Ambiguous images appear everywhere, from psychological studies to the surface of Mars. Is it a butterfly or a splash of sauce? Is it a rabbit or a duck? Is there a vase, or are two human faces facing each other? Based on the information transmitted by the eyes, the human brain can make some very curious interpretations. One of the most striking cases is the one that confuses mountains with valleys, and vice versa. What is happening? In essence, the conflict comes down to light, shadows, and their location.
Observe this image captured by NASA from the International Space Station. Do you notice anything odd? Now, take a copy of that image, place it in your favorite graphics editor, and rotate it 180 degrees. Boom. The image changed completely, and all you did was turn it upside down. The illusion dissipates as you spend more time looking at it, but its first impression is powerful.
Mountain or valley?
"Mountain or valley?" is a valid question in all those examples that present limited information. Basically, I cheated by setting aside the detail that the image belongs to the Grand Canyon; however, what we want to know is why that illusion works so well.
The folks at MinutePhysics on YouTube explain it. The Sun illuminates us from above, causing shadows to appear on the lower part of an object. This relationship has been simulated many times, including in works of art, and something as simple as the Windows cursor.
Placing the light source in the upper-left corner and the shadow directly opposite also conveys a certain "3D effect". Given the lack of context, the combo "light above-shadow below" makes us see things as convex, while "shadow above-light below" conveys a sense of concavity.
The problem arises when you change the position of the light. When placed at the correct angle, something naturally convex could be misinterpreted as concave. When close, there is no room for error, but if you were in orbit, your brain has no choice but to switch to a "safe mode" and use the position of shadows as a reference. The mountain and valley illusion is so strong that cartographers are forced to "fake" a light source in the upper-left corner of their maps to convey correct information.
https://old.neoteo.com/funcionan-las-ilusiones-opticas/
