I recently finished the 5th or 6th or 800th (I can't remember the exact count) draft of my YA Fantasy Novel UNSEEN. (click on the link to see the trailer or it's on the sidebar too.) I think it's finally ready to start sending out.
Although this story is complete fantasy, there are several things that inspired certain aspects of the story. One important object is very much like the cylindric anamorphism shown in the pictures below. The idea is that there is a distorted image on paper but when a cylinder is place in the middle, the reflected image is non distorted.
picture borrowed from Mighty Optical Illusions moillusions.com
It is said that this art form was invented by the Chinese (I think they deserve the trophy for The Culture that has Produced the Coolest Things Award.) and was brought over to Italy in the 16th century, influencing many of the great artists of the time.
this technique was often used to disguise confidential images.