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 stationary train with another train alongside.  One of the trains begins to move, but unless you feel movement, it can be hard to be sure which train is moving, as you lookat the other train through the window.  It’s a bit like that with this demo:  the brain selects a set of cells at one orientation as “figure”, and they then appear to move in front of  the cells at the other orientation, seen as “background”.  But as with other figure/ground illusions, figure and ground can “flip”.  The whole pattern then appears to change direction.

However, for the train effect, one of the trains does actually have to move.  In this movie, the edges of the ‘cells’ of the tessellations don’t really move at all:  only the internal textures do.  The novement sonehow captures the edges of the cells seen as figure.

I think this is also 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.