Can ffmpeg encode only the beginning part of a video for cutting?

Question

I am a really beginner in video editing. Sorry if this question is silly.

I heard videos consist of key frames (I-frames) and inter-frames (P-frames and B-frames).

Say I have a video of 10 seconds, and I ask ffmpeg to extract from 4.3 to 10.

ffmpeg -ss 4.3 -i input.mp4 output.mp4

From my observation, ffmpeg seems to reencode the whole output file.

Assume the original video has I frames at 1, 3, 5, 7, 9 seconds. I think ffmpeg can just insert a keyframe at 4.3, reencode frames from 4.3 to 5, and leave all data afterwards intact.

Is my idea realistic? Can ffmpeg do so?

I ask this is because if my input video is very long, my method can cut the video very fast.

Answer 1

heavy scripting

#!/bin/bash
cd /tmp
f="/mnt/public/upload/test/tg/2024-07-20_10꞉11꞉34.MOV"
# f="/mnt/public/upload/test/tg/2024-07-19_13꞉39꞉18.mp4"
t="tmp.mp4"
o="output.mp4"

IFS=, read vid wid hei sar fmt fps tbn <<< $(ffprobe -v 0 -select_streams v:0 -show_entries stream=codec_name,width,height,sample_aspect_ratio,r_frame_rate,time_base,pix_fmt -of csv=p=0 "$f")
if [ "$sar" = "N/A" ]; then sar=1; fi
tbn=${tbn#*/}
echo $vid $wid $hei $sar $fmt $fps $tbn

beg=4.3
end=$( echo "$beg + 10" | bc -l )
ofs=$(ffprobe -read_intervals ${beg}%${end} -skip_frame nokey -select_streams v:0 -show_entries frame=pts_time -of default=nw=1:nk=1 "$f" -v 16 | head -n 2 | tail -n 1)
echo "---> convert from $beg to $ofs"
ffmpeg -noautorotate -ss $beg -to $ofs -i "$f" -c:v $vid -video_track_timescale $tbn -an "$t" -y -hide_banner

echo "file '$t'
file '$f'
inpoint $ofs" > 1.txt
ffmpeg -f concat -safe 0 -i 1.txt -ss $beg -i "$f" -map 0:v:0 -map 1:a:0 -c copy "$o" -y -hide_banner
mpv --no-config --keep-open "$o"

this ↑ variant doesn't work sometimes with mp4, so you can try variant with using temporary mkv segments ↓

#!/bin/bash
cd /tmp
# f="/mnt/public/upload/test/tg/2024-07-20_10꞉11꞉34.MOV"
f="/mnt/public/upload/test/tg/2024-07-19_13꞉39꞉18.mp4"
t="tmp.mkv"
o="output.mp4"
beg=4.3
ffmpeg -i "$f" -c copy -an -f segment -segment_times $beg -reset_timestamps 1 tmp_%d.mkv -hide_banner -y
echo "---> convert tmp_0.mkv from $beg"
vid=$(ffprobe -v 0 -select_streams v:0 -show_entries stream=codec_name -of csv=p=0 "$f")
ffmpeg -noautorotate -ss $beg -i "tmp_0.mkv" -c:v $vid "$t" -y -hide_banner
echo "file '$t'
file 'tmp_1.mkv'
" > 1.txt
ffmpeg -f concat -i 1.txt -ss $beg -i "$f" -map 0:v:0 -map 1:a:0 -c copy "$o" -y -hide_banner
mpv --no-config --keep-open "$o"