使用os.execvp调用ffmpeg时，长filter_complex字符串无法被正确识别

我在编写Python脚本调用ffmpeg拼接多个视频片段时遇到了异常：给filter_complex参数的字符串加引号后代码能正常工作，但不加引号时，ffmpeg会把整个滤镜字符串拆成多个参数解析。

正常工作的代码 #

inputs = sorted(list(askopenfilenames()))
n = len(inputs)

filter = []
for i in range(n):
    filter.append("[{}:v]".format(i))
    filter.append("[{}:a]".format(i))
filter.append("concat={}:v=1:a=1".format(n))
filter.append("[v]")
filter.append("[a]")
filter = " ".join(filter)

fargs = zip(itertools.repeat('-i'), inputs)
fargs = itertools.chain(
    ["ffmpeg"],
    itertools.chain.from_iterable(fargs),
    ["-filter_complex", '"{}"'.format(filter), "-vsync", "vfr", "-map", "[v]", "-map", "[a]"],
    ["-c:v", "libx264", "-crf", "{}".format(quality)],
    ["-c:a", "aac", "-b:a", "192k"],
    [out]
)

os.execvp("ffmpeg", list(fargs))

异常情况的代码片段 #

fargs = itertools.chain(
    ["ffmpeg", "-loglevel", "debug"],
    itertools.chain.from_iterable(fargs),
    #["-filter_complex", '"{}"'.format(filter), "-vsync", "vfr", "-map", "[v]", "-map", "[a]"],
    ["-filter_complex", filter, "-vsync", "vfr", "-map", "[v]", "-map", "[a]"],
    ["-c:v", "libx264", "-crf", "{}".format(quality)],
    ["-c:a", "aac", "-b:a", "192k"],
    [out]
)

ffmpeg错误日志 #

Reading option '-filter_complex' ... matched as option 'filter_complex' (create a complex filtergraph) with argument '[0:v]'.
Reading option '[0:a]' ... matched as output url.
Reading option '[1:v]' ... matched as output url.
Reading option '[1:a]' ... matched as output url.
Reading option '[2:v]' ... matched as output url.
Reading option '[2:a]' ... matched as output url.
Reading option 'concat=3:v=1:a=1' ... matched as output url.
Reading option '[v]' ... matched as output url.
Reading option '[a]' ... matched as output url.

参数列表打印结果 #

['ffmpeg', '-loglevel', 'debug', '-i', 'a.mp4', '-i', 'b.mp4', '-i', 'c.mp4', '-filter_complex', '[0:v] [0:a] [1:v] [1:a] [2:v] [2:a] concat=3:v=1:a=1 [v] [a]', '-vsync', 'vfr', '-map', '[v]', '-map', '[a]', '-c:v', 'libx264', '-crf', '20', '-c:a', 'aac', '-b:a', '192k', 'asdf.mp4']

原因解析 #

问题出在两个点：

1. ffmpeg参数解析逻辑：ffmpeg对filter_complex的参数解析会特殊处理内部的空格，即使通过os.execvp传递的是单个字符串参数，ffmpeg仍会把字符串内的空格当作参数分隔符，将原本的滤镜图拆分成多个独立参数。
2. 滤镜图语法错误：你用空格分隔滤镜图的元素，这不符合ffmpeg滤镜图的规范语法——滤镜元素之间应该用逗号分隔，而非空格。

加引号能临时解决问题，是因为ffmpeg会将引号内部的内容视为完整的单个参数，忽略内部空格，但这属于临时 workaround，并非规范用法。

正确解决方法 #

修改滤镜字符串的构建逻辑，用逗号代替空格拼接元素：

filter = []
for i in range(n):
    filter.append("[{}:v]".format(i))
    filter.append("[{}:a]".format(i))
filter.append("concat={}:v=1:a=1".format(n))
filter.append("[v]")
filter.append("[a]")
# 用逗号替代空格拼接滤镜元素
filter = ",".join(filter)

之后直接传递filter参数即可，无需添加引号：

["-filter_complex", filter, "-vsync", "vfr", "-map", "[v]", "-map", "[a]"],

这样ffmpeg就能正确识别整个滤镜图为单个参数，不会拆分内部元素，同时符合ffmpeg的语法规范。

内容的提问来源于stack exchange，提问作者incertia