I extract the part 00:02:00.000 to 00:03:30.000 of a video with ffmpeg:
ffmpeg -i input.mp4 -ss 120 -t 90 -vcodec copy -acodec copy output.mp4
It works but the output video is jerky/choppy, i.e. not really smooth/fluid. Why?
The only log I have is:
[mp4 @ 000000000041c700] track 1: codec frame size is not set
[mp4 @ 000000000041c700] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
[mp4 @ 000000000041c700] pts has no value
Last message repeated 7894 times
But when watching the original video and navigating to 00:02:00.000, the video is perfectly fluid/smooth. How to fix this?
A video stream consists of chunks of video, a few seconds at a time, or a few frames at a time. At the start of each chunk is a keyframe, which contains the entire frame, and then the subsequent frame data consists of changes to the previous image. Whenever the scene changes or there is too much of a data change between frames, a new keyframe is generated and then subsequent frames are just data changes again.
When you use -c:v copy -ss 123.45, chances are the timestamp you're specifying doesn't coincide with a keyframe, and so your resultant video will assume a black keyframe and the subsequent frame changes will not make much sense, hence giving a choppy output.
The fix for this is to either:
(1) transcode or recode instead of copy
(2) identify where the keyframes are and align your start timestamp to the keyframe. You can write a script to detect the keyframes and set your timestamps from there, but afaik it can't be done in one command.
for example, you could use something like: ffprobe -i INPUT.mp4 -select_streams v -show_entries frame=pts_time -of csv=p=0 -skip_frame nokey > frames.txt
(3) A third option occurs to me that you could: