[F8]Making user repeat/trace a contour/drawing with mouse

Hello, every1!
I received a task which I have no idea how to approach… I hope someone here can help me.
The specifications where as follows: there should be a black square in which there is a faint picture of a sign. The user must be able to draw in the square and when he repeats the contours of the sign correctly - a popup appears.
I made myself familiar with the ways you can allow a user to draw. However I have no idea how would you know if the two shapes/signs match and whether the user at least approximately drew the correct contour/sign.
Can anyone help? Or at least point in a direction of functions I can use? :cyclops:
Thanks in advance!