FFmpeg: Cutting 3 seconds at the beginning of a video leaves 5 seconds blank

Question

I have an MKV video, and want to cut the first 3 seconds. I tried with FFmpeg:

ffmpeg -i /input.mkv -ss 00:00:3.0 -c copy -t 00:09:58.0 output.mkv

But the result (when played in VLC) is that there are 5 seconds blank video with the original sound, and after that, the video shows.

I think it has something to do with a missing keyframe at the beginning. So I tried:

ffmpeg -i /input.mkv -ss 00:00:3.0 -c copy -t 00:09:58.0 -force_key_frames 00:00:00.000 output.mkv

But this has no effect - same result.

How can I cut a video properly so that it begins exactly where cut?

Answer 1

This is answered in FFmpeg's Seeking wiki page. The behavior is expected: If the time hits to a space between 2 key frames, it copies only audio up to the next keyframe and fills the video with black.

There are some methods to get also the beginning.

• One is to overlay the copied part with few seconds of re-encoded original.
• Or, simply, seek to a bit earlier frame.
• Or, find the previous keyframe and clip from that.
• Or, reencoding the whole clip.

It's a bit pity that FFmpeg (AFAIK) doesn't have a feature for this built in.