UIImage size doubled when converted to CGImage?

Question

I am trying to crop part of an image taken with the iPhone's camera via the cropping(to:) method on a CGImage but I am encountering a weird phenomenon where my UIImage's dimensions are doubled when converted with .cgImage which, obviously, prevents me from doing what I want.

The flow is:

1. Picture is taken with the camera and goes into a full-screen imageContainerView
2. A "screenshot" of this imageContainerView is made with a UIView extension, effectively resizing the image to the container's dimensions
3. imageContainerView's .image is set to now be the "screenshot"

---

let croppedImage = imageContainerView.renderToImage()
imageContainerView.image = croppedImage
print(imageContainerView.image!.size) //yields (320.0, 568.0)
print(imageContainerView.image!.cgImage!.width, imageContainerView.image!.cgImage!.height) //yields (640, 1136) ??

extension UIView {

    func renderToImage(afterScreenUpdates: Bool = false) -> UIImage {

        let rendererFormat = UIGraphicsImageRendererFormat.default()
        rendererFormat.opaque = isOpaque
        let renderer = UIGraphicsImageRenderer(size: bounds.size, format: rendererFormat)

        let snapshotImage = renderer.image { _ in
            drawHierarchy(in: bounds, afterScreenUpdates: afterScreenUpdates)
        }
        return snapshotImage
    }
}

I have been wandering around here with no success so far and would gladly appreciate a pointer or a suggestion on how/why the image size is suddenly doubled.

Thanks in advance.

Answer 1

This is because print(imageContainerView.image!.size) prints the size of the image object in points and print(imageContainerView.image!.cgImage!.width, imageContainerView.image!.cgImage!.height) print the size of the actual image in pixels.

On iPhone you are using there are 2 pixels for evert point in both horizontal and vertical. The UIImage scale property will give you the factor which in your case will be 2.

See this link iPhone Resolutions