Delvo,
That tic-tac-toe pattern is just a flaw in the individual frames with
horizontal and vertical lines. Those odd lines are two pixels wide.
Old Grapejuice Stain,
The grid certainly is one grid rotating as a whole, but the thing which
causes the illusion is that it jumps between frames, exactly the same
as the
backward-rotating wagon wheel illusion. Exactly the same
cause, very similar effect. The reason for the difference of course is
that the grid is rectangular while wagon wheel spokes are radial.
I see quite sharp boundaries between adjacent rotating areas. However,
if I follow along the boundary of one rotating area, I see the boundary in
one place, but if I follow the (same) boundary of the immediately adjacent
rotating area, the boundary isn't necessarily in the same place!
I made a slightly spiffier version. I kept the overal dimensions and grid
spacing, but antialiased the lines and made the steps between frames a
bit smaller. I also reduced the number of frames so that the file is about
40% smaller. The original has 50 frames in 7.2-degree steps; mine has
only 15 frames in 6-degree steps, covering 90 degrees of rotation. The
original is 4 times the size it needed to be.
Rotating grid illusion animation
I haven't noticed so far that the smaller step size reduces the illusion,
though it might.
Edit to add: Uggh. My browser, on my slow computer, adds an "illusion" of
its own. The screen is broken up into sections very much like the illusion
that is supposed to be there. Maybe if I slow the animation down a bit my
browser will be able to keep up. Anyone else have this problem?
I slowed it down a tiny bit but that wasn't enough. I hope I'm the only
one who can't view my animation properly.
-- Jeff, in Minneapolis