Tracking geometric shapes with webcam

Jan Wagner's icon

Hi! :)
I would like to track basic geometric shapes using a webcan which is pointed at a piece of paper witch those shapes drawn on them. (Triangle, Circle, Rectangle,..)
I want to track parameters like: how big is it? how many cornes does it have? how round is the object? etc.
Does anyone know a way to do that or have some tips where i could start?
anks in advance :)