Image sensing

[Obmar]

Hi there,
Anyone has idea of a simple way of detecting a printed image? Printing presses use registration marks for paper alignment. How is the image of the registration mark detected and how can it be compared with a reference image? I am trying to stack printed papers for accurate punching but can't find way of aligning bottom paper with the top one. (the sheets are laminated, so the printing is not in correct position compared to the edges.)

 

[dknguyen]

Move the sheets around until the image contains the least amount of black?

 

[Obmar]

Move the sheets around until the image contains the least amount of black?

Thats a great idea. I will try to get bottom and top image by webcams, digitise and superimpose them (I don't know how, but I will find out) and on screen find position of least black.

Please tell me if I can do anything better.