ios - IOS设置cgcontext路径透明边框

Question

我在上下文中添加了一条贝塞尔路径，如下所示

for (Face* face in facesArray) {

    UIBezierPath* customPath = [UIBezierPath bezierPath];
    [customPath moveToPoint:face.rightEyePosition];
    [customPath addQuadCurveToPoint:face.leftEyePosition controlPoint:face.foreheadPosition];
    [customPath addQuadCurveToPoint:face.mouthPosition controlPoint:face.lowerLeftContrlPoint];
    [customPath moveToPoint:face.rightEyePosition];
    [customPath addQuadCurveToPoint:face.mouthPosition controlPoint:face.lowerRightContrlPoint];

    UIGraphicsBeginImageContextWithOptions(originalImage.size, NO, 0.0);

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextBeginPath (context);
    CGContextAddPath(context,customPath.CGPath);

    CGContextClosePath (context);
    CGContextClip (context);
    [imageView.image drawInRect: CGRectMake(0, 0, imageView.image.size.width, imageView.image.size.height)];

    UIImage* resultImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
}

上面的代码根据贝塞尔形状裁剪图像。我希望裁剪图像的边框透明。我应该如何设置角ob bezier路径的透明度