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The Matroska muxer has a pair of functions designed to write master elements whose exact length is not known in advance: start_ebml_master() and end_ebml_master(). The first one of these would write the EBML ID of the master element that is about to be started, reserve some bytes for the length field and record the current position as well as how many bytes were used for the length field. When writing the master's contents is finished, end_ebml_master() gets the current position (at the end of the master element), seeks to the length field using the recorded position, writes the length field and seeks back to the end of the master element so that one can continue writing other elements. But if one wants to modify the content of the master element itself, then the seek back is superfluous. This is the scenario that presents itself when writing the trailer: One wants to update several elements contained in the Segment master element (this is the main/root master element of a Matroska file) that were already written when writing the header. The current approach is to seek to the beginning of the file to update the elements, then seek to the end, call end_ebml_master() which immediately seeks to the beginning to write the length and seeks back. The seek to the end (which has only been performed because end_ebml_master() uses the initial position to determine the length of the master element) and the seek back are of course superfluous. This commit avoids these seeks by no longer using start/end_ebml_master() to write the segment's length field. Instead, it is now written manually. The new approach is: Seek to the beginning to write the length field, then update the elements (in the order they appear in the file) and seek back to the end. This reduces the ordinary amount of seeks of the Matroska muxer to two (ordinary excludes scenarios where one has big Chapters or Attachments or where one writes the Cues at the front). Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides a mean to alter decoded Audio and Video through chain of filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.