Tinting an image

Question

I have an image which is a black ring:

black ring http://imageshack.us/a/img51/2029/blackcb.png

In my view, I need to display it as a white ring. So, to tint the image, I have written the below method:

- (UIImage *)getRingImage
{
 UIImage *ringImage = [UIImage imageNamed:@"ring"];
 CGFloat scale = [[UIScreen mainScreen] scale];
 UIGraphicsBeginImageContext(CGSizeMake(ringImage.size.width * scale, ringImage.size.height * scale));
 CGContextRef ctx = UIGraphicsGetCurrentContext();
 CGRect rect = CGRectMake(0, 0, ringImage.size.width * scale, ringImage.size.height * scale);
 // Converting a UIImage to a CGImage flips the image,
 // so apply a upside-down translation
 CGContextScaleCTM(ctx, 1, -1);
 CGContextTranslateCTM(ctx, 0, -rect.size.height);
 // Set the fill color space
 CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
 CGContextSetFillColorSpace(ctx, colorSpace);
 // Set the mask to only tint non-transparent pixels
 CGContextClipToMask(ctx, rect, ringImage.CGImage);
 // Set the fill color
 CGContextSetFillColorWithColor(ctx, [UIColor colorWithRed:1.0f green:1.0f blue:1.0f alpha:1].CGColor);
 UIRectFillUsingBlendMode(rect, kCGBlendModeColor);
 ringImage = UIGraphicsGetImageFromCurrentImageContext();
 CGColorSpaceRelease(colorSpace);
 UIGraphicsEndImageContext();
 return ringImage;
}

This code works fine. I want to know if there is a better way to do this.

• Robert Harvey: What do you mean by "better?"
• Munish Poonia: By "better" I mean, use of a different API which could achieve the same result but in less number of steps which in turn could lead to reduced execution time and CPU cycles.
• Jano: I doubt it. Even when it took you 20 steps, the code is simple. If you are interested in other blend modes see robots.thoughtbot.com/post/46668544473/…

Answer 1

Try this function that I adapted. It creates a mono-color "mask" from an input image. If you use white, it turns every non-transparent pixel to white:

- (UIImage*)convertToMask: (UIImage *) image
{
 UIGraphicsBeginImageContextWithOptions(image.size, NO, image.scale);
 CGRect imageRect = CGRectMake(0.0f, 0.0f, image.size.width, image.size.height);
 CGContextRef ctx = UIGraphicsGetCurrentContext();
 // Draw a white background (for white mask)
 CGContextSetRGBFillColor(ctx, 1.0f, 1.0f, 1.0f, 1.0f);
 CGContextFillRect(ctx, imageRect);
 // Apply the source image's alpha
 [image drawInRect:imageRect blendMode:kCGBlendModeDestinationIn alpha:1.0f];
 UIImage* outImage = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();
 return outImage;
}

Used like this:

self.imageView.image = [self convertToMask:self.imageView.image];