extract stills using ffmpeg

getstills

function getstills {
    if [[ ! -z $1 ]]; then
        OUT=${2:-.}
        START=${3:-00:00:00}
        DUR=${4:-99:59:59}
        ffmpeg -skip_frame nokey -i $1 -vsync vfr -ss $START -t $DUR "$OUT/still-%02d.png"
    else
        echo "usage: getstills FILENAME START (hh:mm:ss) DURATION OUTPUT_PATH";
    fi
}