CGContextClipToMask返回空白图像 [英] CGContextClipToMask returning blank image

问题描述

我是Quartz的新手。我有2张图像，一个背景和一个具有剪切形状的蒙版，我想将其放置在背景上以剪切出一部分。生成的图像应为切口的形状。这是我的面具（中间的形状是0 alpha）：

I'm new to Quartz. I have 2 images, a background, and a mask with cutout shape that I want to lay over the background in order to cut out a section. The resulting image should be the shape of the cutout. This is my mask (the shape in the middle is 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...知道我要去哪里了吗？非常感谢。

Except nothing appears... Any idea where I'm going wrong? Many thanks.

推荐答案

用蒙版剪切上下文后，您实际上应该在该上下文上绘制一些东西。

After you clip context with mask you should actually draw something on that context.

此外：

• 您应该使用 UIGraphicsBeginImageContextWithOptions 支持缩放


• 您正在创建两个上下文，一个使用 UIGraphicsBeginImageContext ，另一个使用 CGBitmapContextCreate 。一个就足够了：）


• 您应该翻转蒙版的Alpha，因为根据文档，蒙版的源样本确定剪切区域的哪些点已更改。换句话说，透明将变得透明

• you should use UIGraphicsBeginImageContextWithOptions to support scale factor.
• you're creating two context, one with UIGraphicsBeginImageContext and the other with CGBitmapContextCreate. One is enough :)
• you should "flip" alpha of your mask, because according to docs The source samples of mask determine which points of the clipping area are changed. The other words transparent will become transparent

简单的示例代码，在其中我将图像剪辑在imageView中，然后将其重新设置：

Simple example code, where I clip image inside imageView and then set it back:

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;

使用 CGImageCreateWithMask 的示例：

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);

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