Hi I'm confused about sensors. I am currently doing my senior project and need to detect 4 black circles about 2 inches in diameter in a 6 foot by 6 foot space that is an off white color, with a border of any color. I can not go outside this border. I have been looking up sensors and can not determine exactly how it can detect whether it is looking at a circle or a line? Also I'm confused what sensors to choose. All all seem to do the same thing. Any help will greatly be appreciated.