How to determine image coordinates for a large cropped image within a UIImageView?

Question

With regards to the iPhone SDK:

I have a 512 x 512 pixel .png image, and I want to display it within a 200 x 200 pixel UIImageView WITH cropping (at native resolution, without zooming, fitting, or rotation). Is there a way to determine the image's coordinates that are visible inside the UIImageView?

For example, if the image in the exact center of the UIImageView, then I want to know what the coordinates of the image are at UIImageView's (0,0) and (200,200) points, which will be (156,156) and (356,356), respectively for the image being displayed.

I plan on implementing "pan" and "zoom" functions at some point, but I want to know how to address this issue first. The UIImageView class reference did not seem helpful... I was hoping to avoid using a UIView, but if that is necessary I will go that route.

Thanks!

Answer 1

You're right about what you've seen from the UIImageView reference - there is no easy way to do that using UIImageView alone. You would need to take into account things like the contentMode of the UIImageView or limit yourself to only calculating the position for a single UIViewContentMode.

However, when you eventually get to implementing pan and zoom, you'll be using a UIScrollView with a UIImageView as a subview of that UIScrollView. At that point, it would be most sane to size your UIImageView initially to the size of whatever image you're display (could be dynamic). You can then determine where the image is by determining where the image view is, which is something you can accomplish much more simply and with just a bit of math.

Answer 2

Thanks for your help. Working out the pan/zoom features became trivial after implementing the UIScrollView. Implementing the UIScrollView let me determine the coordinates I needed with the myScrollView.bounds.origin.x and myScrollView.bounds.origin.y values... zooming still requires calculation - as you stated above, but I am on the right track now.

here is the code I used to make the UIScrollView (based on multiple sources available on the net and this site - Apple's ScrollViewSuite was particularly helpful. Here is the code I used in the (void)viewDidLoad section of my program, hopefully others can find it useful:

UIImageView *tempImageView = [[UIImageView alloc] initWithImage:[imageArray objectAtIndex:0]]; //imageArray is an array/stack of images...
[self setMyImage:tempImageView]; //myImage is of type UIImageView
[myImage setTag:100]; //set this to an 'arbitrary' value so that when I update the scrollview I can remove the current image...
[tempImageView release];

myScrollView.contentSize = CGSizeMake(myImage.frame.size.width, myImage.frame.size.height);
myScrollView.maximumZoomScale = 10.0;
myScrollView.minimumZoomScale = 0.15;
myScrollView.clipsToBounds = YES;
myScrollView.delegate = self;

[myScrollView addSubview:myImage];