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• How to averagely extract frames from a video using ffmpeg by specifying "fps"

  14 juin 2021, par alanzzz

  I have a job of using ffmpeg to extract the frames averagely from a video, with different fps. I use this command for it.

  


  ffmpeg -i input.mp4 -r specified_fps -q:v 2 image %4d.png

  


  And I have 3 questions about this task.

  


  

  1. what I expect is that if I double the fps, the number of extracted frames will also get doubled. However, that's not the case. Take one of the input videos as an example.

  


  


  video info

  


  

  • Duration : 1min18s

  


  • Total number of frames : 2340

  


  • Frame rate mode : constant (CFR)

  


  • Frame rate : 30.0 FPS

  


  


  Config setting & results

  


  

  • FPS=1 => number of frames=80

  


  • FPS=2 => number of frames=158 (2x80-2)

  


  • FPS=3 => number of frames=236 (3x80-4)

  


  • FPS=5 => number of frames=392 (5x80-8)

  


  


  Is it possible for me to get the exact doubled number of frames when fps get doubled ? In such case, number of frames is 160 for FPS=2, 240 for FPS=3, 400 for FPS=5.

  


  


  2. I check for the output images, the extracted frames in different fps are totally different from each other. In other words, for example, the 1st image for fps=1 is not the same as the 1st image for fps=2. Is that legitimate ? And is it possible for me the get some identical images for different fps ?

     


  


  3. The last problem is that for some videos I use, the difference between the 1st and 2nd image is different from the difference between the 2nd and 3rd. While for the remaining images, the differences become average. To be specific, there is only a slight change from 1st to 2nd frame, while for 2nd to 3rd, 3rd to 4th, and so on, the changes are the same, which is normally distributed according to the specified FPS. I am wondering why such a case happens ? Does it related to the I-frame, B-frame, P-frame, GOP or IDR ?

     


  


  


  I am new to this field and cannot find some useful info from other places. I've tried my best to describe my questions clearly. Feel free to leave some comments. Any help would do me a great favor. Thanks in advance !

  


• Revert "avcodec/nvdec_av1 : fix setting film grain parameters for frames with update_g...

  25 novembre 2020, par James Almer

  Revert "avcodec/nvdec_av1 : fix setting film grain parameters for frames with update_grain == 0"
  This reverts commit f9eec6298387fe72cd8035ff45276cfc3da784a8.
  This does not effectively cover all cases. The values for some frames need
  
  to be inferred by the decoder.
  Signed-off-by : James Almer <jamrial@gmail.com>
  

  • [DH] libavcodec/nvdec_av1.c

• FFMPEG tee muxer giving "Output file #0 does not contain any stream"

  31 août 2020, par Giorgi Aptsiauri

  I am trying to create two streams : one is mpegts UDP stream another - rtmp to Twitch servers.

  


  This command works :

  


    ffmpeg -threads:v 2 -threads:a 16 -filter_threads 2 -thread_queue_size 16 -y \
 -f dshow -video_size 1920x1080 -pixel_format uyvy422 -framerate 25 -rtbufsize 500M -i video="Decklink Video Capture" \
 -f dshow -rtbufsize 100M -i audio="Decklink Audio Capture" \
 -preset ultrafast -c:v libx264 -tune zerolatency -b:v 900k -map 0:v:0 -f mpegts udp://127.0.0.1:5555 \ 
 -pix_fmt yuv420p -c:v libx264 -crf 20 -tune zerolatency -f flv rtmp://live-fra05.twitch.tv/app/stream_key


  


  But it requires double the encoding CPU power.

  


  So, following this, I rewrote the command like this :

  


  ffmpeg -threads:v 2 -threads:a 16 -filter_threads 2 -thread_queue_size 16 -y \
 -f dshow -video_size 1920x1080 -pixel_format uyvy422 -framerate 25 -rtbufsize 500M -i video="Decklink Video Capture" \
 -f dshow -rtbufsize 100M -i audio="Decklink Audio Capture" \
 -preset ultrafast -c:v libx264 -tune zerolatency -b:v 900k \
 -f tee "[select=\'0:v:0\':f=mpegts]udp://127.0.0.1:5555|[select=\'0:v:0,1:a:0\':f=flv]rtmp://live-fra05.twitch.tv/app/stream_key"


  


  By writing -f tee "[select=\'0:v:0\':f=mpegts]udp://127.0.0.1:5555|[select=\'0:v:0,1:a:0\':f=flv]rtmp://live-fra05.twitch.tv/app/stream_key", I mean :

  


  

  • create UDP stream at udp ://127.0.0.1:5555 and only include video stream from "Decklink Video Capture"

  


  • create RTMP stream where we include the same video stream as above and also the audio stream from "Decklink Audio Capture"

  


  


  I get the error message :

  


  Output file #0 does not contain any stream


  


  How do I fix this ? I assume I made a mistake in the command.