Basically, the ideas is that we perceive things by projection.
(or rather, physically speaking light get reflected into our
retina) Now, forget that we have two eyes and the processing to
get into a stereoscopic imagery in our head. But suppose we just
have one eye for simplicity of this account. Now, we see things
by projection. So, essentially what we get is a planar
representation of a 3D image. Besides having two eyes to get
different perspective to reconstruct the original 3D image, we
also heavily relies on past experiences of reality of likely
objects. So, given a drawing on paper (which is 2D), we are able
to interpret its original 3D scene as the drawing is of. The
simplest example is that a drawing of a cube, which on paper is
a hexagonal thing. (and if you view the cube from its corner
perspective, it's a perfect hexagon, which SGI exploited for
their logo design. See: SGI Logo ＆ Visual Illusion.) Now,
however, given a piece of drawing (which is a 2D image), there
are really a myriad ways to interp