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文件和流和下标都是从0开始算。不使用-map，则每种类型(audio, video, subtitle)只会选出一个流。 There are some special stream specifiers:
- -map 0 selects all streams from all types
- -map i:v selects all video streams from the file with a number i (index), -map i:a selects all audio streams, -map i:s selects all subtitle streams, etc.
- special options -an, -vn, -sn exclude all audio, video or subtitle streams respectively