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doc/filters: review introductory example and explanation 

In particular, fix wrong vertical mirroring command, and clarify
and extend explanation.

Based on a patch by littlebat <dashing.meng@gmail.com>.

Should fix trac ticket #2413.
This commit is contained in:
Stefano Sabatini 2013-04-06 01:43:01 +02:00
parent e62fca11a8
commit 215ca86475
1 changed files with 23 additions and 16 deletions
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39
doc/filters.texi
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@ -3,10 +3,10 @@
Filtering in FFmpeg is enabled through the libavfilter library.
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 filtergraph. For example, the following one:
In libavfilter, a filter can have multiple inputs and multiple
outputs.
To illustrate the sorts of things that are possible, we consider the
following filtergraph.
@example
input --> split ---------------------> overlay --> output
@ -15,25 +15,32 @@ input --> split ---------------------> overlay --> output
+-----> 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:
This filtergraph splits the input 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:
@example
ffmpeg -i input -vf "[in] split [T1], [T2] overlay=0:H/2 [out]; [T1] crop=iw:ih/2:0:ih/2, vflip [T2]" output
ffmpeg -i INPUT -vf "split [main][tmp]; [tmp] crop=iw:ih/2:0:0, vflip [flip]; [main][flip] overlay=0:H/2" OUTPUT
@end example
The result will be that in output the top half of the video is mirrored
onto the bottom half.
Filters are loaded using the @var{-vf} or @var{-af} option passed to
@command{ffmpeg} or to @command{ffplay}. Filters in the same linear
chain are separated by commas. In our example, @var{split,
overlay} are in one linear chain, and @var{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 special labels @var{[in]} and @var{[out]} are the points
where video is input and output.
Filters in the same linear chain are separated by commas, and distinct
linear chains of filters are separated by semicolons. In our example,
@var{crop,vflip} are in one linear chain, @var{split} and
@var{overlay} are separately in another. The points where the linear
chains join are labelled by names enclosed in square brackets. In the
example, the split filter generates two outputs that are associated to
the labels @var{[main]} and @var{[tmp]}.
The stream sent to the second output of @var{split}, labelled as
@var{[tmp]}, is processed through the @var{crop} filter, which crops
away the lower half part of the video, and then vertically flipped. The
@var{overlay} filter takes in input the first unchanged output of the
split filter (which was labelled as @var{[main]}), and overlay on its
lower half the output generated by the @var{crop,vflip} filterchain.
Some filters take in input a list of parameters: they are specified
after the filter name and an equal sign, and are separated from each other