Phantom Quadrant presents:

These are essentially geometric binocular illusions. In the first, if the observer looks out from behind his/her right-hand pillar (from, say, a food platform or in front of a rock wall), an odd shape appears in front of him/her, with a dark border in the same location where the light is entering. In the second, if the observer looks back behind his/her right-hand pillar (from, say, a sand dune or behind a stone wall), the same strange shape appears in front of him/her, but with a line of light beneath the dark border.

It turns out that these geometric illusions may form a real quadrant. Suppose you go behind an ordinary wall to