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• Fastest way to extract moving dynamic crop from video using ffmpeg

  5 avril 2024, par NateW

  I'm working on an AI project that involves object detection and action recognition on roughly 30 minute videos.

  


  My pipeline is the following :

  


  

  • determine crops using object detection model

  


  • extract crops using Python and write them to disk as individual images for each frame.

  


  • use action recognition model by inputting a series of the crops.

  


  


  The models are fast but actual writing of the crops to disk is slow. Sure, using an SSD would speed it up but I'm sure ffmpeg would greatly speed it up.

  


  Some of the challenges with the crops :

  


  

  • the output size is always 128x128

  


  • the input size is variable

  


  • the crop moves on every frame

  


  


  My process for extracting crops is simple using cv2.imwrite(output_crop_path, crop) in a for loop.

  


  I've done experiments trying to use sndcommand and filter_complex. I tried this https://stackoverflow.com/a/67508233/4447761 but it outputs an image with black below the crop and the image gets wrapped around on the x axis.

  


  

  


• avformat/matroskadec : force 48kHz sample rate when rescaling Opus inital padding

  6 juin 2016, par James Almer

  avformat/matroskadec : force 48kHz sample rate when rescaling Opus inital padding
  Mkvtoolnix stores the sample rate of the original stream as reported by the
  
  "OpusHead" stream header instead of 48kHz, the actual sample rate of the Opus
  
  stream.
  
  Ignoring the stored sample rate and forcing 48kHz preserves the correct initial
  
  padding when remuxing such files.
  Reviewed-by : Michael Niedermayer <michael@niedermayer.cc>
  
  Signed-off-by : James Almer <jamrial@gmail.com>
  

  • [DH] libavformat/matroskadec.c

• Converting the sample rate of alac files with ffmpeg [on hold]

  23 décembre 2014, par sodiumnitrate

  I am using ffmpeg in terminal in OS X Yosemite. I have alac files I have previously converted from .flac without realizing that the sample rate is too high for my iPod. Thus, I have a bunch of .m4a files that I am trying to reduce the sample rate from 88,200 to 44,100. I tried the following, which I modified from a command I found elsewhere :

  for f in *.m4a; do ffmpeg -i "$f" -ar 44100  -y "${f%.m4a}.m4a"; done

  This gives me the following message :

  buffer @ 0x7ffed8e002a0] Unable to parse option value "-1" as pixel format
  
  Last message repeated 1 times
  
  [buffer @ 0x7ffed8e002a0] Error setting option pix_fmt to value -1.
  
  [graph 0 input from stream 0:0 @ 0x7ffed8e003a0] Error applying options to the filter.
  
  Error opening filters!

  And overwrote the files with gibberish.