Getting the right coordinates of the visible part

I have a UIImage which is inside a UIScrollView so I can zoom-in and zoom-out, crop, move around, etc.

How do I get the coordinates of the visible part of the UIImage inside the UIScrollView. I want to crop the image in it's native resolution (using GPUIImage) but I need to x, y, width and height for the rectangle.

I use a scrollview to enable zoom of a UIImage. A cropping button presents an overlaid resizable cropping view. The following is the code I use to ensure the user defined crop box in the UIScrollView gets added with the correct coordinates to the UIImageView (can then use to crop image).

To find the x and y coordinates use the scrollview contentOffset multiplied by inverse of zoomscale:

float origX = 0;
float origY = 0;
float widthCropper = 0;
float heightCropper = 0;


if (_scrollView.contentOffset.x > 0){
    origX = (_scrollView.contentOffset.x) * (1/_scrollView.zoomScale);
}
if (_scrollView.contentOffset.y > 0){
    origY = (_scrollView.contentOffset.y) * (1/_scrollView.zoomScale);
}

If you need to create a properly sized cropping box for what is displayed in the scrollview you will need to adjust the width and the height for any zoom factor:

widthCropper = (_scrollView.frame.size.width * (1/_scrollView.zoomScale));
heightCropper = (_scrollView.frame.size.height * (1/_scrollView.zoomScale));

and to add this properly sized rectangle as a cropping view to the UIImageView:

CGRect cropRect = CGRectMake((origX + (SIDE_MARGIN/2)), (origY + (SIDE_MARGIN / 2)), (widthCropper - SIDE_MARGIN), (heightCropper  - SIDE_MARGIN));
_cropView = [[UIView alloc]initWithFrame:cropRect];
[_cropView setBackgroundColor:[UIColor grayColor]];
[_cropView setAlpha:0.7];
[_imageView addSubview:_cropView];
[_imageView setCropView:_cropView];