使用CGImageMaskCreate创建蒙版是全黑的(iphone)

Question

我正在尝试创建一个由两个现有图像合成的图像蒙版。

首先，我开始创建合成图像，它由一个小图像和一个与背景相同大小的图像组成：

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)，它看起来是正确的。这是一个完整大小的背景尺寸，它有一个白色的背景，我的蒙版对象是黑色的，在屏幕上的正确位置。

然后，我通过下面的代码传递MaskImageFull UIImage：

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全黑了。如果我发送一个预先制作的UIImage作为遮罩，它工作得很好，只是当我试图从多个图像中制作它时，它被打破了。

我认为这是一个色彩空间的东西，但似乎无法解决它。任何帮助都是非常感谢的！

Answer 1

我用CGImageCreateWithMask做了同样的事情，得到了同样的结果。我找到的解决方案是使用CGContextClipToMask：

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;

Answer 2

- (UIImage *) maskImage:(UIImage *)image {
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    UIImage *maskImage = [UIImage imageNamed:@"MaskFinal.png"];
    CGImageRef maskImageRef = [maskImage CGImage];
    
    // create a bitmap graphics context the size of the image
    CGContextRef mainViewContentContext = CGBitmapContextCreate (NULL, maskImage.size.width, maskImage.size.height, 8, 0, colorSpace, kCGImageAlphaPremultipliedLast);
    
    
    if (mainViewContentContext==NULL)
        return NULL;
    
    CGFloat ratio = 0;
    
    ratio = maskImage.size.width/ image.size.width;
    
    if(ratio * image.size.height < maskImage.size.height) {
        ratio = maskImage.size.height/ image.size.height;
    } 
    
    CGRect rect1 = {{0, 0}, {maskImage.size.width, maskImage.size.height}};
    CGRect rect2 = {{-((image.size.width*ratio)-maskImage.size.width)/2 , -((image.size.height*ratio)-maskImage.size.height)/2}, {image.size.width*ratio, image.size.height*ratio}};
    
    
    CGContextClipToMask(mainViewContentContext, rect1, maskImageRef);
    CGContextDrawImage(mainViewContentContext, rect2, image.CGImage);
    
    
    // Create CGImageRef of the main view bitmap content, and then
    // release that bitmap context
    CGImageRef newImage = CGBitmapContextCreateImage(mainViewContentContext);
    CGContextRelease(mainViewContentContext);
    
    UIImage *theImage = [UIImage imageWithCGImage:newImage];
    
    CGImageRelease(newImage);
    
    // return the image
    return theImage;
}

Answer 3

必须使用alpha通道创建要被遮罩的图像。Alpha通道可能不是从代码创建的。