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• FFMPEG : Reversing only a segment of a video file

  25 juillet 2019, par N. Johnson

  I am writing a script that takes a piece of video and then reverses the same piece of video and then concats the two together so the final video plays forwards and then loops backwards. I should note that eventually I want to be able to pull an unequal length for the reverse part.

  I can get the entire file to do this, but getting just a segment is not working as expected.

  See code below

  I’ve tried :

  %1 = timecode to seek to (the video file is only 20 seconds and never any longer)

  %2 = length of segment to pull out

  %3 = usually the same as %2 but may be different if we want to only reverse 2 seconds instead of the full 5 for example.

  ffmpeg -ss 00:00:%1 -an -i test.mp4 -t 00:00:%2 out.mp4
  
  :: the above works as expected
  
  
  
  :: this doesn't, no matter what I put into -ss. I've also tried moving -ss out front of the -i as suggested in the documentation? It gives me the right length of segment but never starts in the right place. 
  
  
  
  ffmpeg  -an -i test.mp4 -ss 00:00:xx -t 00:00:%3 -vf reverse reversed2.mp4
  
  
  
  :the below works fine
  
  ffmpeg -f concat -safe 0 -i mylist.txt -c copy output.mp4

  When I run this with say %1 = 05 and %2 = 05, I get a segment from 5 seconds in that lasts 5 seconds. Then I get a seemingly random starting point and 5 seconds of reversed video. I’ve tried a number of inputs in "XX" from 10 (which I think is right) to 0 to 19 and all of them produce output. But it’s all wrong.

• How to encode a HEVC video from YUV420 Mat data (from a hardware triggered camera) via FFmpeg in Python

  4 juillet 2023, par QuantumRiver

  I am having problems encoding a HEVC video from a series of YUV420 Mat data via FFmpeg.

  


  

  1. I am using python in Ubuntu-20.04 ;

  


  2. I am retrieving frame data from a hardware triggered camera (BASLER), using pypylon ;

  


  3. I want to write a video from that camera in HEVC codec, using my GPU-NVENC ;

  


  4. I guess I have to use FFmpeg to achieve these ;

  


  


  What I have tried :

  


  

  1. I find that FFmpeg supports encoding from a camera, but it seems to only support webcams, not the camera I use (hardware triggered BASLER cameras with pypylon APIs) ;

  


  2. I find that FFmpeg supports transfering a video from one codec to another, which is not my case ;

  


  3. I find that FFmpeg supports encoding a video from a series of jpeg images. But in my case, it will be inefficient if I first save each frame into a picture and then encode them into a video ;

  


  4. The frame data retrieved from camera can be converted to YUV420 (directly from pypylon), which is suitable for HEVC encoding ;

  


  5. I learnt that the basic unit in FFmpeg to encode a video is AVFrame. I guess I have to first turn my YUV420 data into AVFrame, then encode AVFrames into HEVC ;

  


  6. But I do not know how to achieve that in python.

  


  


  My simplified and expected codes :

  


  camera = pylon.InstantCamera(tlf.CreateFirstDevice())
converter = pylon.ImageFormatConverter()
converter.OutputPixelFormat = pylon.PixelType_YUV420
video_handle = xxxxxx  # HEVC
while True:
    grabResult = camera.RetrieveResult(timeout, pylon.TimeoutHandling_ThrowException)
    image = converter.Convert(grabResult).GetArray()
    video_handle.write(frame)  # encode into a hevc video via ffmpeg in NVENC


  


• avcodec/h264_loopfilter : Fix incorrect function parameter array size

  10 juillet 2022, par Andreas Rheinhardt

  avcodec/h264_loopfilter : Fix incorrect function parameter array size
  filter_mb_mbaff_edgev() and filter_mb_mbaff_edgecv()
  
  have a function parameter whose expected size depends upon
  
  another parameter : It is 2 * bsi + 1 (with bsi always being 1 or 2).
  
  This array is declared as const int16_t[7], yet some of the callers
  
  with bsi == 1 call it with only an const int16_t[4] available.
  
  This leads to -Wstringop-overread warnings from GCC 12.1.
  This commit fixes these by replacing [7] with [/* 2 * bsi + 1 */],
  
  so that the expected range and its dependence on bsi is immediately
  
  visible.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/h264_loopfilter.c