Comment by somat

A spinning mirror is certainly an option, there are many projects around using them as projectors e.g. [0]. It would need precision faces and be a larger volume than the flapping mirror approach. Because the mirrors are spring-mounted and designed to resonate at ~50Hz they actually take very little power to drive - there's an optosensor on the back used to stabilise the oscillation amplitude, which is why the VB and Private Eye display widths vary during startup.

Can't see the video from this location, so may be just restarting stuff in it.

[0] https://hackaday.com/2018/04/20/laser-projector-ditches-galv...

I'd guess a 6-sided mirror would be too big. The mirror needs a certain width for its reflection range to span across the viewport, and so a hexagon of that side length would be too big, particularly considering both eyes (the hexagons would have to overlap across the center line.)

Another problem would be that 60º between sides is too little angular separation. When the current frame approaches the right edge of your vision field, the next side of the mirror is already in position to be reflecting the LEDs into the left side of the vision field.

A 2-sided rotating mirror might work, but that's more complicated to manufacture and mount, such that I'd guess they found the back-and-forth simpler. Or else they found that rotating mirrors would impose a torque on the whole device and that wasn't workable. You could rotate the two mirrors in opposite directions, but that would mean the viewports are scanning in opposite directions (one right-to-left), and I don't know what that would do for the perceptions of 3d and persistence-of-vision.