During a break n a seminar today, I noticed that the chairs in front of me all had slightly transparent black mesh fabric. The backs of the chairs were in the shape of a hyperbolic paraboloid. The fabric covering it had two layers a couple of centimeters apart (it was just a fabric sheath over a frame), and the mesh was a hexagonal tiling with hole size approximately equal to thread size, both small.
I noticed that the interference between the two meshes made a bullseye pattern. The circles in the bullseye moved inwards or outwards as I moved my head, but the center of the bullseye seemed to be constantly at the critical point of the hyperbolic paraboloid (although I couldn't move my head much from side to side).
Why did this shape appear?