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• avcodec/vp3 : Make parsing Theora Huffman tables more spec-compliant

  20 octobre 2020, par Andreas Rheinhardt

  avcodec/vp3 : Make parsing Theora Huffman tables more spec-compliant
  Theora allows to use custom Huffman tables which are coded in the
  
  bitstream as a tree : Whether the next node is a leaf or not is coded
  
  in a bit ; each node itself contains a five bit token. Each tree can
  
  contain at most 32 leafs ; typically they contain exactly 32 with the 32
  
  symbols forming a permutation of 0..31. Yet the standard does not impose
  
  either of these requirements. It explicitly allows less than 32 leafs
  
  and multiple codes with the same token.
  But our decoder used an algorithm that required the codes->token mapping
  
  to be injective and that also presumed that there be at least two leafs :
  
  Instead of using an array for codes, tokens and code lengths, the
  
  decoder only had arrays for codes and code lengths. The code and length
  
  for a given token were stored in entry[token]. As no symbols table was
  
  used when initializing the VLC, the default one applied and therefore
  
  the entry[token] got the symbol token (if the length of said entry is >0).
  
  Yet if multiple codes had the same token, the codes and lengths from the
  
  later token would overwrite the earlier codes and lengths.
  Furthermore, less than 32 leafs could also lead to problems : Namely if
  
  this was not the first time Huffman tables have been parsed in which
  
  case the array is not zeroed initially so that old entries could make
  
  the new table invalid.
  libtheora seems to always use 32 leafs and no duplicate tokens ; I am not
  
  aware of any existing valid files that do not.
  This is fixed by using a codes, symbols and lengths array when
  
  initializing the VLC. In order to reduce the amount of stuff kept in the
  
  context only the symbols and lengths (which both fit into an uint8_t)
  
  are kept in the context ; the codes are derived from the lengths
  
  immediately before creating the tables.
  There is now only one thing left which is not spec-compliant : Trees with
  
  only one node (which has length zero) are not supported by
  
  ff_init_vlc_sparse() yet.
  Reviewed-by : Peter Ross <pross@xvid.org>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/vp3.c

• PHP get screenshot from YouTube video

  26 août 2014, par drizzy

  I have a website that has a variety of embedded YouTube videos. When a user pauses a given video I want a screenshot to be taken of the playing video. Now, I’ve taken many approaches in tackling this problem such as copying the video frame to canvas (this doesn’t work because the videos are external to my site), and also through the use of FFMpeg, and FFMpeg-PHP. The latter two- although very powerful- also do not work as the given piece of media has to be hosted on my server.

  I’m at my wits end about what to do as I’ve spent countless hours trying to do this, and I’m ready to accept defeat.

  Any ideas ?

  Regards,

  Andre.

• movenc : Don’t write any tfdt atom for ismv files

  30 octobre 2014, par Martin Storsjö

  movenc : Don’t write any tfdt atom for ismv files
  The tfdt atom shouldn’t be needed in those cases, we already
  
  write tfxd atoms for ismv anyway, which is roughly equivalent.
  This avoids having to declare the iso6 brand for ismv files.
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DBH] libavformat/movenc.c