The example of appearance of 12 hidden images at the background of the main image.
The main image:
The image may be printed out either on the paper or on the other surface. The color model CMY. The hidden images are coded in the layers CYAN and MAGENTA. The enlarged fragment of the raster:
A special optical key, usually in the shape of the covering film, is used for development of the hidden images. 12 different hidden images in turn get developed as the result of turning the key.
Aleksey Zaitsevsky, 2007