sub mandelbrot {
    ( $cx, $cy ) = @_;
    $cy -= 0.5;
    $zx = $zy = 0;
    for (1 .. $MAX_ITERATIONS) {
        $zx2  = $zx * $zx;
        $zy2  = $zy * $zy;
        return 1 if $zx2 + $zy2 > $BAILOUT;
        $zx = $zx * $zy * 2 + $cx;
        $zy = $zy2 - $zx2 + $cy;
    }
}