(0,0)      (1/3,0)        (2/3,0)        (1,0)



(0,1/3)    (1/3,1/3)      (2/3,1/3)      (1,1/3)



(0,2/3)    (1/3,2/3)      (2/3,2/3)      (1,2/3)



(0,1)      (1/3,1)        (2/3,1)        (1,1)



(0,0)-->(1/3,0)-->(1/3,1/3)-->(0,1/3)

Pass in the upper left coordinates as your parameter.

(x,y) (x+1/3,y)  (x+1/3,y+1/3)  (x,y+1/3)