Every page of the PDF should be displayed for one second. Quality should be rather good, so that text and line graphics can still be read clearly. Additionally I want to be able to seek every page, so keyframes(?) should be (exactly) every second.

  


  Im using poppler-tools to extract the pdf-pages as png's -> works

  


  Im using ffmpeg to convert the single images to an x264 encoded mp4 video -> works (but maybe problematic)

  


  string.Format("-r 1 -i \"{0}.page_%d.png\" -r 24 -vcodec libx264 -pix_fmt yuv420p {0}.output.mp4", basePath)


  


  Im using MP4Box to convert that video to dash -> works (but needs improvement)

  


  string.Format("-dash 1000 -frag 1000 -rap -segment-name {1}_dash_$RepresentationID$_$Number$ -url-template {0}.output.mp4 -out \"{0}.{2}\"", basePath, tempDataId, STREAM_MANIFEST_FILE_POSTFIX)


  


  Now I have 2 Problems/Questions

  


  

  1. Are the settings for framerate, quality, outputformat correct for the goals I want to achieve

  


  2. When displaying the resulting DASH-Stream in the Demo-Player from https://github.com/Dash-Industry-Forum/dash.js/wiki it plays the stream, but as soon as I try to seek the player starts loading forever. Setting the position to start and let it play again works. The network tab in my browser-debugger shows that the player tries to load the segments and gets them successfully.

  


  


  I guess its a problem with keyframes, fragments or similar. Can somebody correct my conversion steps ?

  


  kind regards