CGImageCreateWithMask works great but the masked out area is black in my resulting image, how can I set it to be white?

Question

I've masked out my image thusly:

    CGImageRef maskRef = [[UIImage imageNamed:@"testMask2.png"] CGImage];

CGImageRef mask = CGImageMaskCreate(CGImageGetWidth(maskRef),
                                    CGImageGetHeight(maskRef),
                                    CGImageGetBitsPerComponent(maskRef),
                                    CGImageGetBitsPerPixel(maskRef),
                                    CGImageGetBytesPerRow(maskRef),
                                    CGImageGetDataProvider(maskRef), nil, YES);

UIImage *image = [UIImage imageWithContentsOfFile:path];

CGImageRef masked = CGImageCreateWithMask([image CGImage], mask);

imageView.image = [UIImage imageWithCGImage:masked];

And it works great, but the resulting image has BLACK where it was masked out, how can I set it to have WHITE where its masked out?

Answer 1

If you are masking JPEG image which does not have alpha channel this will happen (black background instead of transparent).

So you need to do something like this before masking:

    CGImageRef imageNoAlpha = [UIImage imageNamed:@"noAlphaImage.jpg"].CGImage;

CGColorSpaceRef cs = CGColorSpaceCreateDeviceRGB();

CGFloat width = CGImageGetWidth(imageNoAlpha);
CGFloat height = CGImageGetHeight(imageNoAlpha);

CGContextRef ctxWithAlpha = CGBitmapContextCreate(nil, width, height, 8, 4*width, cs, kCGImageAlphaPremultipliedFirst);

CGContextDrawImage(ctxWithAlpha, CGRectMake(0, 0, width, height), imageNoAlpha);

CGImageRef imageWithAlpha = CGBitmapContextCreateImage(ctxWithAlpha);

CGImageRef masked = CGImageCreateWithMask(imageWithAlpha, mask);

...

Be sure to release created images, context and colorspace ...