Illusionism 101

I've used this section of a failed painting in all kinds of variations because, with a simple light-for-dark inversion, it looks even more like terrain than some of my other paintings.  See above how it looks "right side up" (left); rotated 180 degrees (middle) and inverted in Photoshop (right).

I've noticed that not all eyes read the same 3D relief from the shading in a picture.  At a certain distance the relief in the gold painting to the left flops inside out or looks flat to my eyes, but the relief emerges for other people I've had look at it from the same place.

This difference in the way people perceive illusion made me wonder if there were cases where I saw relief and others saw nothing.