function cut_round_corner() {
  IN_FILE=$1
  OUT_FILE=$2
  CORNER=${3:-30}

  convert $IN_FILE \
    \( +clone -alpha extract \
    -draw 'fill black polygon 0,0 0,'$CORNER' '$CORNER',0 fill white circle '$CORNER','$CORNER' '$CORNER',0' \
    \( +clone -flip \) -compose Multiply -composite \
    \( +clone -flop \) -compose Multiply -composite \
    \) -alpha off -compose CopyOpacity -composite $OUT_FILE
}