如何使用QuartzCore绘制一段虹膜快门

Question

任何人都知道如何绘制一段看起来至少与原始苹果相机快门类似的虹膜？

http://stickertagapp.com/_files/_iris.png

我正在尝试复制此功能：

CATransition *animation = [CATransition animation];
[animation setDelegate:self];
[animation setDuration:0.3];
animation.timingFunction = UIViewAnimationCurveEaseInOut;
[animation setType:@"cameraIris"];
[cameraView.layer addAnimation:animation forKey:nil];