Scan image for what part of original image is not there?

It’s hard to describe but say I had a piece of paper with a design on it. If I were to drop little drops of paint on that paper, I would like to be able to scan the image and somehow get the information of what on my image target is being covered up by the paint and at what location on the image target. IE 1 inch left and 2 inches up from the center of the image target there is a mark .25 inches in diameter. I’m not really sure where to look for this so I was wondering if anyone had any resources.