Calculate position in image after ScaleAspectFill in UIImageView

Question

I'm loading images of various sizes inside an UIImageView. Some one the images come with annotations, each annotation has a unique position in the image. The annotation coordinates are relative to the original image size.

How do I transform the coordinates so the annotation show up at the same position in the UIImageView after resizing?

I could use AVMakeRectWithAspectRatioInsideRect to calculate the new position, but this only works with ScaleAspectFit and not ScaleAspectFill which I must use.

The UIImageView is scaled to fit all of the screen using AutoLayout / Storyboard — petard, Aug 18 '17 at 17:08

score 3 · Answer 1 · answered Aug 18 '17 at 19:18

You need to:

calculate the aspectRatio for aspectFill
multiply the x,y coords for your annotation by the aspectRatio
adjust that point by the amount of the image that extends outside the view frame

This is one way to do it (a little verbose, for clarity):

func aspectFillPoint(for point: CGPoint, in view: UIImageView) -> CGPoint {

    guard let img = view.image else {
        return CGPoint.zero
    }

    // imgSize will be modified
    var imgSize = img.size

    let viewSize = view.frame.size

    let aspectWidth  = viewSize.width / imgSize.width
    let aspectHeight = viewSize.height / imgSize.height

    // calculate aspectFill ratio
    let f = max(aspectWidth, aspectHeight)

    // scale imgSize
    imgSize.width *= f
    imgSize.height *= f

    // unless aspect ratio of view is the same as image,
    // it will either extend above and below or left and right
    // of the view frame
    let xOffset = (viewSize.width - imgSize.width) / 2.0
    let yOffset = (viewSize.height - imgSize.height) / 2.0

    // scale the original point, and adjust for offsets
    return CGPoint(
        x: (point.x * f) + xOffset,
        y: (point.y * f) + yOffset
        )

}

Assuming you have an image assigned to theImageView, which is set to .scaleAspectFill, you can call it like this:

let annotationPoint = CGPoint(x: 232, y: 148)

var newPoint = aspectFillPoint(for: annotationPoint, in: theImageView)

// Note: newPoint is relative to the View Bounds, so unless the
// imageView is at 0,0 we need to adjust for position
newPoint.x += theImageView.frame.origin.x
newPoint.y += theImageView.frame.origin.y

thanks a lot! don't you need to account for the difference between original image size and destination uiimageview? like in this [post](https://stackoverflow.com/questions/43720720/how-to-crop-a-uiimageview-to-a-new-uiimage-in-aspect-fill-mode) — petard, Aug 21 '17 at 14:53
"Accounting for the difference" is exactly what this code is doing. Put it into a project - or a storyboard - and see for yourself. — DonMag, Aug 21 '17 at 15:21

Calculate position in image after ScaleAspectFill in UIImageView

1 Answers1