Thinking of relationships is exactly how I conceptualize what I do too. I can manipulate geometric objects in my mind even though I'm not seeing them, because I

Thinking of relationships is exactly how I conceptualize what I do too. I can manipulate geometric objects in my mind even though I'm not seeing them, because I'm manipulating the relationships between the objects (or between the vertices/edges within an object). The downside of this is all this modeling of relationships is a lot to keep in my brain at once, so there is a limit to how far I can push this. That said, I've yet to encounter a situation where this is actually any better than what my wife can do, who does have a mind's eye (and is frighteningly good at tetris / packing). The part you describe about seeing "boundary conditions" sounds to me like what I do with seeing edge cases and potential unwanted interactions in programming, which is completely non-visual.