A shadow cloud is a three-dimensional object consisting of multiple shadow-casting elements seemingly-randomly arranged in three dimensions so that, depending on the direction of illumination, the shadow of the cloud displays various images encoded in it.
The idea of shadow clouds can be summarized as the changing shadow cast by flat, thin elements with the changing 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 shifted along the illumination without changing the overall shadow cast by all the elements allowing for a random, cloud-like placement of elements in space.
A video explains more than words.
More on how shadow clouds work
It will be interesting to see how Drzach & Suchy extend and apply the idea. Apparently they are working on a larger installation that will display an entire three-word message and they are also considering using tiny solar panels as building blocks to allow the clouds to turn and adjust to the light source.
For more information, please visit www.drzachsuchy.ch