Array that contains points of polygon. Can we iterate through it's border?

Question

Let's say we have an array filled with points(x,y)(tile), all in pseudocode, that form a closed and filled 2d polygon. How can we iterate only through the points that form the polygon's border?

Answer 1

Apply the gift-wrapping algorithm also known as the Jarvis march, which will output only those points that form the border.

jarvis(S)

    // S is the set of points
    pointOnHull = leftmost point in S // which is guaranteed to be part of the CH(S)

   i = 0
   repeat
      P[i] = pointOnHull
      endpoint = S[0]      // initial endpoint for a candidate edge on the hull
      for j from 1 to |S|
         if (endpoint == pointOnHull) or (S[j] is on left of line from P[i] to endpoint)
            endpoint = S[j]   // found greater left turn, update endpoint
      i = i+1
      pointOnHull = endpoint
   until endpoint == P[0]      // wrapped around to first hull point