lavfi: add concat filter.

Changelog

@@ -35,6 +35,7 @@ version next:
 - Opus decoder using libopus
 - caca output device using libcaca
 - alphaextract and alphamerge filters
+- concat filter
 
 
 version 0.11:

doc/filters.texi

@@ -4013,6 +4013,81 @@ tools.
 
 Below is a description of the currently available transmedia filters.
 
+@section concat
+
+Concatenate audio and video streams, joining them together one after the
+other.
+
+The filter works on segments of synchronized video and audio streams. All
+segments must have the same number of streams of each type, and that will
+also be the number of streams at output.
+
+The filter accepts the following named parameters:
+@table @option
+
+@item n
+Set the number of segments. Default is 2.
+
+@item v
+Set the number of output video streams, that is also the number of video
+streams in each segment. Default is 1.
+
+@item a
+Set the number of output audio streams, that is also the number of video
+streams in each segment. Default is 0.
+
+@end table
+
+The filter has @var{v}+@var{a} outputs: first @var{v} video outputs, then
+@var{a} audio outputs.
+
+There are @var{n}×(@var{v}+@var{a}) inputs: first the inputs for the first
+segment, in the same order as the outputs, then the inputs for the second
+segment, etc.
+
+Related streams do not always have exactly the same duration, for various
+reasons including codec frame size or sloppy authoring. For that reason,
+related synchronized streams (e.g. a video and its audio track) should be
+concatenated at once. The concat filter will use the duration of the longest
+stream in each segment (except the last one), and if necessary pad shorter
+audio streams with silence.
+
+For this filter to work correctly, all segments must start at timestamp 0.
+
+All corresponding streams must have the same parameters in all segments; the
+filtering system will automatically select a common pixel format for video
+streams, and a common sample format, sample rate and channel layout for
+audio streams, but other settings, such as resolution, must be converted
+explicitly by the user.
+
+Different frame rates are acceptable but will result in variable frame rate
+at output; be sure to configure the output file to handle it.
+
+Examples:
+@itemize
+@item
+Concatenate an opening, an episode and an ending, all in bilingual version
+(video in stream 0, audio in streams 1 and 2):
+@example
+ffmpeg -i opening.mkv -i episode.mkv -i ending.mkv -filter_complex \
+  '[0:0] [0:1] [0:2] [1:0] [1:1] [1:2] [2:0] [2:1] [2:2]
+   concat=n=3:v=1:a=2 [v] [a1] [a2]' \
+  -map '[v]' -map '[a1]' -map '[a2]' output.mkv
+@end example
+
+@item
+Concatenate two parts, handling audio and video separately, using the
+(a)movie sources, and adjusting the resolution:
+@example
+movie=part1.mp4, scale=512:288 [v1] ; amovie=part1.mp4 [a1] ;
+movie=part2.mp4, scale=512:288 [v2] ; amovie=part2.mp4 [a2] ;
+[v1] [v2] concat [outv] ; [a1] [a2] concat=v=0:a=1 [outa]
+@end example
+Note that a desync will happen at the stitch if the audio and video streams
+do not have exactly the same duration in the first file.
+
+@end itemize
+
 @section showwaves
 
 Convert input audio to a video output, representing the samples waves.

libavfilter/Makefile

@@ -199,6 +199,7 @@ OBJS-$(CONFIG_MP_FILTER) += libmpcodecs/vf_yvu9.o
 OBJS-$(CONFIG_MP_FILTER) += libmpcodecs/pullup.o
 
 # transmedia filters
+OBJS-$(CONFIG_CONCAT_FILTER)                 += avf_concat.o
 OBJS-$(CONFIG_SHOWWAVES_FILTER)              += avf_showwaves.o
 
 TOOLS     = graph2dot

libavfilter/allfilters.c

@@ -136,6 +136,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (NULLSINK,    nullsink,    vsink);
 
     /* transmedia filters */
+    REGISTER_FILTER (CONCAT,      concat,      avf);
     REGISTER_FILTER (SHOWWAVES,   showwaves,   avf);
 
     /* those filters are part of public or internal API => registered