I used a section of a failed painting in all kinds of digital experiments because, with a simple light-for-dark inversion, it looks even more like terrain than some of my other paintings. See, left to right above, how it looks right side up, rotated 180 degrees, and then inverted in Photoshop.
I've noticed that not all eyes read the same 3D relief from shading in a picture. At a certain distance the relief in the gold painting below 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.