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• FFmpeg : negative video duration

  19 octobre 2020, par quento

  FFmpeg is used to combine images into video and then duration is taken out of this video.

  


  Surprisingly I found out that FFmpeg could return negative duration.

  


  Command to get duration. This command is the most reliable according to this spec

  


  


  You can also use ffmpeg to get the duration by fully decoding the file. The null muxer is used so no output file is created. Refer to time= in the next-to-last line of the console output. In this example the input has a duration of 00:57:28.87.

  


  


  ffmpeg -i input.webm -f null -  


  


  Output ex :

  


  frame=206723 fps=1390 q=-0.0 Lsize=N/A time=00:57:28.87 bitrate=N/A speed=23.2x


  


  Actual example

  


  Generation of mp4 of screenshots :

  


  `/usr/bin/ffmpeg' -framerate 30 -pattern_type glob -i '/app/tmp/ecc0fe7b5ca4d7b20ea54d3ba757cfbf_b4581272-c485-4be2-a210-cbc99d71bd14_0/*.jpeg' -c:v libx264 -vf 'scale=trunc(iw/2)*2:trunc(ih/2)*2' -pix_fmt yuv420p -filter:v 'setpts=1.11*PTS' '/app/tmp/ecc0fe7b5ca4d7b20ea54d3ba757cfbf_b4581272-c485-4be2-a210-cbc99d71bd14_0/ecc0fe7b5ca4d7b20ea54d3ba757cfbf.mp4`


  


  Next, extracting duration :

  


  /usr/bin/ffmpeg' -i '/app/tmp/ecc0fe7b5ca4d7b20ea54d3ba757cfbf_b4581272-c485-4be2-a210-cbc99d71bd14_0/ecc0fe7b5ca4d7b20ea54d3ba757cfbf.mp4  -f null


  


  Output :

  


  time=-577014:32:22.77


  


  Unfortunately I can't reproduce this issue on local machine so far, so there are not many details to share. However, such cases appear on other machines quite frequently. Different videos get exactly the same broken time. There are also enough other inconsistent cases.

  


  One of the suggestions is to parse extraction out of first command, which generates video. I'm not sure that I won't also face negative numbers there, though.

  


  Most probably I miss much more easier solution. The only requirement is that is must be 100% precise.

  


  Thanks in advance !

  


• avcodec/qtrleenc : Fix negative linesizes, don't use NULL + offset

  26 mars 2021, par Andreas Rheinhardt

  avcodec/qtrleenc : Fix negative linesizes, don't use NULL + offset
  Before commit f1e17eb446577180ee9976730aacb46563766518, the qtrle
  
  encoder had undefined pointer arithmetic : Outside of a loop, two
  
  pointers were set to point to the ith element (with index i-1) of
  
  a line of a frame. At the end of each loop iteration, these pointers
  
  were decremented, so that they pointed to the -1th element of the line
  
  after the loop. Furthermore, one of these pointers can be NULL (in which
  
  case all pointer arithmetic is automatically undefined behaviour).
  Commit f1e17eb44 added a check in order to ensure that the elements
  
  never point to the -1th element of the array : The pointers are only
  
  decremented if they are bigger than the frame's base pointer
  
  (i.e. AVFrame.data[0]). Yet this check does not work at all in case of
  
  negative linesizes ; furthermore in case the pointer that can be NULL is
  
  NULL initializing it still involves undefined pointer arithmetic.
  This commit fixes both of these issues : First, non-NULL pointers are
  
  initialized to point to the element after the ith element and
  
  decrementing is moved to the beginning of the loop. Second, if a pointer
  
  is NULL, it is just made to point to the other pointer, as this allows
  
  to avoid checks before decrementing it.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/qtrleenc.c

• avcodec/pgxdec : Fix issue with negative linesizes

  15 avril 2022, par Andreas Rheinhardt

  avcodec/pgxdec : Fix issue with negative linesizes
  The PGX decoder accesses the lines via code like
  
  (PIXEL*)frame->data[0] + i*frame->linesize[0]/sizeof(PIXEL)
  
  where PIXEL is a macro parameter. This code has issues with negative
  
  linesizes, because the type of sizeof(PIXEL) is size_t, so
  
  that on common systems i*linesize/sizeof(PIXEL) will
  
  always be an unsigned type that is very large in case linesize is
  
  negative. This happens to work*, but it is undefined behaviour
  
  and e.g. leads to "src/libavcodec/pgxdec.c:114:1 : runtime error :
  
  addition of unsigned offset to 0x7efe9c2b7040 overflowed to 0x7efe9c2b6040"
  
  errors from UBSAN.
  
  Fix this by using (PIXEL*)(frame->data[0] + i*frame->linesize[0]).
  
  This is allowed because linesize has to be suitably aligned.
  * : Converting a negative int to size_t works by adding SIZE_MAX + 1
  
  to the number, so that the result is off by (SIZE_MAX + 1) /
  
  sizeof(PIXEL). Converting the pointer arithmetic (performed on PIXELs)
  
  back to ordinary pointers is tantamount to multiplying by sizeof(PIXEL),
  
  so that the result is off by SIZE_MAX + 1 ; but SIZE_MAX + 1 == 0
  
  for the underlying pointers.
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/pgxdec.c