Creating mask with CGImageMaskCreate is all black (iphone)

Question

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

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();

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

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];

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.

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

Answer 1

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;

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

The image to be masked MUST be created with an alpha channel. The Alpha channel may not be created from the code.