CGContextClipToMask返回空白图像

Question

我是Quartz的新手.我有2个图像,一个背景和一个带有剪裁形状的面具,我想要在背景上放置以剪切一个部分.生成的图像应该是切口的形状.这是我的面具(中间的形状是0 alpha):

这是我的代码:

UIView *canvas = [[[sender superview] subviews] objectAtIndex:0];

UIGraphicsBeginImageContext(canvas.bounds.size);
CGColorSpaceRef colourSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef cgContext = CGBitmapContextCreate(NULL, canvas.bounds.size.width, canvas.bounds.size.height, 8, 0, colourSpace, kCGImageAlphaPremultipliedLast);
CGColorSpaceRelease(colourSpace);

CGImageRef maskImage = [[UIImage imageNamed:@"Mask.png"] CGImage];
CGContextClipToMask(cgContext, CGRectMake(0, 0, canvas.frame.size.width, canvas.frame.size.height), maskImage);

CGImageRef maskedImageRef = CGBitmapContextCreateImage(cgContext);
CGContextRelease(cgContext);

UIImage *maskedImage = [UIImage imageWithCGImage:maskedImageRef];
UIGraphicsEndImageContext();

[button setBackgroundImage:maskedImage forState:UIControlStateNormal];

除了什么都没有出现......我知道哪里出错了？非常感谢.

Answer 1

在使用蒙版剪辑上下文后,您应该在该上下文中绘制一些内容.

除此之外:

• 你应该UIGraphicsBeginImageContextWithOptions用来支持比例因子.
• 你正在创建两个上下文,一个用于UIGraphicsBeginImageContext另一个用于CGBitmapContextCreate.一个就够了:)
• 你应该"翻转"你的面具的alpha,因为根据文档The source samples of mask determine which points of the clipping area are changed.透明的其他词语将变得透明

简单的示例代码,我在imageView中剪辑图像,然后将其设置回来:

UIGraphicsBeginImageContextWithOptions(self.imageView.frame.size, NO, 0.0);
CGContextRef context = UIGraphicsGetCurrentContext();

CGContextTranslateCTM(context, 0.0, self.imageView.frame.size.height);
CGContextScaleCTM(context, 1.0, -1.0);

CGImageRef maskImage = [[UIImage imageNamed:@"mask.png"] CGImage];
CGContextClipToMask(context, self.imageView.bounds, maskImage); 

CGContextTranslateCTM(context, 0.0, self.imageView.frame.size.height);
CGContextScaleCTM(context, 1.0, -1.0);

[[self.imageView image] drawInRect:self.imageView.bounds];

UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

self.imageView.image = image;

示例使用CGImageCreateWithMask:

UIImage *image = [UIImage imageNamed:@"image"];

CGImageRef originalMask = [UIImage imageNamed:@"mask"].CGImage;
CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(originalMask),
                                    CGImageGetHeight(originalMask),
                                    CGImageGetBitsPerComponent(originalMask),
                                    CGImageGetBitsPerPixel(originalMask),
                                    CGImageGetBytesPerRow(originalMask),
                                    CGImageGetDataProvider(originalMask), nil, YES);

CGImageRef maskedImageRef = CGImageCreateWithMask(image.CGImage, mask);

UIImage *maskedImage = [UIImage imageWithCGImage:maskedImageRef scale:image.scale orientation:image.imageOrientation];

CGImageRelease(mask);
CGImageRelease(maskedImageRef);