====== FFmpeg Pipeline ======

FFmpeg is used to provide a video pipeline which can feed our virtual camera device (created with v4l2loopback). It is able to directly transmit videos or to gather images into a video and transmit them.

Installed on Ubuntu 14.04 through PPA found in https://ffmpeg.org/:
  sudo add-apt-repository ppa:mc3man/trusty-media
  sudo apt-get update
  sudo apt-get install ffmpeg

===== FFmpeg Parameters =====

FFmpeg has a wide array of parameters that can be used to create / modify a video stream. The ones we use are listed below. A more comprehensive list can be found [[https://ffmpeg.org/ffmpeg.html#Options|here]].

  * **-i (filename)** : input file
  * **-re** : Read input at native frame rate. Mainly used to simulate a live input stream
  * **-vcodec (codec)** : set the video codec
  * **-t (duration)** : limit input reading to (duration) or limit output when (duration) is reached
  * **-r (fps)** : duplicate or drop input frames to achieve constant output frame rate fps (regardless of the input video framerate)
  * **-pix_fmt (format)** : set the pixel format
  * **-vf (filter)** : video filter, here we use scale=640x480, forcing the input video to such resolution
  * **-f (format)** : Force input or output file format.
===== FFmpeg Recipes =====


Read video at original speed and deliver it to the v4l2 device:
  ffmpeg -re -i input.mp4 -r 30 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0

Same as above, video scaled to 640x480:
  ffmpeg -re -i input.mp4 -vf scale=640:480 -r 25 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0

Capture screen and direct it to v4l2 device:
  ffmpeg -f x11grab -r 15 -s 1280x720 -i :0.0+0,0 -vcodec rawvideo -threads 0 -f v4l2 /dev/video0

Sequencially composes images into a mp4 video - 5fps:
  ffmpeg -framerate 5 -start_number 1 -i data%d.png -r 5 -pix_fmt yuv420p out.mp4

Concat the videos listed in "desc" file:
  ffmpeg -f concat -i desc -codec copy output.mp4

  Content of "desc" file:
  file 'out1.mp4'
  file 'out2.mp4'


Grab our images (starting at index 1) and directly transmit them to the v4l2 device/ save them into a.mp4 file:
  ffmpeg -re -framerate 2 -start_number 1 -i data%d.png -vcodec rawvideo -r 15 -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0
  
  ffmpeg -re -framerate 2 -start_number 1 -i data%d.png -r 15 -pix_fmt yuv420p out.mp4

Same as above, but loops through images until -t is fulfilled:
  ffmpeg -loop 1 -framerate 2 -start_number 1 -i data%d.png -t 50 -r 15 -pix_fmt yuv420p out.mp4
  

===== FFmpeg Recipes w/ overlays =====
Play vid on udp + grab it and overlay it

  ffmpeg -re -i AMGS.mp4 -vcodec copy -vbsf h264_mp4toannexb -an -f rawvideo udp://127.0.0.1:1234

  ffmpeg -re -i ../Salvador.mp4 -i udp://127.0.0.1:1234 -filter_complex "overlay=0:0" -strict -2 -threads 0 -f v4l2 /dev/video0

——Overlay stops encoding if udp stream is stopped.


Play frames fed by pipe - Each overlay frame is on by 5s

  cat frag*.png | ffmpeg -re -y -i Salvador.mp4 -r 1/5 -f image2pipe -i - -filter_complex "overlay=0:0" -strict -2 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0


Create MJPEG stream to be overlayed on background video while it is running

  ffmpeg -re -i Salvador.mp4 -vcodec mjpeg -q:v 1 -strict -2 stream.mjpeg

  ffmpeg -re -y -debug 1 -i Salvador.mp4 -r 5 -i stream.mjpeg -filter_complex "overlay=0:0" -strict -2 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0


Stream MJPEG to namedPipe - Main background stream can overlay the MJPEG stream being generated

  ffmpeg -re -i Salvador.mp4 -vcodec mjpeg -q:v 1 -strict -2 -f mjpeg pipe:1 > anyPipe

  ffmpeg -re -y -debug 1 -i Salvador.mp4 -i anyPipe -filter_complex "overlay=0:0" -strict -2 -vcodec rawvideo -pix_fmt yuv420p -threads 0 -f v4l2 /dev/video0