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Seeking


Version 13 (modified by c-14, 4 years ago) (diff)

As of FFmpeg 2.1, when transcoding with ffmpeg (i.e. not streamcopying), -ss is now accurate even when used as an input option.

Introduction

If you need to extract only a specific part of your input, you'll need to use the seeking option to get to that specific part in the input first. The parameter -ss is used to seek into the input and it can be used in several ways.

Fast seeking

The -ss parameter needs to be specified before -i:

ffmpeg -ss 00:23:00 -i Mononoke.Hime.mkv -frames:v 1 out1.jpg

Example output for "Seeking with FFmpeg".

This example will produce one image frame (out1.jpg) at the third minute from the beginning of the movie. The input will be parsed using keyframes, which is very fast. As of FFmpeg 2.1, when transcoding with ffmpeg (i.e. not streamcopying), -ss is now accurate even when used as an input option. Previous behavior can be restored with the -noaccurate_seek option.

Slow seeking

The -ss parameter needs to be specified after -i:

ffmpeg -i Mononoke.Hime.mkv -ss 00:23:00 -frames:v 1 out2.jpg

Example output for "Seeking with FFmpeg".

This example will also produce one image frame (out2.jpg) precisely at the third minute from the beginning of the movie.

Here, the input will be decoded until it reaches the position given by -ss. This will be done very slowly, frame by frame. As of FFmpeg 2.1, the main advantage is that when applying filters to the output stream, the timestamps aren't reset prior to filtering (i.e. when burning subtitles into a video, you don't need to modify the subtitle timestamps), but the drawback is that it will take a lot of time until it finally reaches that time point. The bigger the seeking time is, the longer you will have to wait.

Combined seeking

For this we specify the -ss parameter before and after -i:

ffmpeg -ss 00:22:30 -i Mononoke.Hime.mkv -ss 00:00:30 -frames:v 1 out3.jpg

Example output for "Seeking with FFmpeg".

As of FFmpeg 2.1, combined seeking is still possible but I have yet to find a valid use case for it since -ss as an input option is now both fast and accurate.

This approach uses keyframes to seek until 00:02:30, and then seeks frame-by-frame until it reaches 00:03:00 (00:02:30 + 00:00:30)

Note that both all images created by the previous commands should be identical.

There is no general rule on how to correctly set both time points for -ss options, because those depend on the keyframe interval used when the input was encoded. To give some orientation, the x264 encoder by default uses a GOP size of 250 (which means 1 keyframe each 10 seconds if the input frame rate is 25 fps).

Notes

Cutting small sections

To extract only a small segment in the middle of a movie, it can be used in combination with -t which specifies the duration, like -ss 60 -t 10 to capture from second 60 to 70. Or you can use the -to option to specify an out point, like -ss 60 -to 70 to capture from second 60 to 70. -t and -to are mutually exclusive. If you use both, -t will be used.

Note that if you specify -ss before -i only, the timestamps will be reset to zero, so -t and -to have the same effect:

ffmpeg -ss 00:01:00 -i video.mp4 -to 00:02:00 -c copy cut.mp4
ffmpeg -i video.mp4 -ss 00:01:00 -to 00:02:00 -c copy cut.mp4

Here, the first command will cut from 00:01:00 to 00:03:00 (in the original), whereas the second command would cut from 00:01:00 to 00:02:00, as intended.

If you cut with stream copy (-c copy) you need to use the `-avoid_negative_ts 1` option if you want to use that segment with the concat demuxer .

Example:

ffmpeg -ss 00:03:00 -i video.mp4 -t 60 -c copy -avoid_negative_ts 1 cut.mp4

Timestamp syntax

Note that you can use two different formats, 00:02:30 or 150, i.e. [HH:MM:SS] or just seconds. They mean the same thing. Also, if you use a fraction, like 02:30.05 this is interpreted as "5 100ths of a second" not as frame 5. For instance, 02:30.5 would be 2 minutes, 30 seconds, and a half a second.

Doing a bitstream copy gives me a broken file?

If you use -ss with -c:v copy, the resulting bitstream might end up being choppy, not playable, or out of sync with the audio stream, since ffmpeg is forced to only use/split on i-frames.

Multiple cuts at once

You can do do multiple splits with the same file as well, see How to split video efficiently, which may or may not save you time/CPU.

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