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• FFMPEG - How to reduce frame rate without slow motion effect ? [closed]

  4 avril 2024, par crater33

  I have an MP4 video that's 21 minutes long, that was shot on a GoPro Hero 9 Black at 2.7k and 240 frames per second using HEVC.

  


  I would like to use FFMPEG to reduce the frame rate, with the output being a video that is also 21 minutes long and 2.7k ... but with a frame rate of 120 frames per second. But when I use the command "ffmpeg -i input.mp4 -filter:v fps=120 output.mp4 I get a video that is 42 minutes long.

  


  I would like to tell FFMPEG to simply delete every other frame of the input video, while keeping the total length of the video the same. What command would I use to do that ?

  


  Also, how would I modify this command if I wanted the output to be encoded in AVC ?

  


  Thank you.

  


• Why is random video seeks with OpenCV slow ?

  31 octobre 2020, par Aravind Battaje

  Seeking to random points in a video file with OpenCV seems to be much slower than in media players like Windows Media Player or VLC. I am trying to seek to different positions on a video file encoded in H264 (or MPEG-4 AVC (part10)) using VideoCapture and the time taken to seek to the position seems to be proportional to the frame number queried. Here's a small code example of what I'm trying to do :

  



  import cv2

cap = cv2.VideoCapture('example_file')   

frame_positions = [200, 400, 8000, 200000]
for frame_position in frame_positions:
   cap.set(cv2.cv.CV_CAP_PROP_FRAMES, frame_position)
   img = cap.read()
   cv2.imshow('window', img)
   cv2.waitKey(0)


  



  The perceived times for when the images are displayed from above are proportional to the frame number. That is, frame number 200 and 400, barely have any delay, 8000 some noticeable lag, but 200000 would take almost half a minute.

  



  Why isn't OpenCV able to seek as "quickly" as say Windows Media Player ? Could it be that OpenCV is not using the FFMPEG codecs correctly while seeking ? Would building OpenCV from sources with some alternate configuration for codecs help ? If so, could someone tell me what the configuration could be ?

  



  I have only tested this on Windows 7 and 10 PCs, with OpenCV binaries as is, with relevant FFMPEG DLLs in system path.

  



  Another observation : With OpenCV (binaries) versions greater than 2.4.9 (Example 2.4.11, 3.3.0), the first seek works, but not the subsequent ones. That is, it can seek to frame 200 from above example, but not to 400 and the rest ; the video just jumps back to frame 0. But since it works for me with 2.4.9, I'm happy for now.

  


• Anomalie #2020 : Rendre optionnelles les extensions

  8 juin 2011, par guytarr °

  Bon, à présent ça me parait peu réaliste de vouloir un spip fonctionnel sans extensions. Par contre, si certaines sont nécessaires, d’autres devraient être vraiment optionnelles. Je pense notamment à breves, sites, petitions, mots, organiseur, grenier, squelettes_par_rubrique, statistiques... Or, (...)