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• Synchronization of two RTMP streams with ffmpeg

  6 July 2022, by Strub3l

  I have the problem right now that I need to merge two or more streams into one, the problem is that they are out of sync.

  


  Is there any way to calculate or compare Stream 1 with Stream 2 and get the desync times and desync them automatically with ffmpeg?

  


  I know there is an option -itsoffset, but for that I have to set the delay manually for each stream, which would be a lot of hassle.

  


• How to specify how lossy/lossless a .webm conversion will be (in ffmpeg)?

  16 February 2021, by DistantDigit

  I can't seem to understand how to make the conversion lossless (or at least visually lossless)? The outputs have some fast moving parts at times, and the output would become blocky; I would like to keep it as lossless as possible, while still maintaining some compression. What would I have to tweak at the command line? Thanks you 

  


  ffmpeg -c:v libvpx-vp9 -i in.webm -c:v libvpx -vf scale=400:416,hue=h=45:s=1 -auto-alt-ref 0 out.webm

  


• LibVLC: Retrieving current frame number

  2 April 2015, by Solidus

  I am doing a project which involves a bit of video recording and editing and I am struggling to find a good C++ library to use. I am using QT as my framework and their video player is not working properly for me to use (seeking crashes some times, e.g.). Also, I need to record video and audio from my camera and QCamera does not work in windows (for recording).

  On my program the user can draw on top of the video and I need to store the start frame and the end frame of those drawings.

  Right now I’ve been testing Libvlc which almost does what I want. From what I can see they don’t have a way to just jump to a certain frame as this can only be done by time or position.

  The first solution that I came up with was to capture the time change event and then calculate the frame using the FPS. The problem is that, as far as I can tell, the interval of this event is around 250ms, which for a 15fps video is almost 4 frames.

  So, the second solution was to use libvlc_video_set_callbacks to make my own "lock, unlock and display" and count the frames there. This works for recording from the camera, as there is no going back and the frames go from 0 until the video stops. The problem is when playing a video. Since there is no timestamp, as far as I can tell, there is no way for me to know in which frame number I am (the user can be seeking for example). My "hacky" solution was to have a "lastTime" and "numTimes" on the struct I pass into these callbacks and this is what I do:

  lastTime represents the "last new time" received and numTimes represents the number of times lastTime was received.

  get_the_current_time
  
  calculate_frame_num_with_fps
  
  if current_time is equal to lastTime: 
  
       frameNum += numTimes
  
       numTimes++
  
  else 
  
       lastTime = current_time
  
       numTimes = 1

  This kinda works but I hate the solution. I’m not sure if when doing seeking the time changes if the difference is less than 250ms. That would maybe be kinda hard for a user to do but I’d prefer not to implement it like that.

  So my question is if there is another solution for this? If not, any libraries that could help me on this? I know about FFMPEG which seems would solve me this problem as it’s more low level and I could implement this solution. The problem is my deadline is approaching and that would still me take some time (learning the library and doing all the work). So I was thinking of it as a last resort.

  Thank you for your time.