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• saving frames from webcam stream

  30 décembre 2018, par Alex Witsil

  I would like a routine that systematically extracts and saves the frames from webcam footage to a local directory on my personal computer.

  Specifically, I am trying to save frames from the webcam at Old Faithful geyser in Yellowstone Natl. Park. (https://www.nps.gov/yell/customcf/geyser_webcam_updated.htm)

  Ideally, I would like to :

  1. be able to control the rate at which frames are downloaded (e.g. take 1 frame every minute)
  2. use FFMPEG or R
  3. Save the actual frame and not a snapshot of the webpage

  Despite point 3 above, I’ve tried simply taking a screenshot in R using the package webshot :

  library(webshot)
  
  i=1
  
  while(i<=2) { 
  
  webshot('https://www.nps.gov/yell/customcf/geyser_webcam_updated.htm',delay=60,paste(i,'.png',sep=""))
  
  
  
  i=i+1
  
  }

  However, from the above code I get these two images :

  
  
  Despite the delay in the webshot() function (60 seconds) the two images are the same not to mention the obvious play button in the middle. This method also seems a bit of a hack as it is saving a snapshot of the website and not the frames themselves.

  I am certainly open to to using more appropriate command line tools (I am just unsure of what they are). Any help is greatly appreciated !

• Play UDP live video stream in UWP

  19 avril 2018, par Nicolas Séveno

  I need to display a live video stream in a UWP application.

  The video stream comes from a GoPro. It is transported by UDP messages. I think it is a MPEG-2 TS stream.

  I can play it successfully using FFPlay with the following command line :

  ffplay -fflags nobuffer -f:v mpegts udp://:8554

  I would like to play it with MediaPlayerElement without using a third party library.

  According to the following page :
  https://docs.microsoft.com/en-us/windows/uwp/audio-video-camera/supported-codecs
  UWP should be able to play it. (I installed the "MPEG 2 video extension" in the Windows Store).

  I tried using DatagramSocket and the MessageReceived event to receive the UDP packets, it works without problem :

  _datagramSocket = new DatagramSocket();
  
  _datagramSocket.MessageReceived += (s, args) =>
  
  {
  
      Debug.WriteLine("message received");
  
  };
  
  await _datagramSocket.BindServiceNameAsync(8554);

  Then I create a MseStreamSource :

  _mseStreamSource = new MseStreamSource();
  
  _mseStreamSource.Opened += (_, __) =>
  
  {
  
      _mseSourceBuffer = _mseStreamSource.AddSourceBuffer("video/mp2t");
  
  };
  
  this.MediaSource = MediaSource.CreateFromMseStreamSource(_mseStreamSource);

  And in the DatagramSocket.MessageReceived event I send the messages to the MseStreamSource :

  using (IInputStream stream = args.GetDataStream())
  
  {
  
      _mseSourceBuffer.AppendStream(stream);
  
  }

  The AppendStream method fails with error HRESULT 0x8070000B for some packets.
  If I catch the error, the MediaPlayerElement displays the message "video not supported or incorrect file name". (not sure of the message, my Windows is in French).

  Is the MseStreamSource the correct way to display the stream ? Is there a better solution ?

• How to parallelize this for loop for rapidly converting YUV422 to RGB888 ?

  16 avril 2015, par vineet

  I am using v4l2 api to grab images from a Microsoft Lifecam and then transferring these images over TCP to a remote computer. I am also encoding the video frames into a MPEG2VIDEO using ffmpeg API. These recorded videos play too fast which is probably because not enough frames have been captured and due to incorrect FPS settings.

  The following is the code which converts a YUV422 source to a RGB888 image. This code fragment is the bottleneck in my code as it takes nearly 100 - 150 ms to execute which means I can’t log more than 6 - 10 FPS at 1280 x 720 resolution. The CPU usage is 100% as well.

  for (int line = 0; line < image_height; line++) {
  
      for (int column = 0; column < image_width; column++) {
  
          *dst++ = CLAMP((double)*py + 1.402*((double)*pv - 128.0));                                                  // R - first byte           
  
          *dst++ = CLAMP((double)*py - 0.344*((double)*pu - 128.0) - 0.714*((double)*pv - 128.0));    // G - next byte
  
          *dst++ = CLAMP((double)*py + 1.772*((double)*pu - 128.0));                                                            // B - next byte
  
  
  
          vid_frame->data[0][line * frame->linesize[0] + column] = *py; 
  
  
  
          // increment py, pu, pv here
  
  
  
      }

  ’dst’ is then compressed as jpeg and sent over TCP and ’vid_frame’ is saved to the disk.

  How can I make this code fragment faster so that I can get atleast 30 FPS at 1280x720 resolution as compared to the present 5-6 FPS ?

  I’ve tried parallelizing the for loop across three threads using p_thread, processing one third of the rows in each thread.

  for (int line = 0; line < image_height/3; line++) // thread 1
  
  for (int line = image_height/3; line < 2*image_height/3; line++) // thread 2
  
  for (int line = 2*image_height/3; line < image_height; line++) // thread 3

  This gave me only a minor improvement of 20-30 milliseconds per frame.
  What would be the best way to parallelize such loops ? Can I use GPU computing or something like OpenMP ? Say spwaning some 100 threads to do the calculations ?

  I also noticed higher frame rates with my laptop webcam as compared to the Microsoft USB Lifecam.

  Here are other details :

  • Ubuntu 12.04, ffmpeg 2.6
  • AMG-A8 quad core processor with 6GB RAM
  • Encoder settings :
    • codec : AV_CODEC_ID_MPEG2VIDEO
    • bitrate : 4000000
    • time_base : (AVRational)1, 20
    • pix_fmt : AV_PIX_FMT_YUV420P
    • gop : 10
    • max_b_frames : 1