If it's bottomless it's damned inconvenient. I think it just paint. Anyone care to test it?
Umm, ya. 3 shots of tequila and that would be a 'Nope thing.
Is someone at the bottom with a telephoto lens taking pictures? If it was a painting they didn't need to paint the cross members. A glass floor would make sense to have cross members.
Who changes the light bulbs?
There is a chalk art contest in Savannah, GA every year and I have seen some of these 3-D illusions done.
It is pretty amazing.