IOS: Masking an image keeping retina scale factor in account

6
votes

I want to mask an image by passing another image as mask. I am able to mask the image but the resulting image doesn't look good. It is jagged at borders.

I guess the problem is related to retina graphics. The scale property for the two images are different as:

  1. The image from which I want to mask has a scale value 1. This image generally has a resolution greater than 1000x1000 pixels.
  2. The image according to which I want the resulting image(image having black and white colors only) has scale value 2. This image is generally of resolution 300x300 pixels.

The resulting image has a scale value of 1.

The code I am using is:

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

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);
CGImageRelease(mask);
UIImage *maskedImage = [UIImage imageWithCGImage:masked ];
CGImageRelease(masked);
return maskedImage;
}

How can I get a masked image which follows retina scale?

2
iosuiimagecore-graphicscgimage
You should consider validate an answer if it helped you. – kokluch
I asked this question on July 3rd, 2013 and first answer was on Jan 8. Between that time I have moved to different projects and I haven't checked if the answer helped or not and if I am not sure then how can I accept any answer. I just want to close the question but don't know how to do that. – Gaurav Singh
Not that hard to find... – kokluch

2 Answers

5
votes

You can do 

UIImage * maskedImage = [UIImage imageWithCGImage:masked scale:[[UIScreen mainScreen] scale] orientation:UIImageOrientationUp];
5
votes

I had the same issue. It appears that this line ignore scale factor.

UIImage *maskedImage = [UIImage imageWithCGImage:masked];

So you should draw the image by yourself. Replace it by the following :

UIGraphicsBeginImageContextWithOptions(image.size, NO, 0.0);

CGContextRef context = UIGraphicsGetCurrentContext();

CGRect rect = CGRectMake(0, 0, image.size.width, image.size.height);

CGContextDrawImage(context, rect, masked);

UIImage * maskedImage = UIGraphicsGetImageFromCurrentImageContext();

UIGraphicsEndImageContext();

Works fine by me.

EDIT

OR

UIImage * maskedImage = [UIImage imageWithCGImage:masked 
                                            scale:[[UIScreen mainScreen] scale] 
                                      orientation:UIImageOrientationUp];