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• Play video with multiple audio tracks - different languages to separate devices

  8 février, par Sandre

  I want watch movie with my wife, listening to different audio channels - one language for me, another for my wife.

  


  I found solution and it works fine but not optimal.

  


  I know nothing about audio conversion, so asking to make it better.

  


  I'm using ffmpeg to combine separate stereo tracks to 5.1, then playing using IINA to Agregate device defined in Audio MIDI Setup (Speakers and bluetooth headphone). This for MacOS, but I'm pretty sure something similar will work for windows, taking in count IINA under hood use MPV.

  


  

  


  ffmpeg -i eng.mp3 -filter_complex "[0:a]channelsplit=channel_layout=stereo[left][right]" -map "[left]" en_left.wav -map "[right]" en_right.wav
ffmpeg -i ru.ac3 -filter_complex "[0:a]channelsplit=channel_layout=stereo[left][right]" -map "[left]" ru_left.wav -map "[right]" ru_right.wav

ffmpeg -i ru_left.wav -i ru_right.wav -i ru_left.wav -i ru_right.wav -i en_left.wav -i en_right.wav \
-filter_complex "[0:a][1:a][2:a][3:a][4:a][5:a]join=inputs=6:channel_layout=5.1[a]" -map "[a]" output.wav



  


  What I want to improve :

  


  

  1. I added 2 useless channels (2 and 3) - just a copy of 1 and 2.

  


  2. All tracks converted - probably it's possible just copy them ?

  


  3. Would be good to have possibility extract channels embedded into video.

  


  


• avformat/matroskaenc : Check allocations implicit in dynamic buffers

  29 avril 2020, par Andreas Rheinhardt

  avformat/matroskaenc : Check allocations implicit in dynamic buffers
  Failures of the allocations that happen under the hood when using dynamic
  
  buffers are usually completely unchecked and the Matroska muxer is no
  
  exception to this.
  The API has its part in this, because there is no documented way to
  
  actually check for errors : The return value of both avio_get_dyn_buf()
  
  as well as avio_close_dyn_buf() is only documented as "the length of
  
  the byte buffer", so that using this to return errors would be an API
  
  break.
  Therefore this commit uses the only reliable way to check for errors
  
  with avio_get_dyn_buf() : The AVIOContext's error flag. (This is one of
  
  the advantages of avio_get_dyn_buf() : By not destroying the AVIOContext
  
  it is possible to inspect this value.) Checking whether the size or the
  
  pointer vanishes is not enough as it does not check for truncated output
  
  (the dynamic buffer API is int based and so has to truncate the buffer
  
  even when enough memory would be available ; it's current actual limit is
  
  even way below INT_MAX).
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavformat/matroskaenc.c

• How to check if OpenCV VideoCapture uses multithreading or multiprocessing internally when connecting to a camera [closed]

  20 décembre 2024, par Chen

  I'm using OpenCV's cv::VideoCapture in C++ (or Python) to connect to a camera stream. I would like to know if OpenCV internally uses multithreading or multiprocessing when opening and capturing frames from the camera.

  


  Specifically, I'm curious about :

  


  

  • Whether OpenCV spawns new threads or processes when using cv::VideoCapture.

  


  • If FFmpeg is enabled as the backend, does it use multithreading or multiprocessing under the hood ?

  


  • How can I verify the threading or process behavior during camera streaming ?

  


  


  What I Tried :
I checked the current process using psutil (in Python) and printed the number of threads :

  


  import cv2
import psutil

cap = cv2.VideoCapture("rtsp://camera_stream", cv2.CAP_FFMPEG)
process = psutil.Process()

print(f"Process ID: {process.pid}, Name: {process.name()}")
print(f"Thread count: {process.num_threads()}")

for thread in process.threads():
print(f"Thread ID: {thread.id}, CPU Time: {thread.user_time + thread.system_time}")

cap.release()


  


  

  • Does VideoCapture use multithreading or multiprocessing internally ?

  


  • Is there a reliable way to verify whether the underlying backend (e.g., FFmpeg) has enabled multithreading ?

  


  • Are there specific settings in OpenCV or FFmpeg that can control this behavior ?