Quelques exemples :

def f(t, i, x, y): return sin(y/8+t)
def f(t, i, x, y): return random()
def f(t, i, x, y): return sin(t)
def f(t, i, x, y): return i / 256
def f(t, i, x, y): return x / 16
def f(t, i, x, y): return y - 7.5
def f(t, i, x, y): return y - t
def f(t, i, x, y): return [1, 0, -1][i%3]
def f(t, i, x, y): return sin(t-sqrt((x-7.5)**2+(y-6)**2))
def f(t, i, x, y): return (y > x) and (14-x < y)
def f(t, i, x, y): return (int(t*10)) & (1<<x) and y==8
def f(t, i, x, y): return random() * 2 - 1
def f(t, i, x, y): return cos(t + i + x * y)
def f(t, i, x, y): return sin(x/2) - sin(x-t) - y+6
def f(t, i, x, y): return -.4/(.01+hypot(x-t%10,y-t%8)-t%2*9)
def f(t, i, x, y): return .02/sin(3**(x+1)-atan2(x+1,(x+1)*y)*3-t)
def f(t, i, x, y): return sin(atan2(x-8,y+5)*8+sin(y)+t*4)
def f(t, i, x, y): return x & 1 if cos((i|17)**2+t)>0 else y & 1
def f(t, i, x, y): return (16*(1+sin(t+x))<(x+y))*sin(x-t)
def f(t, i, x, y): return sin((x & y)/(.01 + sin((t/3)%3)*38-y-x))