FFmpeg -ss 奇怪的行为
我一直在使用 FFmpeg 将单帧提取到图像中。尽管进行了一些谷歌搜索，结果发现运行：
ffmpeg.exe -i video.avi -ss 00:30:00 -y -an -vframes 1 test.png
... 运行速度比以下命令慢得多，这几乎相同，但是是瞬时的：
ffmpeg.exe -ss 00:30:00 -i video.avi -y -an -vframes 1 test.png
唯一的区别是 -i 和 -ss 的顺序。这是故意的“功能”吗？这里的差异是否有某种技术原因？
I've been using FFmpeg to extract single frames into an image. Though some Googling, it turns out that running:
ffmpeg.exe -i video.avi -ss 00:30:00 -y -an -vframes 1 test.png
...runs SIGNIFICANTLY slower than the following, which is nearly identical, but instantaneous:
ffmpeg.exe -ss 00:30:00 -i video.avi -y -an -vframes 1 test.png
The only difference is the order of -i and -ss. Is this an intentional 'feature'? Is there some sort of technical reason for the difference here?
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这是一个有根据的猜测。当
-ss出现在-i之前时，它被视为输入指令，因此视频流的第一帧是30秒处的帧。当-ss出现在-i之后时，它被视为效果，并且前30秒的帧被读取并丢弃，导致性能差异。This is an educated guess. When
-ssoccurs before-i, it is treated as instructions for the input, so the first frame of the video stream is the one at 30 seconds. When-ssoccurs after-i, it is treated as an effect, and the first 30 seconds of frames are read and dropped, leading to a performance difference.wberry的回答确实很有教养。阅读文档可以进一步提供帮助：
我目前正在编写一个音频采集应用程序，并且，因此，我正在使用较慢但更准确的方法。您可以选择最佳方法。
wberry's answer is indeed a very educated one. Reading the documentation can help even further:
I'm currently writing an audio grabber application, and, as such, I'm using the slower but more accurate method. It is up to you to choose the best approach.
我还刚刚完成了一个从视频内容生成缩略图的应用程序。
你应该检查一下这个
http://ffmpeg.org/trac/ffmpeg/wiki/Seeking%20with%20FFmpeg
它描述了一种将
-ss标志（来自两个位置）组合成一个统一命令的方法，该命令提供帧时间精度，并具有更快的帧选择。ffmpeg -ss 00:02:30 -i Underworld.Awakening.avi -ss 00:00:30 -vframes 1 out3.jpg还包含其他可能技巧的链接，例如单个拇指的多个拇指视频。
I have also just finished an app that generates thumbnails from video content.
You should check this out,
http://ffmpeg.org/trac/ffmpeg/wiki/Seeking%20with%20FFmpeg
It describes a method of combining the
-ssflag (from both locations) into a unified command that provides frametimeaccuracy, with a quicker frame selection.ffmpeg -ss 00:02:30 -i Underworld.Awakening.avi -ss 00:00:30 -vframes 1 out3.jpgand also includes links to other possible tricks, like multiple thumbs from a single video.
当 -ss 出现在 -i 之前时，它会转到最近的关键帧（在 25 fps 的 H.264 文件中每 10 秒一次，因为 H.264 将使用 250 的 GOP）。这使得查找速度非常快，因此您可以在 -i 之后添加另一个 -ss 以转到第一个 -ss 之后的小数位置。
在 -i 之后加上 -ss 将寻找确切的位置，但速度非常慢。
有关示例和更多信息，请参阅 https://trac.ffmpeg.org/wiki/Seeking
When -ss occurs before the -i, it goes to the closest key frame (which is every 10 seconds in H.264 files for 25 fps, since H.264 will use a GOP of 250). This makes seeking really fast, so you can add another -ss after the -i to go to a fractional location after the first -ss.
Having -ss after -i will seek to the exact location, but it is very slow.
See https://trac.ffmpeg.org/wiki/Seeking for examples and more information