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• How to extract video clip from larger video based on specific time and duration

  26 juillet 2022, par user19019404

  I have video clips that get created. Each video is recorded for 5 minutes and starts at a time e.g. 10:01:20 to 10:06:19 then the next video from 10:06:20 etc. These videos are recorded at a specific frame rate, be it 5 frames or 30 frames (its dependent on the platform making the recording). The net result is NOT a 5 minute video clip but might be a 2 minute video clip (where everyone moves very quickly in the view as a result of the frame rates). I cannot restrict a 1 to 1 recording as these are generated by external systems.

  


  I need to extract specific portions out of the video. For example I need to extract from 10:03:10 to 10:03:35 (25 seconds). This would equate to 10 seconds into the video up to 14 seconds into the video, in those 4 seconds of video, 25 real world seconds are displayed.

  


  My question is do you have any guidance as to how I can calculate that each second of recording actually means 10 or 12 seconds in real life, therefore go to this frame and record to this frame for example.

  


  I have been looking at cv2.CAP_PROP_FPS to get the video frame rates, CAP_PROP_POS_MSEC and CAP_PROP_FRAME_COUNT and believe the answer might lie there, but not sure.

  


  The thinking being if I work out the frame rate of the video, the total frames then I can divide the one by the other to get to how many frames make up real world seconds (this is where I fail). This way I can run from frame to frame as a result.

  


  Thank you

  


• FFmpeg C++, H.264 parser build

  18 mai 2015, par Yoohoo

  I currently working on a project simulate webcam video transmission in C++, at sender side, I capture the raw webcame video with v4l2, encoded with FFmpeg, video file are put into an array and transmitted. And at decoder side, video data received to an array, decoded and play. The program works fine with codec_id AV_CODEC_ID_MPEG1VIDEO, but when I try replace it with AV_CODEC_ID_H264, some problem happen in decoding, please refer to FFmpeg c++ H264 decoding error. Some people suggest me to use parser but I have no idea how is a parse in ffmpeg looks like. Any simple example of how to build a parser for H.264 in FFmepg ? I cannot find such tutorial in google.....

• ffplay got extra delay seconds than ffmpeg

  14 juillet 2017, par jiandingzhe

  I am working on low-latency video streaming. When I was making some tests, I noticed ffplay got much extra latency than ffmpeg, though they are the same software package.

  The video is encoded in H.264, size 320x240, 15fps. The stream is packed in FLV format, uploading and receiving are both done via RTMP. ffmpeg is called fairly simple :

  $ ffmpeg -i 'rtmp://my.path.to/my_rtmp_stream' -f sdl window_title

  And ffplay is also called in simple way :

  $ ffplay 'rtmp://my.path.to/my_rtmp_stream'

  The ffmpeg would starts to show the video instantly, and plays the video with minor latency (<0.5s) ; while ffplay would start the video in 5 seconds, and introduce a delay of 8 seconds.

  I further optimized the ffplay way, by minimizing probe size :

  $ ffplay -probesize 32 'rtmp://my.path.to/my_rtmp_stream'

  In this way, the startup goes quickly, but the delay is still 3 seconds.

  Why they behave so differently ?