Recherche avancée

Sur d’autres sites (10547)

• How to map frame extracted with ffmpeg and subtitle of a video ? (frame accuracy problem)

  14 novembre 2019, par Abitbol

  would like to generate text files for frames extracted with ffmpeg, containing subtitle of the frame if any, on a video for which I have burn the subtitles using ffmpeg also.

  I use a python script with pysrt to open the subrip file and generate the text files.
  What I am doing is that each frames is named with the frame number by ffmpeg, then and since they are extracted at a constant rate, I can easily retrieve the time position of the frame using the formula t1 = fnum/fps, where fnum is the number of the frame retrieved with the filename, and fps is the frequency passed to ffmpeg for the frame extraction.

  Even though I am using the same subtitle file to retrieve the text positions in the timeline, that the one that has been used in the video, I still get accuracy errors. Most I have some text files missing or some that shouldn’t be present.

  Because time is not really continuous when talking about frames, I have tried recalibrating t using the fps of the video wih the hardcoded subtitles, let’s call that fps vfps for video fps (I have ensured that the video fps is the same before and after subtitle burning). I get the formula : t2 = int(t1*vfps)/vfps.
  It still is not 100% accurate.

  For example, my video is at 30fps (vfps=30) and I extracted frames at 4fps (fps=4).
  The extracted frame 166 (fnum=166) shows no subtitle. In the subrip file, the previous subtitle ends at t_prev=41.330 and the next subtitle begins at t_next=41.400, which means that t_sub should satisfy : t_prev < t_sub and t_sub < t_next, but I can’t make this happen.

  Formulas I have tried :

  t1 = fnum/fps  # 41.5 > t_next
  
  t2 = int(fnum*vfps/fps)/vfps  # 41.5 > t_next
  
  # is it because of a indexing problem? No:
  
  t3 = (fnum-1)/fps  # 41.25 < t_prev
  
  t4 = int((fnum-1)*vfps/fps)/vfps  # 41.23333333 < t_prev
  
  t5 = int(fnum*vfps/fps - 1)/vfps  # 41.466666 > t_next
  
  t6 = int((fnum-1)*vfps/fps + 1)/vfps  # 41.26666 < t_prev

  Command used :

  # burning subtitles
  
  # (previously)
  
  # ffmpeg -r 25 -i nosub.mp4 -vf subtitles=sub.srt withsub.mp4
  
  # now:
  
  ffmpeg -i nosub.mp4 -vf subtitles=sub.srt withsub.mp4
  
  # frames extraction
  
  ffmpeg -i withsub.mp4 -vf fps=4 extracted/%05.bmp -hide_banner

  Why does this happen and how can I solve this ?

  One thing I have noticed is that if I extract frames of the original video and the subtitle ones, do a difference of the frames, the result is not only the subtitles, there are variations in the background (that shouldn’t happen). If I do the same experience using the same video two times, the difference is null, which means that the frame extraction is consistant.

  Code for the difference :

  ffmpeg -i withsub.mp4 -vf fps=4 extracted/%05.bmp -hide_banner
  
  ffmpeg -i no_sub.mp4 -vf fps=4 extracted_no_sub/%05.bmp -hide_banner
  
  for img in no_sub/*.bmp; do
  
      convert extracted/${img##*/} $img -compose minus -composite diff/${img##*/}
  
  done

  Thanks.

• avformat/dashdec, hls : Update correct pointer to AVDictionary

  7 septembre 2020, par Andreas Rheinhardt

  avformat/dashdec, hls : Update correct pointer to AVDictionary
  open_url() in the DASH as well in the hls demuxer share a common bug :
  
  They modify an AVDictionary (i.e. set a new entry) given to them as
  
  AVDictionary *, yet if this new entry leads to reallocation and
  
  relocation of the AVDictionary, the caller's pointer will become
  
  dangling, leading to use-after-frees. So pass an AVDictionary **.
  (With the current implementation of AVDictionary the above can only
  
  happen if the AVDictionary was empty initially (in which case the
  
  new AVDictionary leaks) ; furthermore if the I/O is ordinary (i.e. opened
  
  by avio_open2() or ffio_open_whitelist()), the dict is never empty (it
  
  contains an rw_timeout entry from save_avio_options()). So this issue
  
  could only happen if the caller sets a nondefault io_open callback, but
  
  no AVIOContext (the AVFMT_FLAG_CUSTOM_IO flag won't be set in this
  
  case). In case of the HLS demuxer, it was also necessary that setting
  
  the "seekable" entry failed. Yet one should simply not rely on internals
  
  of the AVDict API.)
  Reviewed-by : Steven Liu <lq@chinaffmpeg.org>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavformat/dashdec.c
  • [DH] libavformat/hls.c

• Perspective transformations

  4 février 2009, par Mikko Koppanen — Imagick, PHP stuff

  Finally (after a long break) I managed to force myself to update the PHP documentation and this time it was distortImage code example. Things have been hectic lately but that does not quite explain the 6 months(?) break between this and the previous post. As a matter of a fact there is no excuse for such a long silence so I will try to update this blog a bit more often from now on.

  Back in the day I used to blog the examples and update the documentation if I remembered but I am trying to fix this bad habit. Most of the latest examples have been updated in to the manual. In the case of the two last examples I updated the documentation first and then blogged on the subject.

  I took some time to actually understand the perspective transformations properly using the excellent ImageMagick examples (mainly created by Anthony Thyssen) as a reference. The basic idea of perspective distortion seems simple : to distort the control points to new locations. Grabbing the syntax for Imagick was easy, an array of control point pairs in the form of :

  php

  < view plain text >

  1. array(source_x, source_y, dest_x, dest_y ... )

  The following example uses the built-in checkerboard pattern to demonstrate perspective distortion :

  php

  < view plain text >

  1. < ?php
  2. /* Create new object */
  3. $im = new Imagick() ;
  4.  
  5. /* Create new checkerboard pattern */
  6. $im->newPseudoImage(100, 100, "pattern:checkerboard") ;
  7.  
  8. /* Set the image format to png */
  9. $im->setImageFormat(’png’) ;
  10.  
  11. /* Fill background area with transparent */
  12. $im->setImageVirtualPixelMethod(Imagick: :VIRTUALPIXELMETHOD_TRANSPARENT) ;
  13.  
  14. /* Activate matte */
  15. $im->setImageMatte(true) ;
  16.  
  17. /* Control points for the distortion */
  18. $controlPoints = array( 10, 10,
  19.             10, 5,
  20.  
  21.             10, $im->getImageHeight() - 20,
  22.             10, $im->getImageHeight() - 5,
  23.  
  24.             $im->getImageWidth() - 10, 10,
  25.             $im->getImageWidth() - 10, 20,
  26.  
  27.             $im->getImageWidth() - 10, $im->getImageHeight() - 10,
  28.             $im->getImageWidth() - 10, $im->getImageHeight() - 30) ;
  29.  
  30. /* Perform the distortion */ 
  31. $im->distortImage(Imagick: :DISTORTION_PERSPECTIVE, $controlPoints, true) ;
  32.  
  33. /* Ouput the image */ 
  34. header("Content-Type : image/png") ;
  35. echo $im ;
  36.  ?>

  Here is the source image :
  

  And the result :