SCREEN 12
w = 640: h = 480
h = h \ 4
y2 = h - 1
FOR ps = 1 TO 4
col = 0
FOR x = 0 TO (w - ps - 1) STEP ps
LINE (x, y1)-(x + ps - 1, y2), col, BF
col = 15 - col ' col alternate between 0 (black) and 15 (white)
NEXT x
y1 = y1 + h: y2 = y2 + h
NEXT ps