With the new 2.0 release of the PLNAR app, you can measure structures on a wall such as windows, openings, art, or other non-rectangular shapes on a wall.
Step 1) Make sure you have the perimeter of the room already marked out (see here for instructions) using PLNAR
Step 2) Mark sure you have defined the height of the floor to ceiling (see here for instructions) using PLNAR
Step 3) Just tap the corners of the structure and join the beginning and the end. This is great for non-rectangular structures like special shaped windows.