The challenge was to decide whether a 2D point is within a given distance to a rectangle, which might be rotated. I first solved this in C# but then decided to remake it in Python to graph/test my solution.

## Initial Thoughts

The rectangle is described by center position, dimensions, and the local X and Y axes which describe rotation. For no rotation, these are `(1,0)`

and `(0,1)`

respectively. A 45 degree rotation is represented by `(1,1)`

, `(-1,1)`

for localX and localY.

Immediately, I considered distance equations involving points and lines. Naively, I could test the point to see if it’s within all 4 line segments of the rectangle, and if it isn’t, find the distance to the edges and corners. This seemed unwieldy so I considered the fact that the problem is simpler if the rectangle is not rotated and is centered at origin. Thus I set out rewind the rectangle’s transformations by applying them in reverse to the point (e.g if the rect was rotated 10 degrees CCW, rotate the point 10 CW). I found this transformation by finding the change of basis matrix, which, simply enough, is just the column vectors of the rectangle’s local X and Y axes.

Reversing this is as simple as inverting the matrix and applying it.

After finding the point relative to the rectangle, I took the absolute value of the point’s coordinates. If the point was at `(-20,-10)`

, the distance is the same as if it were `(20,10)`

. This greatly simplifies the logic. The rest of the code is testing whether the point is inside, to the right, above, or right & above, of the rectangle. Finding the distance in each case is trivial.

Great, so the initial plots look correct for a normal rectangles. My plan to visualize it worked it, meaning I can see many results at once. The third picture shows the points close to an infinitely small rectangle. As predicted, it’s a circle. Lastly, I tried changing the rectangle’s axes so they weren’t 90 degrees from each other. This results in a parallelogram, as seen in the last image. Success!

My code is a little more complex than necessary since I wanted practice in documentation, to use Numpy for matrix inversion, and proper command line support. My initial implementation in C# didn’t use external libraries and was much more efficient, but that wasn’t a goal for this version.