what is Flat image in 3D image? is most of the time a big question for a beginner but we are here to make you understand the what is flat and what is 3D, the position and intensity of light make more prominent affects on a surface therefore it creates two effects on surface 1) gradient. 2) detach. which you must have observed in above image.  In curve( such as sphere ,cylinder,elliptical )objects  it creates the gradient and detach in such as rectangle, cube objects which have share edges .