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• How to get non-decoded h264 stream from the webcam using ffmpeg ?

  14 mars 2018, par irobo

  I want to get the file which is non-decoded h264 format to use in another client application. I know how to stream to disk using below command from the docs.

  Example to encode video from /dev/video0 :

  ffmpeg -f v4l2 -framerate 25 -video_size 640x480 -i /dev/video0 output.mp4

  High level Diagram

  This is typical producer and consumer problem -

  Webcam  =============> ffmpeg to  video stream into file. (producer)                                    
  
                                         ^
  
                                         |
  
                                         |
  
  Client ________________________________|
  
  (consumer)
  
  // reads only Non-decoded h264 format from a file. 

• How to change mjpeg to yuyv422 from a webcam to a v4l2loopback ?

  3 janvier 2020, par Dev Null

  Backstory : One livestreaming site I use isn’t smart enough to detect the capabilities of my webcam (Logitech Brio, 4k), and instead just uses the default frames per second settings, which is 5fps.

  (full solution walk-through in the answer)

  The best solution I could think of (besides changing livestream providers) was to create a loopback virtual webcam using v4l2loopback that I could force to have the exact settings I wanted to use on that livestream site.

  For the brio, the higher frame rates come with mjpeg, not the default yuyv.

  Problem 1 :

  I could easily read mjpeg, but unfortunately kept banging my head against the wall because v4l2loopback evidently only wanted yuyv.

  I tried things like :

  ffmpeg -f v4l2              \
  
         -input_format mjpeg  \
  
         -framerate 30        \
  
         -video_size 1280x720 \
  
         -i /dev/video0       \
  
         -vcodec copy         \
  
         -f v4l2 /dev/video6

  and

  ffmpeg -f v4l2              \
  
         -input_format mjpeg  \
  
         -framerate 30        \
  
         -video_size 1280x720 \
  
         -i /dev/video0       \
  
         -vcodec yuyv422      \ # this line changed (even tried "copy")
  
         -f v4l2 /dev/video6

  But they wouldn’t work. I got errors like :

  Unknown V4L2 pixel format equivalent for yuvj422p

  and

  ...deprecated pixel format used, make sure you did set range correctly...

  ...V4L2 output device supports only a single raw video stream...

  Eventually I got this to work :

  ffmpeg -f v4l2              \
  
         -input_format mjpeg  \
  
         -framerate 30        \
  
         -video_size 1280x720 \
  
         -i /dev/video0       \
  
         -pix_fmt yuyv422     \ # The winning entry
  
         -f v4l2 /dev/video6

  Problem 2

  The next problem was getting chrome to see the virtual webcam. It worked correctly with guvcview, and on firefox I could use webcam testing sites and it would pick the virtual camera up without a problem.

  Turns out google, in it’s overly-protective nature (while it’s siphoning off all our data, btw), doesn’t want to use webcams that can be read and written to.

  So when starting v4l2loopback you have to tell it to announce that it’s "read only" to consumers like chrome.

  Here’s the exact modprobe I use that works :

  sudo modprobe v4l2loopback devices=1 exclusive_caps=1

• very low latency streaming with ffmpeg using a webcam

  14 août, par userDtrm

  I'm trying to configure ffmpeg to do a real-time video streaming using a webcam. The ffmpeg encoder command I use is as follows.

  



  ffmpeg -f v4l2 -input_format yuyv422 -s 640x480 -i /dev/video0 -c:v libx264 -profile:v baseline -trellis 0 -subq 1 -level 32 -preset superfast -tune zerolatency -me_method epzs -crf 30 -threads 0 -bufsize 1 -refs 4 -coder 0 -b_strategy 0 -bf 0 -sc_threshold 0 -x264-params vbv-maxrate=2000:slice-max-size=1500:keyint=30:min-keyint=10: -pix_fmt yuv420p -an -f mpegts udp://192.168.1.8:5001


  



  The ffplay command used to display the video feed is,

  



  ffplay -analyzeduration 1 -fflags -nobuffer -i udp://192.168.1.8:5001


  



  However, I'm experiencing a latency of 0.5 - 1.0s latency in the video stream. Is there a way to reduce this to a number less than 100ms. Also, when I replace the v4l2 camera capture with a screen capture using x11grab, the stream is almost real-time and I experience no noticeable delays. Moreover, changing the encoder from x264 to mpeg2 had no effect on the latency. In addition, the statistics from the ffmpeg shows that the encoder is performing at a 30fps rate, which I believe indicates that the encoding is real-time. This leaves me with only one reason for the experienced delay.

  



  

  • Is there a significant delay in buffers when using v4l2 during video capturing in a webcam ?

  


  • I don't think the transmission delay is in effect in this case as I see no latencies when screen capture is used under the same conditions.

  


  • Can this latency be further reduced ?. Can someone think of a different encoder configuration to be used instead of the one that I've been using ?