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\input texinfo @c -*- texinfo -*-
@settitle Libavfilter Documentation
@center @titlefont{Libavfilter Documentation}
@end titlepage
@chapter Introduction
Libavfilter is the filtering API of FFmpeg. It is the substitute of the
now deprecated 'vhooks' and started as a Google Summer of Code project.
Audio filtering integration into the main FFmpeg repository is a work in
progress, so audio API and ABI should not be considered stable yet.
@chapter Tutorial
In libavfilter, it is possible for filters to have multiple inputs and
multiple outputs.
To illustrate the sorts of things that are possible, we can
use a complex filter graph. For example, the following one:
input --> split --> fifo -----------------------> overlay --> output
| ^
| |
+------> fifo --> crop --> vflip --------+
@end example
splits the stream in two streams, sends one stream through the crop filter
and the vflip filter before merging it back with the other stream by
overlaying it on top. You can use the following command to achieve this:
ffmpeg -i input -vf "[in] split [T1], fifo, [T2] overlay=0:H/2 [out]; [T1] fifo, crop=iw:ih/2:0:ih/2, vflip [T2]" output
@end example
The result will be that in output the top half of the video is mirrored
onto the bottom half.
Video filters are loaded using the @var{-vf} option passed to
@command{ffmpeg} or to @command{ffplay}. Filters in the same linear
chain are separated by commas. In our example, @var{split, fifo,
overlay} are in one linear chain, and @var{fifo, crop, vflip} are in
another. The points where the linear chains join are labeled by names
enclosed in square brackets. In our example, that is @var{[T1]} and
@var{[T2]}. The magic labels @var{[in]} and @var{[out]} are the points
where video is input and output.
Some filters take in input a list of parameters: they are specified
after the filter name and an equal sign, and are separated each other
by a semicolon.
There exist so-called @var{source filters} that do not have a video
input, and we expect in the future some @var{sink filters} that will
not have video output.
@chapter graph2dot
The @file{graph2dot} program included in the FFmpeg @file{tools}
directory can be used to parse a filter graph description and issue a
corresponding textual representation in the dot language.
Invoke the command:
graph2dot -h
@end example
to see how to use @file{graph2dot}.
You can then pass the dot description to the @file{dot} program (from
the graphviz suite of programs) and obtain a graphical representation
of the filter graph.
For example the sequence of commands:
echo @var{GRAPH_DESCRIPTION} | \
tools/graph2dot -o graph.tmp && \
dot -Tpng graph.tmp -o graph.png && \
display graph.png
@end example
can be used to create and display an image representing the graph
described by the @var{GRAPH_DESCRIPTION} string.
@include filters.texi
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