- (UIImage *)maskImage:(UIImage *)originalImage toPath:(UIBezierPath *)path {
    UIGraphicsBeginImageContextWithOptions(originalImage.size,NO,0);
    [path addClip];
    [originalImage drawAtPoint:CGPointZero];
    UIImage *maskedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return maskedImage;
}

如果要使用多个路径的并集,则更难,因为Quartz没有任何直接计算两个路径的并集的函数.一种方法是将每个路径逐个填入蒙版,然后通过掩码绘制图像：

- (UIImage *)maskedImage
{
    CGRect rect = CGRectZero;
    rect.size = self.originalImage.size;
    UIGraphicsBeginImageContextWithOptions(rect.size,YES,0.0); {
        [[UIColor blackColor] setFill];
        UIRectFill(rect);
        [[UIColor whiteColor] setFill];
        for (UIBezierPath *path in self.paths)
            [path fill];
    }
    UIImage *mask = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    UIGraphicsBeginImageContextWithOptions(rect.size,0.0); {
        CGContextClipToMask(UIGraphicsGetCurrentContext(),rect,mask.CGImage);
        [self.originalImage drawAtPoint:CGPointZero];
    }
    UIImage *maskedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return maskedImage;
}