From the documentation VLC HowTo/Make thumbnails
vlc C:\video\to\process.mp4 --rate=1 --video-filter=scene --vout=dummy --start-time=10 --stop-time=11 --scene-format=png --scene-ratio=24 --scene-prefix=snap --scene-path=C:\path\for\snapshots\ vlc://quit
VLC 3.0.3 MacOS 10.12.6
'/Applications/VLC.app/Contents/MacOS/VLC' '/Users/larry/Desktop/VLC Video/video.mp4' --rate=1 --video-filter=scene --vout=dummy --start-time=10 --stop-time=11 --scene-format=png --scene-ratio=24 --scene-prefix=snap --scene-path='/Users/larry/Desktop/VLC Snapshots/' vlc://quit
Have also tried adding -I dummy
'/Applications/VLC.app/Contents/MacOS/VLC' '/Users/larry/Desktop/VLC Video/video.mp4' -I dummy --rate=1 --video-filter=scene --vout=dummy --start-time=10 --stop-time=11 --scene-format=png --scene-ratio=24 --scene-prefix=snap --scene-path='/Users/larry/Desktop/VLC Snapshots/' vlc://quit
Just stalls with this message
[00007ff18c56e9e0] dummy interface: using the dummy interface module...
Thanks for your help