Pepper's Ghost is a trick that has been frightening theater goers since Victorian times, but recently it has been receiving some technological upgrades. The latest is Pepper's Cone, a dynamic 3D display that is simple enough to be implemented by almost anyone with some programming and almost no hardware skills.

Pepper's Ghost refers to a technique in which a sheet of glass is used on stage to make a brightly lit actor appear as a ghostly image. The glass acts as a partial reflector because it is set at an angle to the audience, which can both see through it to the scene behind and see the offstage actor due to the reflection. 

A modern variation is to use a curved surface on a table to reflect the image present on a 2D display. With some work, it is possible to create an illusion that a 3D object is floating above the table. In this case the curved surface is provided by a cone - hence the name Pepper's Cone.

Take a look at it in action:

The hardware needed for the display is ridiculously simple - a cone made from a suitable plastic sheet, a rotatable base and a tablet with a gyro sensor - an iPad Pro in this case, but there is no reason why it couldn't be made to work with other well known tablets. The idea is to compute the transformation needed to the 2D image for the reflection in the cone to look undistorted.

As the cone is rotated the tablet detects the rotation using the gyro sensor and recomputes the transformation to show the correct view of the object.

The code is written in Unity with a custom shader. It will be available as open source on Github, but the challenge is - can you create your own code to run natively on a low cost Android tablet?

More Information

Pepper's Cone: An Inexpensive Do-it-yourself 3d Display

Xuan Luo, Jason Lawrence and Steven M. Seitz

University of Washington and  Google

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