gistfile1.m

// call when zoom level or page size changes (i.e. after zooming or after rotation)
- (void)updateContentInsetForPageScrollView:(UIScrollView *)pageScrollView {
	UIImageView *imageView = (UIImageView *) [pageScrollView viewWithTag:TAG_IMAGE_VIEW];
	CGFloat zoomScale = pageScrollView.zoomScale;
	
	CGSize imageSize = imageView.bounds.size;
	CGSize zoomedImageSize = CGSizeMake(imageSize.width * zoomScale, imageSize.height * zoomScale);
	CGSize pageSize = pageScrollView.bounds.size;
	
	UIEdgeInsets inset = UIEdgeInsetsZero;
	if (pageSize.width > zoomedImageSize.width) {
		inset.left = (pageSize.width - zoomedImageSize.width) / 2;
		inset.right = -inset.left;
	}
	if (pageSize.height > zoomedImageSize.height) {
		inset.top = (pageSize.height - zoomedImageSize.height) / 2;
		inset.bottom = -inset.top;
	}
	pageScrollView.contentInset = inset;
}

-(void)scrollViewDidZoom:(UIScrollView *)pageScrollView {
	[self updateContentInsetForPageScrollView:pageScrollView];
}

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation {
	// loop through all pages, adjusting their sizes
	// and calling updateContentInsetForPageScrollView for each
}