How to skip start frames with ffmpeg

I have using ffmpeg to create short preview of videos...

$length = $length_video/5;

$ffmpeg_path." -i ".$content." -vf select='lt(mod(t\,".$length.")\,1)',setpts=N/FRAME_RATE/TB -af aselect='lt(mod(t\,".$length.")\,1)',asetpts=N/SR/TB -an ".$content_new.";

Actually this will create 5sec. video. and problem is because first frame is extracted from 0-1 sec.

There is any chance to skip first frame and extract next 5?

Problem is because first frame is actually useless because of video intro

Solution

This select filter should skip the first 5 frames:

$ ffmpeg -i input_file -vf select="gte(n\, 5)" ...

For timespan it's even easier. Here's how you skip the first 30 seconds:

$ ffmpeg -i input_file -ss 30 ...

To skip the first 30 seconds and keep the next 120 seconds of the original video, add -t:

$ ffmpeg -t input_file -ss 30 -t 120 ...

-ss  time_off        set the start time offset
-t   duration        record or transcode "duration" seconds of audio/video

Both -ss and -t take a time duration:

The following examples are all valid time duration:

55 55 seconds

12:03:45 12 hours, 03 minutes and 45 seconds

23.189 23.189 seconds

None of these methods will be perfectly accurate due to the very mechanics of how video compression works (P and B frames can not stand on their own). ffmpeg will try to adjust accordingly:

Note that in most formats it is not possible to seek exactly, so ffmpeg will seek to the closest seek point before position. When transcoding and -accurate_seek is enabled (the default), this extra segment between the seek point and position will be decoded and discarded. When doing stream copy or when -noaccurate_seek is used, it will be preserved.

Refer to the ffmpeg documentation for more.