How to limit iPhone tap gesture recognition to inside a circular image?

I'm trying to place a circular image on an iPhone view and 开发者_JAVA百科then consume taps inside the circle but not outside of it. The problem I'm running into is that when I place a UIImageView on the screen view in Interface Builder, I seem to be constrained to a rectangular shape. I tried using an image of a circle with the area outside the circle left as transparent but the image in total is still rectangular, so when it's placed on a UIImageView and hooked up to recognize tap gestures it still picks up the taps outside the circle itself.

The image below shows what I mean. The blue dots represent the outer border of the UIImageView that holds the image. The tap gesture recognition is currently linked to that UIImageView but as you can see, there is a bit of space at the corners of the UIImageView that are not covered by the circular image. Is there any way to either conform a UIImageView to a non-rectangular shape or to place an image on a View without using a UIImageView and still be able to hook up tap recognition?

image of walter with transparent background on a UIImageView http://img237.imageshack.us/img237/2164/walterheadshot.png

I'm pretty new to iPhone graphics but does anyone have some ideas on this or can point me in the right direction?

Thanks!

---

Assumes a circle, and not just an oval:

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
    CGFloat halfSize = [self bounds].size.width * 0.5f;
    CGPoint location = [[touches anyObject] locationInView:self];
    location.x -= halfSize;
    location.y -= halfSize;
    CGFloat squaredDistanceFromCenter = location.x * location.x + location.y + location.y;
    if (squaredDistanceFromCenter < (halfSize * halfSize)) {
        NSLog(@"Within circle :)");
    } else {
        NSLog(@"Not within circle :(");
    }
}