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106 lines
3.4 KiB
Plaintext
106 lines
3.4 KiB
Plaintext
\input texinfo @c -*- texinfo -*-
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@settitle Libavfilter Documentation
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@titlepage
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@center @titlefont{Libavfilter Documentation}
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@end titlepage
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@top
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@contents
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@chapter Introduction
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Libavfilter is the filtering API of Libav. It is the substitute of the
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now deprecated 'vhooks' and started as a Google Summer of Code project.
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Integrating libavfilter into the main Libav repository is a work in
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progress. If you wish to try the unfinished development code of
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libavfilter then check it out from the libavfilter repository into
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some directory of your choice by:
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@example
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svn checkout svn://svn.libav.org/soc/libavfilter
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@end example
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And then read the README file in the top directory to learn how to
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integrate it into ffmpeg and avplay.
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But note that there may still be serious bugs in the code and its API
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and ABI should not be considered stable yet!
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@chapter Tutorial
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In libavfilter, it is possible for filters to have multiple inputs and
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multiple outputs.
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To illustrate the sorts of things that are possible, we can
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use a complex filter graph. For example, the following one:
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@example
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input --> split --> fifo -----------------------> overlay --> output
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| ^
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+------> fifo --> crop --> vflip --------+
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@end example
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splits the stream in two streams, sends one stream through the crop filter
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and the vflip filter before merging it back with the other stream by
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overlaying it on top. You can use the following command to achieve this:
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@example
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./ffmpeg -i in.avi -s 240x320 -vf "[in] split [T1], fifo, [T2] overlay= 0:240 [out]; [T1] fifo, crop=0:0:-1:240, vflip [T2]
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@end example
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where input_video.avi has a vertical resolution of 480 pixels. The
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result will be that in output the top half of the video is mirrored
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onto the bottom half.
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Video filters are loaded using the @var{-vf} option passed to
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ffmpeg or to avplay. Filters in the same linear chain are separated by
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commas. In our example, @var{split, fifo, overlay} are in one linear
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chain, and @var{fifo, crop, vflip} are in another. The points where
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the linear chains join are labeled by names enclosed in square
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brackets. In our example, that is @var{[T1]} and @var{[T2]}. The magic
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labels @var{[in]} and @var{[out]} are the points where video is input
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and output.
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Some filters take in input a list of parameters: they are specified
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after the filter name and an equal sign, and are separated each other
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by a semicolon.
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There exist so-called @var{source filters} that do not have a video
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input, and we expect in the future some @var{sink filters} that will
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not have video output.
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@chapter graph2dot
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The @file{graph2dot} program included in the Libav @file{tools}
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directory can be used to parse a filter graph description and issue a
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corresponding textual representation in the dot language.
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Invoke the command:
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@example
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graph2dot -h
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@end example
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to see how to use @file{graph2dot}.
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You can then pass the dot description to the @file{dot} program (from
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the graphviz suite of programs) and obtain a graphical representation
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of the filter graph.
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For example the sequence of commands:
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@example
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echo @var{GRAPH_DESCRIPTION} | \
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tools/graph2dot -o graph.tmp && \
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dot -Tpng graph.tmp -o graph.png && \
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display graph.png
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@end example
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can be used to create and display an image representing the graph
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described by the @var{GRAPH_DESCRIPTION} string.
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@include filters.texi
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@bye
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