iphone - 圆形蒙版效果

Question

我用这段代码做了一个圆形 mask 效果:

CGContextRef context = CGBitmapContextCreate(NULL, self.bounds.size.width, self.bounds.size.height, 8, 4 * self.bounds.size.width, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaPremultipliedFirst);
CGContextAddArc(context, 1024/2, 768/2, size, 0, 6.3, 0);
CGContextClosePath(context);
CGContextClip(context);
CGContextDrawImage(context, self.bounds, imageView.image.CGImage);
CGImageRef imageMasked = CGBitmapContextCreateImage(context);
CGContextRelease(context);
UIImage *newImage = [UIImage imageWithCGImage:imageMasked];
CGImageRelease(imageMasked);

[imageView setImage:newImage];
UIGraphicsEndImageContext();

当我设置size--时，圆随着值缩小，但是当我设置size++时圆没有随着值放大。这是怎么回事？

Answer 1

我认为不先设置原始图像是你的问题。我对您的代码进行了一些测试，并使其以两种方式工作:

- (IBAction)minus:(id)sender {
    self.radius = @([self.radius floatValue]+10.0f);
    [self redrawImage];
}

- (IBAction)plus:(id)sender {
    self.radius = @([self.radius floatValue]-10.0f);
    [self redrawImage];
}

-(void)redrawImage
{
    //set the original image first
    [_imageView setImage:[UIImage imageNamed:@"your image goes here"]];

    CGFloat size = [self.radius floatValue];
    CGContextRef context = CGBitmapContextCreate(NULL, self.view.bounds.size.width, self.view.bounds.size.height, 8, 4 * self.view.bounds.size.width, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaPremultipliedFirst);
    CGContextAddArc(context, 320/2, 460/2, size, 0, 2*M_PI, 0);
    CGContextClosePath(context);
    CGContextClip(context);
    CGContextDrawImage(context, self.view.bounds, _imageView.image.CGImage);
    CGImageRef imageMasked = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    UIImage *newImage = [UIImage imageWithCGImage:imageMasked];
    CGImageRelease(imageMasked);

    [_imageView setImage:newImage];
    UIGraphicsEndImageContext();
}

另外，不用设置 6.3，使用 2 * M_PI 作为角度更容易。