[Libav-user] ReZound Integration Question
ddurham at davyandbeth.com
Mon Oct 8 19:25:19 CEST 2012
I need some advice on integration.
I'm experimenting with changing rezound (a destructive audio editor) to
use libavcodec/libavformat instead of its current use of many other
libraries (libsndfile, libaudiofile, lame, manual coding in some places,
etc). I did all of that work a LONG time ago.
So, loading audio files that might have multiple streams is rather
straight forward in concept.. I don't think users of rezound would
normally load files with multiple streams (e.g. video files), but I can
certainly enumerate the audio streams and present the user with a choice
of which audio stream to load. Let's call that chosen stream "stream X".
So then if the user chooses the save the file after doing their editing,
is it feasible (with libavformat) to replace stream X in the original
file leaving all other streams intact? I'm guessing this could be done
(in concept) by opening a new temp file for write, and re-opening the
original file for reading. Then as I read the packets from the file, if
the packet was for some other stream, then I just write it verbatim to
the new file, but if the packet was for stream X, then I write the new
encoded packet to the new file. Then when it's all done, I move the tmp
file back to the original file. This would generally preserve the
interlacing of packets within the file. I assume it would be a bad idea
to simply drop all the original file's packets for !stream X and then
simply append stream X's new packets to the end of the file.
So that, or is that kind of functionality already built into
libavformat? (i.e. functionality to /rewrite/ one of the streams within
It's daunting to consider the type and extent of UI that needs to be in
place if a user chooses to save a file in a chosen format with a chosen
codec. That is, just how many little options and their sensible
defaults would need to be presented to the user for all the minutia of
all the supported audio codecs. Does ffmpeg provide any sort of
programatic enumeration of the parameters (along with their name,
data-type, defaults, etc) that need to be specified in the encode
operation? I don't immediately see something like a struct for each
supported codec that describes all the options for that codec. [Did I
Also, is there a way to know from the library what codecs are supported
by a given muxer? (just beginning to look at how -formats, -decodes,
As you can tell, I'm just beginning to look at the idea of switching to
ffmpeg. I'm looking at the ffplay code (which only helps with
understanding how to read). I've found the doxygen code and the
api-example.c (which only helps with libavcodec). Are there other good
resources for figuring this stuff out?
Thanks. Any suggestions, guidance, ideas, warnings, etc are appreciated.
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