ffmpeg给视频添加字幕
在网上下载了一些美剧，但是下载下来发现都是没有字幕的，就打算自己给视频添加字幕了。
需要准备的东西：
- 视频文件：最好是官方原版的，避免视频被剪辑过导致和字幕匹配不上
- 字幕文件：可以在http://www.subhd.tv/网站上找到，一般为ass或者srt格式
- ffmpeg工具：ffmpeg是一个非常牛逼的视频处理工具，很多视频软件（如暴风影音等）都是通过它实现的。支持windows、linux以及macos等多个平台。
给视频添加字幕的操作非常简单，执行一行命令即可：
ffmpeg -i video.mkv -i subtitle.ass -c copy output.mkv
替换掉已有字幕：
ffmpeg -i input.mkv -i input.ass -c copy -map 0 -map -0:s -map 1 output.mkv
参数说明：
-map 0：选择第一个输入文件的所有流
-map -0:s：删除第一个输入文件中的subtitles流（即删除字幕）
-map 1：选择第二个输入文件中的流
如果ffmpeg要实现添加字幕的功能需要在编译时开启--enable-filter=subtitles --enable-libass
ffmpeg -i plane.mp4 -vf subtitles=p.srt psrt.mp4 #合并为视频，而不是增加字幕流。
ffmpeg中srt相关用例
1 1 2 00:00:02,940 --> 00:00:31,060 3 <font face="Microsoft YaHei" size="30" color="#eddfc1">测试波音727哪个座位最安全！^M 4 <font size="30">It's a test!</font></font>
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Title: Default Aegisub file
ScriptType: v4.00+
WrapStyle: 0
ScaledBorderAndShadow: yes
YCbCr Matrix: TV.709
PlayResX: 1920
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[Aegisub Project Garbage]
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[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:02.94,0:00:10.06,Subtitle,,0,0,0,,降落伤亡测试！\N{\fs40}It's a test!
Dialogue: 0,0:00:10.94,0:00:13.06,Subtitle,,0,0,0,,降落在沙漠的！\N{\fs40}On the !
Dialogue: 0,0:00:20.94,0:00:23.06,Subtitle,,0,0,0,,瓦列利安家族会找你算账的！\N{\fs40}House Velaryon is coming for you!
Dialogue: 0,0:00:23.98,0:00:26.73,Subtitle,,0,0,0,,海蛇会砍下你那该死的麻脸脑袋！\N{\fs40}The Sea Snake will have your poxy fucking head!使用ffmpeg可以很轻松的把ass/vtt/lyric转换为srt文件，命令如下：
ffmpeg -i a.ass b.srt
ffmpeg -i c.vtt d.srt
ffmpeg -i e.lyric f.srt
./ffplay test.mp4 -vf subtitles=test.srt
srt字幕以其简单、体积小、易查看、易掌握等优点，深得人们的喜爱，但便利的代价就是样式少，无法实现复杂的特效。
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