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 t

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.