Problem when masking UIImage

Question

I took a sample code from the following URL: how to mask an image in order to mask an image.

The code is working perfectly on the iPhone simulator but works incorrectly on iPhone 4 simulator (that's when high-res images are loaded...)

Here is my code and the mask function:

- (void)someMethod {

    UIImage* image = [UIImage imageNamed:@"image.png"];  // [email protected] is loaded for high-res device

    UIImage* mask = [UIImage imageNamed:@"mask.png"];  // [email protected] is loaded for high-res device

    UIImage* maskedImage = [self maskImage:image withMask:mask];

    // ... Some code here displaying maskedImage

}

- (UIImage *)maskImage:(UIImage *)image withMask:(UIImage *)maskImage {

    UIImage* maskedImage = nil;

    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);
    maskedImage = [UIImage imageWithCGImage:masked];

    return maskedImage;
}

On the iPhone 4 simulator the [email protected] and [email protected] are loaded and then the resulted maskedImage is cropped.

Any idea what am I doing wrong?

Answer 1

My guess would be to not trust UIImage to deliver the highrez format to CGImageRefs etc. I think you will need to do it manually.

Answer 2

Use this method instead of above

• (UIImage*) maskImage:(UIImage *)image withMask:(UIImage *)maskImage {

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

  //UIImage *maskImage = [UIImage imageNamed:@"mask.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

@ganesh, Hi, i i have masked a colored uiimage (a normal color image from photos album), with its image negative version. thanks to your chunk of code dude.

The result is, i have a negativeImage with the white areas of the negative image filled with colors from the Colored version.

now i wanted to achieve an effect that is just the opposite of what i have now. i.e. I wanted my colors to fill in the black areas of the negative image.

I went through apple developer docs, Bitmap Image Masking with MaskImage and i couldnt help myself achieve this.

Could someone help me ?? please...