iOS UIImage 剪辑到路径

我正在处理用户使用 UIBezierPath 选择部分图像的图像。如何删除/清除/透明化不属于该选择的所有内容？

通过一条路径，这非常容易。只需将路径设置为剪切路径：

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

如果你想使用多个路径的并集，那就更难了，因为 Quartz 没有任何直接计算两个路径的并集的函数。一种方法是将每条路径一一填充到蒙版中，然后通过蒙版绘制图像：

- (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, NO, 0.0); {
        CGContextClipToMask(UIGraphicsGetCurrentContext(), rect, mask.CGImage);
        [self.originalImage drawAtPoint:CGPointZero];
    }
    UIImage *maskedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return maskedImage;
}