[OT] Poll: What does it mean for 1 rect to be 'within' a certain distance of another rect?

[Paul Dupuis]

This is an Off Topic informal poll of sorts, but related to LiveCode as 
I am writing a LiveCode expression to determine if 2 arbitrary 
rectangles (r1,r2) are with some distance d (in px) of one another. In 
considering this problem, the questions comes up: What is meant by 
rectangles being within a distance d of one another. What is the 'd' 
measured from?

center to center? Easiest is many ways, but I don't think this is what 
most people would think of.

adjacent edge to adjacent edge? This is harder (I think), but I think 
this is what more people intuitively think of. To me, implicit in the 
visual concept of 2 rects being within some distance of one another is 
that they are NOT overlapping, but that some gap exists between the 
nearest adjacent edges?

Something else? What does 2 rects  being 'within' d pixels of one 
another mean to you, if not one of the two above options?

Maybe there is a exact mathematical definition of what 2 rectangles 
being within distance d of one another is, but, if there is, I am 
unfamiliar with it.