Export keyframes from video at intervals
Seconds:
time for i in {0..42} ; do ffmpeg -accurate_seek -ss `echo $i.0 | bc` -i input.mp4 -frames:v 1 period_down_$i.jpg ; done
Minutes:
time for i in {0..42} ; do ffmpeg -accurate_seek -ss `echo $i*60.0 | bc` -i input.mp4 -frames:v 1 period_down_$i.jpg ; done