float viewWidth = 80;
UIBezierPath * path = [UIBezierPath bezierPath];
path.lineWidth = 2;
[[UIColor whiteColor] setStroke];
[path moveToPoint:CGPointMake((sin(M_1_PI / 180 * 60)) * (viewWidth / 2), (viewWidth / 4))];
[path addLineToPoint:CGPointMake((viewWidth / 2), 0)];
[path addLineToPoint:CGPointMake(viewWidth - ((sin(M_1_PI / 180 * 60)) * (viewWidth / 2)), (viewWidth / 4))];
[path addLineToPoint:CGPointMake(viewWidth - ((sin(M_1_PI / 180 * 60)) * (viewWidth / 2)), (viewWidth / 2) + (viewWidth / 4))];
[path addLineToPoint:CGPointMake((viewWidth / 2), viewWidth)];
[path addLineToPoint:CGPointMake((sin(M_1_PI / 180 * 60)) * (viewWidth / 2), (viewWidth / 2) + (viewWidth / 4))];
[path closePath];
CAShapeLayer * shapLayer = [CAShapeLayer layer];
shapLayer.lineWidth = 2;
shapLayer.strokeColor = [UIColor whiteColor].CGColor;
shapLayer.path = path.CGPath;
_headerImageView.layer.mask = shapLayer;
同理其他多边形也可以实现。
圆形的：_headerImageView.layer.cornerRadius = viewWidth / 2;
_headerImageView.clipsToBounds = true;
也可以设置线的宽度和颜色。
ios开发 UIView（UIImageView）添加六边形的遮罩
原文：http://blog.csdn.net/zhouleizhao/article/details/46226161
