It is an optical illusion. There are other versions—silhouettes (cast shadows with depth cues removed)—and most people tend to see one direction or the other.
With a little practice, you can swit...
By the way, do you play any 3D games? Do you also have trouble with the brain-manufactured depth perception there?
There is a visual processing technique called SSAO (Screen Space Ambient Occlusion) which does something to the image that is stencilled over the outline of 3D objects to make them jump forward. There is a specialisation in painting called trompe-l'œil which uses similar exaggerated shading techniques to make images leap off the painted surface by exploiting these light-based depth cues.
Drop shadows are a similar visual element as well, used on almost all GUIs these days.
There is a visual processing technique called SSAO (Screen Space Ambient Occlusion) which does something to the image that is stencilled over the outline of 3D objects to make them jump forward. There is a specialisation in painting called trompe-l'œil which uses similar exaggerated shading techniques to make images leap off the painted surface by exploiting these light-based depth cues.
Drop shadows are a similar visual element as well, used on almost all GUIs these days.
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