A shadow cloud is a three-dimensional object, consisting of multiple shadow-casting elements semi-randomly arranged in three dimensions in such a way, that depending on the direction of illumination the overall shadow of the cloud displays various images encoded in it. A shadow cloud can be viewed as a generalization of shadow casting panels, but of course the basic idea of multiple shadows from one three-dimensional structure is already present in well-known GEB-triples. However, in contrast to GEB-triplets, a shadow cloud can encode up to four arbitrary images and display them under appropriate illumination without any distortions.
The idea of shadow clouds can be summarized as follows: the shadow cast by flat, thin elements depends on their relation to the direction of illumination: elements perpendicular to illumination cast clear shadows, while the shadows of elements parallel to illumination are practically invisible. Moreover, the elements perpendicular to the illumination can be arbitrary shifted along the illumination without changing the overall shadow cast by all the elements. This allows for a random, cloud-like placement of elements in space.
Figure 1. The idea of shadow clouds: the floating horizontal elements are perpendicular to the illumination and jointly cast a clear shadow, while the shadow of vertical elements is hardly visible.
In a real, physical realization of a shadow cloud the floating elements must be somehow kept together at appropriate locations. One obvious way of achieving this could via a solid transparent material, like plexiglas. Another option is the use of a scaffolding, which allows for much larger and lighter installations with greater flexibility, and offers a different look-and-feel.
Figure 2. Shadow-casting elements held in place by a regular scaffolding.
The rectangular grid as described above allows for encoding of three independent images. For encoding of four images we use a hexagonal grid and shadow casting elements with a more complex structure, as shown in Figure 3.
Figure 3. Shadow-casting elements for encoding of four independent images.
Shadow clouds can be used to make animated signs and displays, as shown in the video above.