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• avutil/bprint : Allow size == 0 in av_bprint_init_for_buffer()

  6 août 2023, par Andreas Rheinhardt

  avutil/bprint : Allow size == 0 in av_bprint_init_for_buffer()
  The AVBPrint API guarantees that the string buffer is always
  
  zero-terminated ; in order to honour this guarantee, there
  
  obviously must be a string buffer at all and it must have
  
  a size >= 1. Therefore av_bprint_init_for_buffer() treats
  
  passing a NULL buffer or size == 0 as invalid data that
  
  leads to undefined behaviour, namely NPD in case NULL is provided
  
  or a write to a buffer of size 0 in case size == 0.
  But it would be easy to support this, namely by using the internal
  
  buffer with AV_BPRINT_SIZE_COUNT_ONLY in case size == 0.
  There is a reason to allow this : Several functions like
  
  av_channel_(description|name) are actually wrappers
  
  around corresponding AVBPrint functions. They accept user
  
  provided buffers and are supposed to return the required
  
  size of the buffer, which would allow the user to call
  
  it once to get the required buffer size and call it once
  
  more after having allocated the buffer.
  
  If av_bprint_init_for_buffer() treats size == 0 as invalid,
  
  all these users would need to check for this themselves
  
  and basically add the same codeblock that this patch
  
  adds to av_bprint_init_for_buffer().
  This change is in line with e.g. snprintf() which also allows
  
  the pointer to be NULL in case size is zero.
  This fixes Coverity issues #1503074, #1503076 and #1503082 ;
  
  all of these issues are about providing NULL to the channel-layout
  
  functions that are wrappers around AVBPrint versions.
  Reviewed-by : Nicolas George <george@nsup.org>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavutil/bprint.c
  • [DH] libavutil/bprint.h
  • [DH] libavutil/version.h

• How to calculate ffmpeg output file size ?

  25 septembre 2011, par poundifdef

  I am using ffmpeg to convert home videos to DVD format and want to calculate the output file size before doing the conversion.

  My input file has a bit rate of 7700 kbps and is 114 seconds long. The audio bitrate is 256 kbit (per second ?) The input file is 77MB. To get this information I ran :

  mplayer -vo null -ao null -frames 0 -identify input.MOD

  So in theory, the input file should have (roughly) a file size of :

  ((7700 / 8) * 114) / 1024

  That is, (7700 / 8) is kilobytes/second, multiplied by 114 seconds, and then converted to megabytes. This gives me 107MB, which is way beyond my 77. Thus I am skeptical of his formula.

  That said, after converting the video :

  ffmpeg -i input.MOD -y -target ntsc-dvd -sameq -aspect 4:3 output.mpg

  The numbers seem to make more sense. Bitrate is 9000 kbps, and applying the above formula, I get 125MB, and my actual output file size is 126MB.

  So, two questions :

  1. How do I factor the audio bitrate into this calculation ? Is it additive (video file size + audio file size) ?

  2. Do DVDs always have a 9000 kilobit/second rate ? Is that the definition of a DVD ? Or might that change depending on video quality of my input video ? What does "-target ntsc-dvd" guarantee about my video ?

  3. Why does my input file not "match" the calculation, but the output file does ? Is there some other variable I'm not accounting for ?

  What is the correct way to calculate filesize ?

• How to extract the EMBL header size from a webm video stream in C++ ?

  18 septembre 2023, par Anil

  How to extract the header size (highlighted in the image) using ffmpeg apis ?

  


  

  


  (I'm loading the webm video using avio_alloc_context and avformat_open_input)

  


  I tried using extradata_size as follows - but it returns 0.

  


  ...
AVStream* videoStream = formatContext->streams[videoStreamIndex];
AVCodecParameters* videoCodecParams = videoStream->codecpar;
headerSize = videoCodecParams->extradata_size;
...


  


  (PS : This seems to work for mp4 but not for webm/vp9)

  


  How do I get the header size in C++ ?