These three images succeed in using spacial depth for one common reason: despite appearing to be simplistic, they have a very complex design look that keeps people’s attentions. Just by looking at these designs, it keeps people wondering if there is more to see if they analyze them at varying distances.

Sources:

http://1.bp.blogspot.com/_wwL03lhNnEA/S_mzrXKgwWI/AAAAAAAABjk/RnCc6IgNsCA/s1600/depth.jpg

http://images.fineartamerica.com/images-medium-large/illusion-of-depth-14-uma-devi.jpg