Draw perimeter around player range in grid based game

the link photo is not actually drawing lines. they are using a set of prefab images that represent corners and strait lines and such to place in a grid like placement. the images are square with transparent green line parts that make it look like lines. Very good approach. as far as finding the area, there are many alorithms for flood filling grids, edge detection, path finding and so on to detect where boundrys are and what type or direction you might place images like this. if you have a grid like game with grid data and need help with algo's let me know.