I saw a comment today on HN showing voxiebox[1] which seems to have a better volumetric display, unlike this one which has a very small number of discrete layers.
I think it's a spinning plane with a projector beneath. Their limitation is in the DLP projector's refresh rate. Seems they also may have extra limitations in colour space, since most DLP projectors use a rotating colour wheel.
It have to agree with the parent, it definitely looks like a display or a projection surface oscillating in the vertical direction.
I'm really curious if they opted for a display or a projector. A display is heavy but it might be possible if they use a vacuum. A projection surface on the other hand can be very light but you have to calibrate the optics and it _has_ to have a display and not a DLP chip, etc.. it's a really interesting problem to solve.
Looks a front-surface OLED display that's vibrating up and down on its Z-axis very fast.
So the number of layers would be bound by the frame rate of the display, the amount of vertical travel, and the speed of the motor driving it up and down.