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• Is there a way to eliminate seek time when decoding part of a video using ffmpeg ?

  17 décembre 2019, par Babis

  I’ve got some MKV videos encoded with FFV1. For each of the frames, I want to run some complex and time-intensive python or matlab code, so I’m using multithreading, where each thread works on an individual image.

  I’ve tried extracting a single frame from the video using -ss, but it’s terribly inefficient.

  The most efficient way is to decompress everything into images in one go, but then I’m writing to disk, and then I’ll be reading from disk, therefore it’s not ideal either.

  I’ve tried using a ram disk to export images to, and reading them from python/matlab, but it’s not great performance-wise either. Also, I have to split the export into several batches, as the video file is 20GB and all of the exported images will not fit into memory

  Is there a way to rapidly extract individual frames from ffmpeg directly into RAM (or ram disk), so that they can be used by another program ? For example using something like a lookup-table.

  For reference, each video is about 20GB, comprised of 50000 frames, and they are all keyframes (it’s for archival purposes)

• How can I use ffmpeg to crop a part of a video by frames numbers including video and audio ?

  28 mars 2019, par Dubi Duboni

  I have a simple list :

  LReg.start = rise[i];
  
  LReg.end = fall[i];
  
  LR.Add(LReg);

  start and end are int’s in the end I have a List of the frames I want to extract from a video file. For example in index 0 of LR I have start 48 end 51
  In index 1 start 110 end 124

  So I want to loop over the List an save as a video file the part of a video file using ffmpeg according to the frame number start and end by jumping to this frames.
  Jump to frame 48 and create a video file out from frame 48 to 51 including 48 and 51. Then jump forward to the next group of frames 110 and 124 and so on.

  The problem is how to use ffmpeg to extract and save video files ?

• How to replace the video track of a part of a video file ?

  17 juin 2019, par yukashima huksay

  I have an mp4 file like this(same format but longer) :

  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'N1.2.mp4':
  
    Metadata:
  
      major_brand     : mp42
  
      minor_version   : 0
  
      compatible_brands: mp42mp41
  
      creation_time   : 2018-10-31T13:44:21.000000Z
  
    Duration: 00:28:54.21, start: 0.000000, bitrate: 10295 kb/s
  
      Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, bt709), 1920x1080, 9972 kb/s, 50 fps, 50 tbr, 50k tbn, 100 tbc (default)
  
      Metadata:
  
        creation_time   : 2018-10-31T13:44:21.000000Z
  
        handler_name    : ?Mainconcept Video Media Handler
  
        encoder         : AVC Coding
  
      Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 317 kb/s (default)
  
      Metadata:
  
        creation_time   : 2018-10-31T13:44:21.000000Z
  
        handler_name    : #Mainconcept MP4 Sound Media Handler

  I also have another video file that is 3 minutes long. and has no audio. What is the fastest way to encode the other video in a way that it is encoded like my main video and then replace the last three minutes of the video track of my original video with this ?

  In other words.
  I have video A that is 1 hour long. With the encoding shown above.

  I have video B that is 3 minutes long with no audio. with a random encoding.

  I want to have video C with the same encoding and same audio as A. But it’s video track would be the first 57 minutes of A + B(which is 3 minutes).

  I want to do this as fast as possible so I would like to not re encode A.

  I know how to concatenate two videos, I use this command :

  ffmpeg -f concat -i files.txt -c copy res.mp4