使用CGImageMaskCreate创建蒙版是所有黑色（iphone） [英] Creating mask with CGImageMaskCreate is all black (iphone)

问题描述

我试图从两个现有图片的复合材料中创建一个图片蒙版。

I'm trying to create an image mask that from a composite of two existing images.

首先，我创建的复合材料包含一个小图像是掩蔽图像，以及与背景尺寸相同的较大图像：

First I start with creating the composite which consists of a small image that is the masking image, and a larger image which is the same size as the background:

UIImage * BaseTextureImage = [UIImage imageNamed:@"background.png"];
UIImage * MaskImage = [UIImage imageNamed:@"my_mask.jpg"];
UIImage * ShapesBase = [UIImage imageNamed:@"largerimage.jpg"];
UIImage * MaskImageFull;

CGSize finalSize = CGSizeMake(480.0, 320.0);
UIGraphicsBeginImageContext(finalSize);
[ShapesBase drawInRect:CGRectMake(0, 0, 480, 320)];
[MaskImage drawInRect:CGRectMake(150, 50, 250, 250)];
MaskImageFull = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

我可以输出这个UIImage（MaskImageFull），它看起来不错，它是一个全尺寸的背景大小，

I can output this UIImage (MaskImageFull) and it looks right, it is a fullsize background size and it is a white background with my mask object in black, in the right place on the screen.

然后我通过MaskImageFull UIImage通过这个方法：

I then pass the MaskImageFull UIImage through this:

CGImageRef maskRef = [maskImage CGImage];
CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
    CGImageGetHeight(maskRef),
    CGImageGetBitsPerComponent(maskRef),
    CGImageGetBitsPerPixel(maskRef),
    CGImageGetBytesPerRow(maskRef),
    CGImageGetDataProvider(maskRef), NULL, false);

CGImageRef masked = CGImageCreateWithMask([image CGImage], mask);
UIImage* retImage= [UIImage imageWithCGImage:masked];

问题是retImage都是黑色的。如果我发送一个预制的UIImage作为面具，它的工作正常，只是当我尝试从它断裂的多个图像。

The problem is that the retImage is all black. If I send a pre-made UIImage in as the mask it works fine, it is just when I try to make it from multiple images that it breaks.

我想是一个色彩空间的东西，但似乎无法修复它。非常感谢任何帮助。

I thought it was a colorspace thing but couldn't seem to fix it. Any help is much appreciated!

推荐答案

我尝试了同样的事情与CGImageCreateWithMask，并得到相同的结果。我发现的解决方案是使用CGContextClipToMask改为：

I tried the same thing with CGImageCreateWithMask, and got the same result. The solution I found was to use CGContextClipToMask instead:

CGContextRef mainViewContentContext;
CGColorSpaceRef colorSpace;

colorSpace = CGColorSpaceCreateDeviceRGB();

// create a bitmap graphics context the size of the image
mainViewContentContext = CGBitmapContextCreate (NULL, targetSize.width, targetSize.height, 8, 0, colorSpace, kCGImageAlphaPremultipliedLast);

// free the rgb colorspace
CGColorSpaceRelease(colorSpace);    

if (mainViewContentContext==NULL)
    return NULL;

CGImageRef maskImage = [[UIImage imageNamed:@"mask.png"] CGImage];
CGContextClipToMask(mainViewContentContext, CGRectMake(0, 0, targetSize.width, targetSize.height), maskImage);
CGContextDrawImage(mainViewContentContext, CGRectMake(thumbnailPoint.x, thumbnailPoint.y, scaledWidth, scaledHeight), self.CGImage);


// Create CGImageRef of the main view bitmap content, and then
// release that bitmap context
CGImageRef mainViewContentBitmapContext = CGBitmapContextCreateImage(mainViewContentContext);
CGContextRelease(mainViewContentContext);

// convert the finished resized image to a UIImage 
UIImage *theImage = [UIImage imageWithCGImage:mainViewContentBitmapContext];
// image is retained by the property setting above, so we can 
// release the original
CGImageRelease(mainViewContentBitmapContext);

// return the image
return theImage;

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