Shadow casting panels is a technique that allows multiple images to be encoded within a single physical object, a white panel, which displays the separate images under appropriate lighting conditions. The underlying principle of our technique is based on a simple observation: the shadow cast by an object depends not only on the object itself, but also on the light; therefore the same object under changing lighting conditions can totally change its appearance.
Shadow Casting Panels (SCP) is a novel technique for storing and presenting multiple images using one physical object, so that separate images become visible under varying illumination. It was invented by Drzach in 2004 (patent pending), and then developed further by Drzach & Suchy. Below a brief introduction to the technique is given, for more details please see the original thesis [pdf].
The images to be stored in a panel are first rasterized and converted to a black-and-white format, and then jointly encoded into the panel. More precisely, the images are considered in a pixel-by-pixel manner, and for each pixel of the an appropriate building block is placed at the corresponding place in the panel. The type of the block depends on the colors of the particular pixel in the images to be encoded. The actual blocks can have various geometries, yielding a variety of possible designs and perceptions.
The main observation underlying the concept of SCP is that the shadow cast by a physical object depends on the shape of the object and the direction of the illumination. This enables the control of the shadow by appropriate design of an object for specific lighting conditions. In particular, it is possible to design objects (building blocks) which depending on the illumination, either cast a uniform shadow, partial shadow, or (almost) no shadow at all. Such objects can be directly used to represent pixels of rasterized images, and by arranging the blocks into panels (arrays) entire images can be represented. Moreover, by using transparent, colored materials, blocks for representing colorful pixels can be obtained.
Panels encoding two images
For simplicity, consider two black-and-white images of same size, like for example image A and image B in the figure below. We compare the pixels at every location in both images one-by-one, and each comparison determines the type of building block to be used, according to the table on the left. As a result of the "merging" of two images we obtain an array of building block types.
To complete the panel, we need to design the building blocks with corresponding properties, i.e. blocks which either cast shadow (black pixel) or cast no shadow (white pixel), depending on the direction of the external illumination. An example design of such blocks is presented in the figure below.
The resulting panel looks as follows. When illuminated from left top side, the panel displays image A as a shadow/light pattern, and under illumination from top right side image B becomes visible.
Applying the same method as above, but using a transparent colored film to obtain colored "shadows", we can create panels for displying colorful images (see a prototype below).
Panels encoding multiple images
The basic principle outlined above applies naturally also for encoding more than two images in a single panel, but then more types of building blocks of more complicated shapes are needed. In particular, for three black-and-white images we need eight building blocks, to cover all eight possible states of a pixel in all three images.
An example design of such blocks is presented in the following figure (showing the top views of the blocks only). This technique was used to build the "coexist"-panel.
Shadow Casting Panels have numerous applications at a wide range of sizes, for example:
Techniques related to shadow casting panels
The following projects also use shadow as a presentation medium, but are encoding only a single image in a physical object.
The following non-shadow-based techniques enable encoding of several images in a single object
The following shadow-based techniques enable encoding of several images in a single 3-d object
Manufacturing methods and prototypes
Shadow Casting Panels can be produced from various materials, and assembled manually or in an automated way. The images and video below present several prototypes and manufacturing methods. Further examples can be found on the page of *relativity-project