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• Streaming Encoded MPEG-4 live video from a web camera using RTP in C++

  6 mars 2017, par Maad A.Galil

  I have been working on building a video streamer from a webcam using RTP protocol in C++ language in Linux. I was able to use opencv to take frames from the webcam and send them frame by frame to a client app that shows those frames one by one using the same frame per second parameter.

  But that is mostly a MJPEG kind of transfer as the encoding used is JPEG encoding with a specific quality for each frame. However, I would like to use MPEG-4 encoding before transferring the video. After a deep search online I found out that ffmpeg is the best for such purpose. But the samples online mostly work through a command line interface to capture a webcam video and save it as a video file with the given encoding and format.

  What I am asking about here. Is there a possibility to encode the webcam "LIVE", and get byte data while encoding the camera stream. I would like to use those byte data to transfer it using RTP for example. And then I would like to decode the received byte data and show the video using opencv imshow function for example.

  I hope that I could explain my question clearly.

  Thanks in advanced,

  Yours,
  Maad

• How to run ffmpeg command from the client side ?

  12 janvier 2017, par Debraj Biswas

  I am trying to make an online examination portal. When students start the exam, their webcam will start automatically and record the stream live and store in the server. Invigilators will either watch the students live or they can watch the saved live streams later.

  After many research on various technologies I came across this link. It uses Node JS with websockets and ffmpeg. I ran the ffmpeg command and it streamed the live video successfully.

  But the problem is, in order to live stream, the students have to have ffmpeg installed in their system and they should run the command directly from the terminal. So how can I change this ? The students will live stream from their browser, because this is a web portal. If we put this command in a PHP script, then the command will run at the server side. But client side command should run in this case. How can I run command from client side ?

  Any suggestions will be helpful.

• Debugging in Visual Studio without Pdb files (C++ Access Voilation)

  2 décembre 2016, par Prakash M

  I built OpenCV binaries(.dll) using Cmake & visual studio which generated .pdb file which helped me to find the issue in code (Partially !)

  How this Crash is being caused. .
  I’m using a software with which we can set internet download speed limit (transfer rate) for any particular program.

  Now if i connect IP camera to the code below, i noticed that my app needs around 100Kb/s of internet usage (transfer rate) - only then i can watch live stream seamlessly.
  Lets say i cut down (set) my application internet usage to 10Kb/s [This is the reason behind crash]
  in this case, i should be able to see a new frame once in 4+ seconds.

  I’m getting access violation error probably because (cap>>img;) cap is trying to reach a location in ram & get the frame but there is no frame YET because its still being downloaded due to low internet speed.
  Clearly the pointer is reaching some location in ram to grab a frame which is not yet present.

  Some interesting behaviour . . .

  Void OpenCamera()
  
  {
  
      VideoCapture cap("http://192.168.1.3:8080/video?x.xmjpeg");
  
      Mat img;
  
      while(true)
  
      {
  
        try
  
        {
  
          if(cap.isOpened()) //also tried grab + retrieve, crashes at grab
  
          cap>>img; //code crashes here
  
        }
  
        catch(...)
  
        {
  
          cout<<"Camera Disconnected"<code>

  If i use the entire code in same class (within same header file), there is no problem at all(new frame is displayed after 4+ seconds without crashing the program) but if i put the code into a separate class(different header file), then call the function to open camera from a class object, then it crashes if internet speed is cut down.
  weird behavior - if i debug step by step, it never crashes !
  

  when i build opencv library with ffmpeg , i get .pdb file only for opencv (opencv_world310.pdb)- so no issue debugging using call stack
  but i do not get pdb for ffmpeg (because Opencv_ffmpeg.dll is precompiled and that is where its crashing)

  hence its getting hard to debug, building ffmpeg doesn’t produce pdb file cause its built using MSYS
  so is it possible to debug with what we have ?

  I’m including snapshot from visual studio debugging,
  some of the variables that will help in understanding :

  typedef int (*CvGrabFrame_Plugin)( void* capture_handle );      [cap_ffmpeg_api.cpp]
  
  protected: void* ffmpegCapture;                                 [cap_ffmpeg.cpp]
  
  static CvGrabFrame_Plugin icvGrabFrame_FFMPEG_p = 0;            [cap_ffmpeg.cpp]

  

  Exception thrown at 0x0A0AF6F0 (opencv_ffmpeg310.dll) in Sample.exe :
  0xC0000005 : Access violation reading location 0x00000020. If there is
  a handler for this exception, the program may be safely continued.

  in source code i included below line & compiled & used it in project - didn’t work, crashed again !
  if(ffmpegCapture) - null pointer check

  can we make some changes at line 214 in [cap_ffmpeg.cpp] to avoid crash ?
  other header files are just one folder up.

  Update : I noticed that program crashes immediately when i limit internet consumption speed. I’m using C++/Cli(winforms, target dot net Framework = 4.6), i have CameraClass (in separate header file) & main function in (separate header file)
  Main function has below code
  

  CameraClass ^CC = gcnew CameraClass();
  
  CC->OpenCamera();

  Some clash between .net memory handling & C++ memory handling ?