// A camera shutter that closes as 
// s1 moves.

nLine = 4;
a = pi/4;
r = 1;
offset = s1 * pi/10;
for (i, 1, 20) {
    plot polar(r, 2*pi*t + a);
    a = a + offset;
    r = r * .707 / sin(pi/4 + offset);
}

