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• Seek function of HTML5 video and Maximum Keyframe Interval in video encoding compression

  20 décembre 2019, par Sanxofon

  This is a very specific question about how the seek function (of an HTML5 video element) interprets a video, in this case one in WEBM format and what specific relationship it has with the Maximum Keyframe Interval in the process of coding of said video.

  I’ve made this fiddle to show what I mean :

  https://jsfiddle.net/sanxofon/hctuxo3e/

  What it does : The idea is to control the position of a paused video with the scroll wheel of the mouse ... in addition, each frame of the video is copied to a canvas element, but I think that has no relation. I mention it just in case.

  What to watch : In Chrome (v66) and slightly less in Firefox (v59) the scrolling looks pretty good when the video is encoded with a Maximum Keyframe Interval of 6 or less, but jumps are appreciated when the interval is every 24 frames or more. This is noticeable in the video and even more on the canvas.

  FFMPEG : When encoding a video with FFMPEG this is achieved with the option -g6 or -g24 of Maximum Keyframe Interval. However, the file becomes heavier as we decrease the interval. It can be seen that there is no difference between both formats when the video is in play.

  You can switch the video in the snippet with the buttons.

  • Case 1 : When we use the video encoded with -g 6 the video scrolling is acceptable but the size increases : 6.229 Mb.

  FFMPEG string used :

   ffmpeg -i INPUT.MOV -c: v libvpx -qmin 0 -deadline best -qmax 50 -crf 1 -b: v 100K -g 6 test / video_g6.webm

  • Case 2 : When we use the video encoded with -g 24 the displacement is not smooth and suffers from jumps but the size decreases : 4,477 Mb.

  FFMPEG string used :

   ffmpeg -i INPUT.MOV -c: v libvpx -qmin 0 -deadline best -qmax 50 -crf 1 -b: v 100K -g 24 test / video_g24.webm

  Why does this happen ?

  What about -keyint_min or -force_key_frames ? Do they have any positive effect ? Is it better to use something like cgop (closed gop) ?

  I would appreciate some reference of consultation on this subject or a more or less detailed explanation of this relationship for both the WEBM container and for MP4 and OGG video.

  I am not looking so much for a magical ffmpeg chain (although I would appreciate it) but rather an explanation of how this relationship between the keyframes and the seek of a javascript video works.

  Thank you very much for reading here.

  P.S. One more thing, if the seek function only stops in a keyframe, is it possible that this frame has more quality than the others so that the quality increases when the video stops ?

• How to create a video of a precise duration from a single image or by trimming a video using FFmpeg ? [closed]

  29 mai 2024, par aas

  I'm trying to create a video from a single image with a very specific duration of 0.09375 seconds using FFmpeg. I've tried various commands, but I can't seem to get the exact duration I need. The closest I've gotten is 0.080000 seconds. It doesn't always have to be something like 0.09375, but I wanted to have an example for it.

  


  I've also tried trimming a video, but from what I've read so far, the encoding of the video can be a problem. Even after trying different FFmpeg commands or using MoviePy directly, I've never arrived at the desired result.

  


  Here are the steps I've taken so far :

  


  Initial Attempt :

  


  ffmpeg -loop 1 -i frame_00001.png -c:v libx264 -t 0.09375 -r 240 -pix_fmt yuv420p -vf scale=1920:1080 output.mp4


  


  Resulting duration : 0.080000 seconds

  


  Adjusted Frame Rate :

  


  ffmpeg -y -loop 1 -i frame_00001.png -c:v libx264 -t 0.09375 -r 160 -pix_fmt yuv420p -vf scale=1920:1080 output.mp4


  


  Resulting duration : 0.081250 seconds

  


  I've also tried using the setpts filter, but I didn't get anything satisfying.

  


  Creating a video from a single image or trimming an existing video were my two ideas, but if anyone has another method to create a video with such a precise duration, I'm all ears !

  


  Any guidance or suggestions would be greatly appreciated.

  


• How to convert a CCTV footage into time lapse video by cutting parts of the video by a set interval on FFMPEG [duplicate]

  2 décembre 2020, par mark

  I have bunch of CCTV footages and I want it to look like it was recorded from a time lapse camera. One video file is around 3 hours long capturing scenes in real time (from 1pm-3pm for example). And in one day, I'll get around 8 footages (8files * 3hours = 24hours = 1 day)

  


  I want to convert those 24hours worth of footages to 1min making 1 day = 1min of video not just making it fast but actually cutting some of the scenes by a set interval. Usually, a time lapse camera has an interval of one photo per 10 min and at the end of the day, it will stitch them into one video. How can I do something like that on FFMPEG ?

  


  I'm using FFmpeg Batch converter and here's my code so far. It just makes my videos faster but not cutting it into itervals

  


  -filter:v "setpts=0.25*PTS" -an


  


  I ended up with this code :

  


  -vf framestep=25,select='not(mod(n,1000))',setpts=N/FRAME_RATE/TB -an


  


  The above code will make a 1hr long video into 4sec which is perfect for my needs.