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• Why can't I get a manually modified MPEG-4 extended box (chunk) size to work ?

  15 avril 2019, par Moshe Rubin

  Overview

  As part of a project to write an MPEG-4 (MP4) file parser, I need to understand how an extended box (or chunk) size is processed within an MP4 file. When I tried to manually simulate an MP4 file with an extended box size, media players report that the file is invalid.

  Technical Information

  Paraphrasing the MPEG-4 specification :

  An MP4 file is formed as a series of objects called ’boxes’. All data is contained in boxes, there is no other data within the file.

  Here is a screen capture of Section 4.2 : Object Structure, which describes the box header and its size and type fields :

  

  

  Most MP4 box headers contain two fields : a 32-bit compact box size and a 32-bit box type. The compact box size supports a box’s data up to 4 GB. Occasionally an MP4 box may have more data than that (e.g., a large video file). In this case, the compact box size is set to 1, and eight (8) octets are added immediately following the box type. This 64-bit number is known as the ’extended box size’, and supports a box’s size up to 2^64.

  To understand the extended box size better, I took a simple MP4 file and wanted to modify the moov/trak/mdia box to use the extended box size, rather than the compact size.

  Here is what the MP4 file looks like before modifying it. The three box headers are highlighted in RED :

  

  My plan was as follows :

  1. Modify the moov/trak/mdia box
     • In the moov/trak/mdia, insert eight (8) octets immediately following the box type (’mdia’). This will eventually be our extended box size.
     • Copy the compact box size to the newly-inserted extended box size, adding 8 to the size to compensate for the newly inserted octets. The size is inserted in big-endian order.
     • Set the compact size to 1.
  2. Modify the moov/trak box
     • Add 8 to the existing compact box size (to compensate for the eight octets added to mdia).
  3. Modify the moov box
     • Add 8 to the existing compact box size (again, to compensate for the eight octets in mdia)

  Here’s what the MP4 file looks like now, with the modified octets are in RED :

  

  What have we done ?

  We have told the MP4 parser/player to take the moov/trak/mdia box size from the extended field rather than the compact size field, and have increased all parent boxes by eight (8) to compensate for the newly-inserted extended box size in the mdia box.

  What’s the problem ?

  When I attempt to play the modified MP4 file I receive error messages from different media players :

  

  

  Why do the media players see the modified file as invalid MP4 ?

  • Did I need to alter any other fields ?
  • Does the extended box size have to be greater than 2^32 ?
  • Can it be that only specific box types support extended box size (e.g., Media Data) ?

• How to convert images to video using FFMpeg for embedded applications ?

  19 avril 2019, par zthatch56

  I’m encoding images as video using FFmpeg using custom C code rather than linux commands because I am developing the code for an embedded system.

  I am currently following through the first dranger tutorial and the code provided in the following question.

  How to encode a video from several images generated in a C++ program without writing the separate frame images to disk ?

  I have found some "less abstract" code in the following github location.

  https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/encode_video.c

  And I plan to use it as well.

  My end goal is simply to save video on an embedded system using embedded C source code, and I am coming up the curve too slowly. So in summary my question is, Does it seem like I am following the correct path here ? I know that my system does not come with hardware for video codec conversion, which means I need to do it with software, but I am unsure if FFmpeg is even a feasible option for embedded work because I am yet to compile.

  The biggest red flag for me thus far is that FFmpeg uses dynamic memory allocation. I am unfamiliar with how to assess the amount of dynamic memory that it uses. This is very important information to me, and if anyone is familiar with the amount of memory used or how to assess it before compiling, I would greatly appreciate the input.

• How do I use find and ffmpeg to batch convert a bunch of .flac files to .mp3 ?

  30 avril 2019, par Keith

  I have a directory with a bunch of .flac files that I need to convert to .mp3. I plan to use ffmpeg from the command line to do the conversions and I’d like to avoid doing this manually for every file. I’m familiar with the find command but I’m having difficulty using it with ffmpeg which requires both input and output filenames. I imagine using something like

  find . -name "*.flac" -exec ffmpeg -i {}.flac {}.mp3 +

  But of course this doesn’t work. For one thing it fails to strip prefixes and suffixes from the filename being passed to ffmpeg.

  Please also note that the filenames include whitespace so the solution has to ignore whitespace successfully. I’m also on OS X having built ffmpeg with homebrew.