def create_rectangular_mask(canvas_width, canvas_height, offset_x, offset_y,
mask_width, mask_height):
m = Mask((canvas_width, canvas_height))
for x in range(offset_x, offset_x + mask_width):
for y in range(offset_y, offset_y + mask_height):
m.set_at((x, y), 1)
return m
