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• ffmpeg - extract exact number of frames from video

  29 mars 2017, par Michael B

  I want to create a maximum of 30 images from a video (and tile them for a sprite sheet).

  I’ve tried using the ’select’ with ’mod’ but if the total number of frames does not fit neatly into the desired number of images (30) then I sometimes end up with more images, sometimes less.

  For example if my video is 72 frames long, my ’mod’ would be 72 / 30, which is 2.4.

  I’m running this from a python script so i’m doing something like the following for the filter :

  select='not(mod(n\," + str(mod) + "))'

  I think the mod has to be an integer (?) so I could either round down and use 2 which gives me 36 images or round up which gives me 24 images

  Whats the best way to get exactly 30 ? - obviously the interval wouldn’t be identical but thats fine.

  Maybe I could use a for loop to generate a list of the frames closest to the desired interval and then pass that in as the select filter ?

  e.g. to get the frames I would do something like this :

  nframes = 72 # number of frames in video
  
  outImages = 30 # number of images I want
  
  mod = float(nframes) / outImages # 2.4
  
  
  
  frames = []
  
  
  
  idx = 1
  
  
  
  while i < nframes:
  
      print str(idx) + ": " + str(math.floor(i+0.5)) 
  
      frames.append(int(math.floor(i+0.5)))
  
      idx += 1
  
      i += mod

  Then am I able to pass that (the frames list) into the ffmpeg command ? Or can I tell ffmpeg to do something similar ?

• Round number of bits read to next byte

  4 décembre 2014, par watwat2014

  I have a header that can be any number of bits, and there is a variable called ByteAlign that’s calculated by subtracting the current file position from the file position at the beginning of the file, the point of this variable is to pad the header to the next complete byte. so if the header is taking up 57 bits, the ByteAlign variable needs to be 7 bits in length to pad the header to 64 bits total, or 8 bytes.

  Solutions that don’t work :

  Variable % 8 - 8, the result is the answer, but negative.

  8 % Variable ; this is completely inaccurate, and gives answers like 29, which is blatantly wrong, the largest number it should be is 7.

  how exactly do I do this ?

• ffmpeg produce not expected number of frames

  22 mars 2016, par mrgloom

  I’m using this command to count frames in video.

  FRAME_COUNT=`./ffprobe -v error -count_frames -select_streams v:0 -show_entries stream=nb_read_frames -of default=nokey=1:noprint_wrappers=1 $VIDEO_NAME`

  and this command for video splitting :

  ffmpeg -i $VIDEO_NAME -qscale:v 1 $DIRNAME$SEPARATOR'image%d.jpg'

  but last command produce much more images then expected for some videos, for example calculated number of frames is 597 and produces number of images > 100,000.

  What can be the case of such behaviour ?