Recreate CGAffineTransform of UIImageView inside a CGContext - swift

Question

I have a transformed UIImage inside a UIVIew with clipsToBounds = true and I want to recreate (redraw) it exactly using a CGContext. It's hard to explain, here is the code:
class ViewController: UIViewController {
#IBOutlet weak var topView: UIView!
#IBOutlet weak var topImageView: UIImageView!
#IBOutlet weak var bottomImageView: UIImageView!
override func viewDidLoad() {
super.viewDidLoad()
// the example image
let image = UIImage(named: "image")!
// topImageView is embedded inside topView so that the image is being clipped
topView.clipsToBounds = true
topImageView.image = image
topImageView.contentMode = .center // I set this to center because it seems to be easier to draw it then
// The transformation data
let size = CGSize(width: 800, height: 200)
let scale: CGFloat = 1.5
let offset: (x: CGFloat, y: CGFloat) = (x: 0.0, y: 0.0)
// transform the imageView
topImageView.transform = CGAffineTransform(translationX: offset.x, y: offset.y).scaledBy(x: scale, y: scale)
// Now try to recreate the transform by using a transformed CGContext
UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
let context = UIGraphicsGetCurrentContext()!
// transform the context
// Almost the same as above, but I additionally move the context so that the middle of the image is in the middle of the size defined above
context.concatenate(CGAffineTransform(translationX: (-image.size.width / 2 + size.width / 2) + offset.x, y: (-image.size.height / 2 + size.height / 2) + offset.y).scaledBy(x: scale, y: scale))
image.draw(in: CGRect(x: 0, y: 0, width: image.size.width, height: image.size.height))
bottomImageView.image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
}
}
This is working only when I have scale set to 1.0 (so no scale). Otherwise, I don't get the correct image. Here's a screenshot so that you see it:
It's not the same. It's hard to figure out how to correctly transform the context, I don't entirely understand it. Any ideas?
Thanks!

Answer 1

It's a mystery. I've literally spent hours trying to get this right. 5 minutes after posting this question, I solved it. It's actually not that hard if you draw it (I used photoshop to visualize the coordinate system and played around a little bit. Should have done this earlier..)
UIGraphicsBeginImageContextWithOptions(size, false, UIScreen.main.scale)
let context = UIGraphicsGetCurrentContext()!
context.concatenate(CGAffineTransform.identity.scaledBy(x: scale, y: scale).translatedBy(x: size.width / (2 * scale), y: size.height / (2 * scale)).translatedBy(x: offset.x / scale, y: offset.y / scale))
image.draw(in: CGRect(x: -image.size.width / 2, y: -image.size.height / 2, width: image.size.width, height: image.size.height))
bottomImageView.image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()