lavfi: add join audio filter.

It joins multiple input streams into one multi-channel output.

Changelog

@@ -28,6 +28,7 @@ version <next>:
 - RTMPT protocol support
 - iLBC encoding/decoding via libilbc
 - Microsoft Screen 1 decoder
+- join audio filter
 
 
 version 0.8:

doc/filters.texi

@@ -232,6 +232,43 @@ front_center.wav -map '[LFE]' lfe.wav -map '[SL]' side_left.wav -map '[SR]'
 side_right.wav
 @end example
 
+@section join
+Join multiple input streams into one multi-channel stream.
+
+The filter accepts the following named parameters:
+@table @option
+
+@item inputs
+Number of input streams. Defaults to 2.
+
+@item channel_layout
+Desired output channel layout. Defaults to stereo.
+
+@item map
+Map channels from inputs to output. The argument is a comma-separated list of
+mappings, each in the @code{@var{input_idx}.@var{in_channel}-@var{out_channel}}
+form. @var{input_idx} is the 0-based index of the input stream. @var{in_channel}
+can be either the name of the input channel (e.g. FR for front left) or its
+index in the specified input stream. @var{out_channel} is the name of the output
+channel.
+@end table
+
+The filter will attempt to guess the mappings when those are not specified
+explicitly. It does so by first trying to find an unused matching input channel
+and if that fails it picks the first unused input channel.
+
+E.g. to join 3 inputs (with properly set channel layouts)
+@example
+avconv -i INPUT1 -i INPUT2 -i INPUT3 -filter_complex join=inputs=3 OUTPUT
+@end example
+
+To build a 5.1 output from 6 single-channel streams:
+@example
+avconv -i fl -i fr -i fc -i sl -i sr -i lfe -filter_complex
+'join=inputs=6:channel_layout=5.1:map=0.0-FL\,1.0-FR\,2.0-FC\,3.0-SL\,4.0-SR\,5.0-LFE'
+out
+@end example
+
 @section resample
 Convert the audio sample format, sample rate and channel layout. This filter is
 not meant to be used directly, it is inserted automatically by libavfilter

libavfilter/Makefile

@@ -32,6 +32,7 @@ OBJS-$(CONFIG_ANULL_FILTER)                  += af_anull.o
 OBJS-$(CONFIG_ASPLIT_FILTER)                 += split.o
 OBJS-$(CONFIG_ASYNCTS_FILTER)                += af_asyncts.o
 OBJS-$(CONFIG_CHANNELSPLIT_FILTER)           += af_channelsplit.o
+OBJS-$(CONFIG_JOIN_FILTER)                   += af_join.o
 OBJS-$(CONFIG_RESAMPLE_FILTER)               += af_resample.o
 
 OBJS-$(CONFIG_ANULLSRC_FILTER)               += asrc_anullsrc.o

libavfilter/allfilters.c

@@ -41,6 +41,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (ASPLIT,      asplit,      af);
     REGISTER_FILTER (ASYNCTS,     asyncts,     af);
     REGISTER_FILTER (CHANNELSPLIT,channelsplit,af);
+    REGISTER_FILTER (JOIN,        join,        af);
     REGISTER_FILTER (RESAMPLE,    resample,    af);
 
     REGISTER_FILTER (ANULLSRC,    anullsrc,    asrc);