转自：
https://www.jianshu.com/p/3fa0b04027ce
ffmpeg中的filter
FFmpeg中的libavfilter提供了一整套的基于filter的机制。filter本身是一个插件的形式，可以快速的组装需要的效果。
比如下面的filter，可以实现视频的水平镜像效果。
ffplay.exe sample.rmvb -vf hflip
FFmpeg中filter分为：
source filter （只有输出）
audio filter
video filter
Multimedia filter
sink filter （只有输入）
除了source和sink filter，其他filter都至少有一个输入、至少一个输出。
介绍了这么多，下面也是一个例子，使用filter实现宽高减半显示：
ffplay.exe sample.rmvb -vf scale=iw/2:ih/2
可以用
ffmpeg -filters
查看支持的filter.
[TheFlyingPenguindeMacBook-Pro:gzl Private$ ffmpeg -filters
ffmpeg version 3.1 Copyright (c) 2000-2016 the FFmpeg developers
built with Apple LLVM version 7.0.2 (clang-700.1.81)
configuration: --prefix=/usr/local/Cellar/ffmpeg/3.1 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libxvid --enable-librtmp --enable-libfaac --enable-ffplay --disable-lzma --enable-nonfree --enable-vda
libavutil 55. 27.100 / 55. 27.100
libavcodec 57. 48.101 / 57. 48.101
libavformat 57. 40.101 / 57. 40.101
libavdevice 57. 0.101 / 57. 0.101
libavfilter 6. 46.102 / 6. 46.102
libavresample 3. 0. 0 / 3. 0. 0
libswscale 4. 1.100 / 4. 1.100
libswresample 2. 1.100 / 2. 1.100
libpostproc 54. 0.100 / 54. 0.100
Filters:
T.. = Timeline support
.S. = Slice threading
..C = Command support
A = Audio input/output
V = Video input/output
N = Dynamic number and/or type of input/output
| = Source or sink filter
... abench A->A Benchmark part of a filtergraph.
...
... aresample A->A Resample audio data.
...
... copy V->V Copy the input video unchanged to the output.
...
T.. delogo V->V Remove logo from input video.
...
... format V->V Convert the input video to one of the specified pixel formats.
... fps V->V Force constant framerate.
...
... reverse V->V Reverse a clip.
TSC rotate V->V Rotate the input image.
T.. sab V->V Apply shape adaptive blur.
..C scale V->V Scale the input video size and/or convert the image format.
...
... split V->N Pass on the input to N video outputs.
...
scale
将输入的1920×1080缩小到960×540输出:
./ffmpeg -i input.mp4 -vf scale=960:540 output.mp4
//ps: 如果540不写，写成-1，即scale=960:-1, 那也是可以的，ffmpeg会通知缩放滤镜在输出时保持原始的宽高比。
crop
Crop the input video to given dimensions.
It accepts the following parameters:
- w, out_w
The width of the output video. It defaults to iw.
This expression is evaluated only once during the filter configuration, or when the ‘w’ or ‘out_w’ command is sent.- h, out_h
The height of the output video. It defaults to ih.
This expression is evaluated only once during the filter configuration, or when the ‘h’ or ‘out_h’ command is sent.- x
The horizontal position, in the input video, of the left edge of the output video.
It defaults to (in_w-out_w)/2.
This expression is evaluated per-frame.- y
The vertical position, in the input video, of the top edge of the output video.
It defaults to (in_h-out_h)/2.
This expression is evaluated per-frame.
- Crop area with size 100×100 at position (12,34).
crop=100:100:12:34
- Using named options, the example above becomes:
crop=w=100:h=100:x=12:y=34
References:
https://www.cnblogs.com/tocy/p/ffmpeg-filter-intro.html
http://www.ffmpeg.org/ffmpeg-filters.html#crop
https://blog.csdn.net/newchenxf/article/details/51364105
作者：FlyingPenguin
链接：https://www.jianshu.com/p/3fa0b04027ce
来源：简书
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