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.

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
********

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 -vfilters "[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 -VFILTERS 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.

Available video filters
***********************

When you configure your FFmpeg build, you can disable any of the existing video filters. The configure output will show the video filters included in your build.

Below is a description of the currently available video filters.

crop
====

Crop the input video to x:y:width:height.

     ./ffmpeg -i in.avi -vfilters "crop=0:0:0:240" out.avi

``x'' and ``y'' specify the position of the top-left corner of the output (non-cropped) area.

The default value of ``x'' and ``y'' is 0.

The ``width'' and ``height'' parameters specify the width and height of the output (non-cropped) area.

A value of 0 is interpreted as the maximum possible size contained in the area delimited by the top-left corner at position x:y.

For example the parameters:

     "crop=100:100:0:0"

will delimit the rectangle with the top-left corner placed at position 100:100 and the right-bottom corner corresponding to the right-bottom corner of the input image.

The default value of ``width'' and ``height'' is 0.

format
======

Convert the input video to one of the specified pixel formats. Libavfilter will try to pick one that is supported for the input to the next filter.

The filter accepts a list of pixel format names, separated by ``:'', for example ``yuv420p:monow:rgb24''.

The following command:

     ./ffmpeg -i in.avi -vfilters "format=yuv420p" out.avi

will convert the input video to the format ``yuv420p''.

noformat
========

Force libavfilter not to use any of the specified pixel formats for the input to the next filter.

The filter accepts a list of pixel format names, separated by ``:'', for example ``yuv420p:monow:rgb24''.

The following command:

     ./ffmpeg -i in.avi -vfilters "noformat=yuv420p, vflip" out.avi

will make libavfilter use a format different from ``yuv420p'' for the input to the vflip filter.

null
====

Pass the source unchanged to the output.

scale
=====

Scale the input video to width:height and/or convert the image format.

For example the command:

     ./ffmpeg -i in.avi -vfilters "scale=200:100" out.avi

will scale the input video to a size of 200x100.

If the input image format is different from the format requested by the next filter, the scale filter will convert the input to the requested format.

If the value for ``width'' or ``height'' is 0, the respective input size is used for the output.

If the value for ``width'' or ``height'' is -1, the scale filter will use, for the respective output size, a value that maintains the aspect ratio of the input image.

The default value of ``width'' and ``height'' is 0.

slicify
=======

Pass the images of input video on to next video filter as multiple slices.

     ./ffmpeg -i in.avi -vfilters "slicify=32" out.avi

The filter accepts the slice height as parameter. If the parameter is not specified it will use the default value of 16.

Adding this in the beginning of filter chains should make filtering faster due to better use of the memory cache.

vflip
=====

Flip the input video vertically.

     ./ffmpeg -i in.avi -vfilters "vflip" out.avi