Since you are considering creating an "iris" effect, you can create a more mechanical approach by considering the nature of a mechanical iris. It is not a perfect circle, but rather a apperature that uses overlapping segments to close of more and more of the opening. Suppose you created a six-sided figure at the extreme limits of the screen. Each segment would enclose an angle of 120 degrees. You then rotate the figure by say, five degreesm and make it slightly smaller so that each point touches a line segment on the first. You do a fill to each triangle produced, then rotate another five degrees, and create another figure inside the second, again just touching at a point on the second figure. Then a fourth figure rotated another five degrees and reduced to fit at the points inside the third. You get a spiral effect, and each fill makes the iris smaller and blends it with the previous figures thus created.