Rotate an image, another one appears (w/ Video)

December 23, 2015 by Hillary Sanctuary
Rotate an image, another one appears (w/ Video)

Hidden elements can now be placed into an inkjet printed image, thanks to EPFL software. These hidden elements become visible by simply rotating the print.

Hide an image within an image. EPFL scientists have developed a printing algorithm that does just that onto metallic sheets. Two are printed together using an inkjet printer so that only one image can be seen at a time from a given angle.

The technique could one day be used as a security element in passports and printed money to prevent counterfeit.

Roger Hersch holds a printed image of a colourful butterfly. He then turns the print 90 degrees to reveal an entirely new image, the Mona Lisa. "Elements can only be seen upon rotation, " says Hersch of EPFL's Peripheral Systems Laboratory. "This rotation effect has never been done before."

Messages have been hidden in documents for over two millennia, starting with the chemical properties of substances from lemon juice to bodily fluids. Modern techniques use the fluorescence of substances under ultraviolet light, the precision of nanoparticle printing or even engineered bacteria that glow under certain circumstances. Printing is part of that game.

Printers spray ink as tiny dots into precise patterns, a standard technique called halftoning. Different patterns of cyan, magenta and yellow dots produce a wide range of colours.

When the halftone is printed along lines onto metallic sheets, the researchers noticed that the resulting colour depends on the viewing angle. This is because incoming light traversing the ink lines cast shadows onto the metallic surface. Ink lines perpendicular to the incoming light create a large shadow and appear as "strong colors". Ink lines parallel to the incoming light do not induce a shadow and appear as "weak colors". When the print is rotated by 90 degrees, strong colors become weak and weak colors become strong.

The video will load shortly

This effect would not be noticeable on paper. Unlike the mirror properties of a metallic sheet, paper diffuses light in many directions and gets rid of directional shadows.

Based on these observations, the researchers developed both a mathematical model and associated software that predict the colours that will be observed from different viewing angles. Currently, the software supports standard inkjet prints onto metallic sheets.

 Their algorithm essentially superposes two images during the printing process, creating a print with visible and hidden elements. By changing the viewing angle, and hence the way light casts shadows, hidden elements literally come to light.

Explore further: Inkjet hologram printing now possible

Related Stories

Inkjet hologram printing now possible

November 25, 2015

Vivid holographic images and text can now be produced by means of an ordinary inkjet printer. This new method, developed by a team of scientists from ITMO University in Saint Petersburg, is expected to significantly reduce ...

Researchers print inside gels to create unique shapes

September 30, 2015

(—A team of researchers at the University of Florida has taken the technique of printing objects inside of a gel a step further by using a highly shear-rate sensitive gel. In their paper published in the journal ...

Recommended for you

Dutch unveil giant vacuum to clean outside air

October 25, 2016

Dutch inventors Tuesday unveiled what they called the world's first giant outside air vacuum cleaner—a large purifying system intended to filter out toxic tiny particles from the atmosphere surrounding the machine.

3-D-printed organ-on-a-chip with integrated sensors

October 24, 2016

Harvard University researchers have made the first entirely 3D-printed organ-on-a-chip with integrated sensing. Built by a fully automated, digital manufacturing procedure, the 3D-printed heart-on-a-chip can be quickly fabricated ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.