Everyone loves a kaleidoscope, particularly the ones with a lens at the end, so that as you look through them whilst sweeping the kaleidoscope around, the view becomes a dazzling starburst pattern. (I find Nova Magic Marble kaleidoscopes are inexpensive ones for kids that work pretty well). However, real-world kaleidoscopes can only tile the visual field with a limited repertoire of geometric shapes – typically triangles. Digitally we can tile with any shape that will tessellate – that is, fill the plane by repetition without gaps or overlaps. As with real-world kaleidoscopes with a lens at the end, each tile can enclose a streaming segment of a visual scene, if you are handy with graphics and 2D animation packages. If that all sounds a bit puzzling, I think the movie will make it clearer.
But then there’s a surprise! Illusions of movement may appear, dependent on figure/ground effects.
As you will see in the movie, the fish-shaped tiles and the streaming textures within them are identical, except that some are one way up, the rest upside down. And yet the tiles seen one way up appear as “ground”, and stationary, whilst others, seen as “figure”, seem to drift in relation to them. At the end of the movie, I’ve introduced a colour difference between the figure and ground cells, to make the distinction clearer.
The illusion is analogous to the experience you may have had on a train with another train alongside: through the window, it’s apparent the trains are moving in relation to one another, but unless you feel movement, it can be hard to be sure which train is moving. In this demo, the brain selects a set of cells at one orientation as “window” and another as “compartment”. But as with other figure ground illusions, figure and ground can “flip”, so that the whole pattern appears to change direction.
By the way, that’s a revision at 27/2/15 of my original interpretation of the illusion! I previously attributed it to a “drifting edge” effect, which I thought was new! Wishful thinking.
However, I think this is a debut for digital kaleidoscopes enclosing streaming real world textures! I’ll be adding more about them in a later post.
Coral reef background image in the movie thanks to NOAA picture library.