The light source and the perspective on the letters varies.
26 Implied Edges offers a variety of how-to ideas for fooling the eye. By careful use of the broad-edged pen on flat paper, you can show the side of an imaginary 3-D solid. Not only does the brain believe that there is a real object there, but the eye even supplies the missing edge. Each of the letters here represent an alphabet you can invent for yourself.