CGImageCreateWithMaskingColors Doesn't Work with iOS7

Question

I've developed an app on iOS5 and iOS6. After I upgraded to XCode 5 and iOS7, I have some new bugs to play with.

The main one is the colorMasking no longer works. The exact same code still compiles and works on a phone with iOS6. On iOS7, the masked color is still there. I tried to find the answer on Google, but haven't found an answer. Is it a bug of iOS7, or does anybody know of a better way of doing colormasking?

Here is the code:

- (UIImage*) processImage :(UIImage*) image
{
    UIImage *inputImage = [UIImage imageWithData:UIImageJPEGRepresentation(image, 1.0)];
    const float colorMasking[6]={100.0, 255.0, 0.0, 100.0, 100.0, 255.0};
    CGImageRef imageRef = CGImageCreateWithMaskingColors(inputImage.CGImage, colorMasking);
    UIImage* finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    return finalImage;
}

Here are a couple StackOverflow posts I found that helped me get it working in iOS6 the first place: Transparency iOS iOS color to transparent in UIImage

Answer 1

I have stumbled across some strange behavior of CGImageCreateWithMaskingColors in conjunction with UIImagePNGRepresentation. This may or may not be related to your problem. I have found that if:

1. If use CGImageCreateWithMaskingColors and immediately add that image to an image view, I can see that the transparency appears to have been applied correctly;

2. But in iOS 7, if I then:

   • take this image from CGImageCreateWithMaskingColors and create a NSData using UIImagePNGRepresentation; and

   • if reload the image from that NSData using imageWithData, then the resulting image will no longer have its transparency.

   To confirm this, if I writeToFile for this NSData and examine the saved image in a tool like Photoshop, I can confirm that the file does not have any transparency applied.

   This only manifests itself in iOS 7. In iOS 6 it's fine.

3. But if I take the image in step 1 and roundtrip it through drawInRect, the same process of saving the image and subsequently loading it works fine.

This following code illustrates the issue:

- (UIImage*) processImage :(UIImage*) inputImage
{
    const float colorMasking[6] = {255.0, 255.0, 255.0, 255.0, 255.0, 255.0};
    CGImageRef imageRef = CGImageCreateWithMaskingColors(inputImage.CGImage, colorMasking);
    UIImage* finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);

    // If I put this image in an image view, I see the transparency fine.

    self.imageView.image = finalImage;                           // this works

    // But if I save it to disk and the file does _not_ have any transparency

    NSString *documentsPath           = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
    NSString *pathWithoutTransparency = [documentsPath stringByAppendingPathComponent:@"image-but-no-transparency.png"];
    NSData   *data                    = UIImagePNGRepresentation(finalImage);
    [data writeToFile:pathWithoutTransparency atomically:YES];   // save it so I can check out the file in Photoshop

    // In iOS 7, the following imageview does not honor the transparency

    self.imageView2.image = [UIImage imageWithData:data];        // this does not work in iOS 7

    // but, if I round-trip the original image through `drawInRect` one final time,
    // the transparency works

    UIGraphicsBeginImageContextWithOptions(finalImage.size, NO, 1.0);
    [finalImage drawInRect:CGRectMake(0, 0, finalImage.size.width, finalImage.size.height)];
    UIImage *anotherRendition = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    data = UIImagePNGRepresentation(anotherRendition);
    NSString *pathWithTransparency = [documentsPath stringByAppendingPathComponent:@"image-with-transparancy.png"];
    [data writeToFile:pathWithTransparency atomically:YES];

    // But this image is fine

    self.imageView3.image = [UIImage imageWithContentsOfFile:pathWithTransparency];   // this does work

    return anotherRendition;
}

Answer 2

I was loading a JPEG which for some reason loads with an alpha channel, which won't work when masking, so here I recreate the CGImage ignoring the alpha channel. There may be a better way of doing this but this works!

- (UIImage *)imageWithChromaKeyMasking {
    const CGFloat colorMasking[6]={255.0,255.0,255.0,255.0,255.0,255.0};
    CGImageRef oldImage = self.CGImage;
    CGBitmapInfo oldInfo = CGImageGetBitmapInfo(oldImage);
    CGBitmapInfo newInfo = (oldInfo & (UINT32_MAX ^ kCGBitmapAlphaInfoMask)) | kCGImageAlphaNoneSkipLast;
    CGDataProviderRef provider = CGImageGetDataProvider(oldImage);
    CGImageRef newImage = CGImageCreate(self.size.width, self.size.height, CGImageGetBitsPerComponent(oldImage), CGImageGetBitsPerPixel(oldImage), CGImageGetBytesPerRow(oldImage), CGImageGetColorSpace(oldImage), newInfo, provider, NULL, false, kCGRenderingIntentDefault);
    CGDataProviderRelease(provider); provider = NULL;
    CGImageRef im = CGImageCreateWithMaskingColors(newImage, colorMasking);
    UIImage *ret = [UIImage imageWithCGImage:im];
    CGImageRelease(im);
    return ret;
}