Is this kind of masking possible with UIImage or CGImage API in iOS - iphone

Question

I have an UIImage with some text and would like to apply pattern UIImage as masking. Is this possible ?
I understand that with UILabel we can get this kind of gradient using CAGradientLayer. But can this be done if the source is an UIImage ?
The image may have some symbols/pictures etc other than regular characters and hence UIImage. Also i could reuse the image by applying different masking pattern depending on the context.
is this possible ?
Appreciate your help.
EDIT: Thanks for all your answers.
I understand applying the gradient to a text label or creating an image that has text.
But my goal is to get this.--> Click here
i.e. i have a png with some drawing like a flower with transparent background. I want to apply the gradient to the object inside that picture at runtime with a gradient.png as shown in the picture above. Is that possible with masking ?
Thanks

Answer 1

Looks like you should be able to use CGImageMaskCreate:
- (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);
return [UIImage imageWithCGImage:masked];
}
For a longer discussion check out the comment thread here:
http://iosdevelopertips.com/cocoa/how-to-mask-an-image.html

Answer 2

Yes, it is :)
textField.textColor = [UIColor colorWithPatternImage: [UIImage imageNamed:#"rainbowGradient.png"]];
If you want heavy control, Jason Whyne's idea might work. But I like this one, because it's about 8 lines shorter.

Answer 3

Here's just another way to draw an image of text masking something. It's based on kCGBlendModeSourceIn blending mode: you draw text on a clear background and then draw the fill all over the place.
NSString *theString = ...;
UIFont *theFont = ...;
CGSize stringSize = [theString sizeWithFont:theFont];
// The background must be clear (fully transparent), hence NO as the 2nd argument
UIGraphicsBeginImageContextWithOptions(stringSize, NO, 0);
[theString drawAtPoint:CGPointZero withFont:theFont];
// This effectively colorizes the image. Use a pattern color...
[patternColor set];
UIRectFillUsingBlendMode(CGRectMake(0, 0, stringSize.width, stringSize.height), kCGBlendModeSourceIn);
// ... or an image:
[patternImage drawInRect:CGRectMake(0, 0, stringSize.width, stringSize.height) blendMode:kCGBlendModeSourceIn alpha:1.0f];
UIImage *resultingImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Answer 4

sample using mask is fine and works well, but You are leaking.
CGImageMaskCreate and CGImageCreateWithMask do allocate (following "create" -> retain rule)
so You should release mask & image after using:
CGImageRef masked = CGImageCreateWithMask([image CGImage], mask);
UIImage * result = [UIImage imageWithCGImage:masked];
CGImageRelease(mask);
CGImageRelease(masked);
return result;
As per ADC docs:
...
Return Value
A Quartz bitmap image mask. You are responsible for releasing this object by calling CGImageRelease.