How do I clip or change alpha of an image (pixels) in Quartz?

I'm working on making an iPhone App where there are two ImageViews and when you touch the top one, wherever you tapped, the bottom one shows instead.

Basically what I want to do is cut an ellipse/roundedrect out of an imag开发者_Go百科e. To do this I was thinking on either clipping the image, or changing the alpha pixels in the rect to zero. I am new to Quartz 2D Programming so I am not sure how to do this.

Assuming I have:

UIImageView *topImage;
UIImageView *bottomImage;

How do I delete a CGRect/Ellipse/RoundedRect from these images. This is kind of like those lottery tickets that you have to scratch off to reveal if you won.

I would generally try to make a mask from a path (here containing a rounded rectangle), then masking the image with it, as demonstrated in the apple docs. The one of the benefits of this is that for hit testing all you need to do is CGPathContainsPoint with the point that was touched (as in it will test whether it was in the visible area of the image).

I tried this code:

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGRect frame = CGRectMake(100, 100, 100, 100);
CGPathRef roundedRectPath = [self newPathForRoundedRect:frame radius:5];
CGContextAddPath(ctx, roundedRectPath);
CGContextClip (ctx);
CGPathRelease(roundedRectPath);

(Together with the rounded rect path function you sent) This is on a white view and beneath the view there is a gray Window, so I thought this would just show gray instead of white in CGRect frame but it didn't do anything...