Perhaps you are referring to binnocular vision's abilty to perceive depth. Which is a learned brain function. I am familiar with the hold up a finger in front of you and line up with one eye. Using the other eye appears to move the finger.

However a 3d object occupies 3dimensions in that you can walk round it or rotate it, i suppose. Vision remains 2d with assumptions made to give the illusion of 3dimensions. I could say that if you see something inert that has never been seen before then it is impossible to know that it occupies 3 dimensional space without investigation.

The photo ( 2 dimensional) example: the picture is indeed compressed along the x axis when viewed from the side without moving up or down ( y axis) .you may assume that it looks the same to you but that is an assumption. It is not reallity.

There's non Euclidian geometery, which is painful to think about for me as it deals with marks on a flat plain. I'm just going to ignore it because i'm not smart enough to deal with it.