Introduction
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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.

Integrating libavfilter into the main FFmpeg repository is a work in progress. If you wish to try the unfinished development code of libavfilter then check it out from the libavfilter repository into some directory of your choice by:

        svn checkout svn://svn.ffmpeg.org/soc/libavfilter

And then read the README file in the top directory to learn how to integrate it into ffmpeg and ffplay.

But note that there may still be serious bugs in the code and its API and ABI should not be considered stable yet!

Tutorial
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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 --------+

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 in.avi -s 240x320 -vf "[in] split [T1], fifo, [T2] overlay= 0:240 [out]; [T1] fifo, crop=0:0:-1:240, vflip [T2]

where input_video.avi has a vertical resolution of 480 pixels. 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 -VF option passed to ffmpeg or to ffplay. Filters in the same linear chain are separated by commas. In our example, SPLIT, FIFO, OVERLAY are in one linear chain, and 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 [T1] and [T2]. The magic labels [IN] and [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 SOURCE FILTERS that do not have a video input, and we expect in the future some SINK FILTERS that will not have video output.

graph2dot
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The graph2dot program included in the FFmpeg 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

to see how to use graph2dot.

You can then pass the dot description to the dot program (from the graphviz suite of programs) and obtain a graphical representation of the filter graph.

For example the sequence of commands:

     echo GRAPH_DESCRIPTION | \
     tools/graph2dot -o graph.tmp && \
     dot -Tpng graph.tmp -o graph.png && \
     display graph.png

can be used to create and display an image representing the graph described by the GRAPH_DESCRIPTION string.

     [file filters.texi not included]