That's a nice illusion, fun to view. The visual cue misleading you here is movement parallax.

I'll never forget when I first saw this illusion in Zurich: They had in a hallway two identical faces one protruding as usual the other hollow. The one face looked straight ahead and the other followed you. You couldn't go near the two faces to study them and see the trick. A young boy asked his father why the two heads were so different. The father explained him that he can't understand this yet because it is done by holographic projection.

The son was mightyly impressed and I swallowed down any comment. I couldn't do that to a six year old looking admiringly at his father. The effect by the way was known and used already more than 2000 years ago be the Greek.

Wolfgang