Hi,
you want the fingers to do something "unique", like some movements, press and lift finger on specific positions, and the touch can detect this as "your unique movement pattern" and unlock the electronic.
Cellular phones do this with the unlock pattern. To ease positioning they often show a 3 x 3 dots matrix on the screen.
Or they show a numeric or alpahnumeric keyboard on the screen and you may input your pin or password.
You are free to show a blank screen (which makes it more difficult for you to find the correct positions) or you may show any picture.
Klaus