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• Any other better ways to extract frames from a large size video according to the timecode given ?

  6 avril 2020, par Yong En

  Given a video from youtube with at least 600 MB. The video is annotated with labels that happens in multiple timecodes. The timecodes are in millisecond (SSSS.ss). I am trying to get the frames that fall within a time period (2 timecodes). There are TWO approaches that I used with different tools, one is using openCV in python and FFmpeg in bash script :

  



  I would stick with few variables here,

  



  

  • fps = 25

  


  • timecode (after convert into second) = 333.44 to 334.00 take note that I am dealing with time period that might less than a second.

  


  



  openCV

  



  

  1. Using openCV in python, I read the video as frames into a numpy array. Using the fps from video, 25. I can estimate where do the frames that fall under this time period by diving total video duration in second with length of the numpy array.

  


  2. Problem here is I will miss out some frames here due to the video fps is not really 25 as given by the video meta info, it could be 24.xx. Any solutions ?

  


  



  ffmpeg

  



  

  1. What I did, every time I want to get the frames, I run the script.

  


  2. Problem here is I need to read the video 100 times if I have 100 time periods. Any ways to overcome this ?

  


  



  Thanks for reading it.

  


• ffmpeg from chrome native messaging host app not working

  9 juin 2014, par Moisés Olmedo

  OK, this is what I got, I’m using native messaging to fire a c++ program from a chrome extension in Windows.
  This c++ program (ehost.exe) creates a ffmpeg process for recording the input audio and saving it to mp3.
  I do this with the CreateProcess() function
  This works perfect when I open ehost.exe myself (double clicking it)
  but when I open it from my chrome extension (chrome.runtime.connectNative() ), the ffmpeg process opens but ffmpeg writes no output file, as if it didn’t have permissions...

  Anybody knows why ?
  This is the ehost.exe code for starting ffmpeg.exe

  static HANDLE startFFmpeg()
   {
       char cmd[] = "C:\Users\moizilla\Desktop\native\ffmpeg\bin\ffmpeg.exe -y -f dshow -i audio=\"Microphone (High Definition Aud\" -acodec libmp3lame C:\Users\moizilla\Desktop\native\kasdkddfodkadd.mp3";
       PROCESS_INFORMATION pi = {0};
       STARTUPINFO si={0};
       si.cb = sizeof(STARTUPINFO);
       if(CreateProcess(NULL, cmd, NULL, NULL, TRUE, 0, NULL, NULL, &si, &pi)){
           CloseHandle(pi.hThread);
           return pi.hProcess;
       }
       return NULL;
}

• When creating a Xing or Info tag in an MP3, may I use any MP3 header or does it have to match other frames ?

  13 décembre 2019, par Alexis Wilke

  I have a set of bare MP3 files. Bare as in I removed all tags (no ID3, no Xing, no Info) from those files.

  Just before sending one of these files to the client, I want to add an Info tag. All of my files are CBR so we will use an Info tag (no Xing).

  Right now I get the first 4 bytes of the existing MP3 to get the Version (MPEG-1, Layer III), Bitrate, Frequency, Stereo Mode, etc. and thus determine the size of one frame. I create the tag that way, reusing these 4 bytes for the Info tag and determining the size of the frame.

  For those wondering, these 4 bytes may look like this :

  FF FB 78 04

  To me it felt like you are expected to use the exact same first 4 bytes in the Info tag as found in the other audio frames of the MP3, but when using ffmpeg, they stick an Info tag with a hard coded header (wrong bitrate, wrong frequency, etc.)

  My question is : Is ffmpeg really doing it right ? (LAME doesn’t do that) Could I do the same, skipping the load of the first 4 bytes and still have the greater majority of the players out there play my files as expected ?

  Note : since I read these 4 bytes over the network, it would definitely save a lot of time and some bandwidth to not have to load these 4 bytes on a HEAD request. Resources I could use for the GET requests instead...