How to show a zoomed part of an image in a predefined rectangle?

I'm trying to develop a Photo App in which when the user swipes an image, a particular part (a predefined rectangular region) of the image zooms in and scales to fit the screen's dimensions and on subsequent swipes other (predefined) parts of the image get zoomed in in a similar way.

And also the transitions to the other parts of the image should be seamless.

I'm not quite familiar with Quartz Core framework. Where should I start?

EDIT:

To aspect fit the zoomed part of the image, I did this:

if (recognizer.direction == UISwipeGestureRecognizerDirectionLeft) {
    [UIView animateWithDuration:1.0 animations:^{
        self.myScrollView.frame = CGRectMake((self.view.bounds.size.width - zoomWidth) / 2, (self.view.bounds.size.height - zoomHeight) / 2, zoomWidth, zoomHeight);
        [self.myScrollView zoomToRect:CGRectMake(10, 10, zoomWidth, zoomHeight) animated:NO];
    }];
}

Solution

I don't think you might need to use Quartz Core(which is used for custom drawing). Two ways to achieve your case come to mind. They will be more of hints since we do not know much details about the case/architecture.

UIScrollView can help you zoom in/out to the image and you can "lock" the zoom to the new region with contentSize, contentOffset, zoomScale and so on properties.
CALayer's contentsRect property can be useful since it uses only a portion of the image you supply as the layer's backing image constrained by the provided CGRect