How to Mask an Image

Masking an image enables a developer to create images with irregular shapes dynamically. Masking is often used to create a user interface that is more compelling and less boring.
Take for example the following example …

Creating the mask above is really simple using CoreGraphics on the iPhone. The following is a function that takes two images and uses one to mask the other.

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- (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];   }

Yep … its just that simple!
NOTE: The mask image cannot have ANY transparency. Instead, transparent areas must be white or some value between black and white. The more towards black a pixel is the less transparent it becomes.

http://mobiledevelopertips.com/cocoa/how-to-mask-an-image.html