How to create an image of specific size from UIView

I'm working on iOS app which should enable users to create Instagram stories photos and export them to Instagram. Basically app like Unfold, Stellar, Chroma Stories... I've prepared UI where user can select from prepared templates and add to them own photos with filters, labels etc.

My question is - what is the best way to export created UIView to bigger Image? I mean how to get the best quality, sharp pixels of labels etc? Because the template view with subviews (added photos, labels...) is taking +- half of device's screen. But I need bigger size for exported image. Currently I use:

 func makeImageFromView() -> UIImage {
    let format = UIGraphicsImageRendererFormat()
    let size = CGSize(width: 1080 / format.scale, height: 1920 / format.scale)

    let renderer = UIGraphicsImageRenderer(size: size, format: format)
    let image = renderer.image { (ctx) in
        templateView.drawHierarchy(in: CGRect(origin: .zero, size: size), afterScreenUpdates: true)
    }
    return image
}

The resulted image has size of 1080 x 1920, but labels aren't sharp. Do I need to scale somehow the photo and font size before making it to an image? Thanks!

So actually yes, before capturing image I need to scale whole view and it's subviews. Here are my findings (maybe obvious things but it took me a while to realize that – I'll be glad for any improvements)

Rendering image of the same size

When you want to capture UIView as an image, you can simply use this function. Resulted image will have a same size as a view (scaled 2x / 3x depending on actual device)

 func makeImageFrom(_ desiredView: MyView) -> UIImage {
    let size = CGSize(width: desiredView.bounds.width, height: desiredView.bounds.height)
    let renderer = UIGraphicsImageRenderer(size: size)
    let image = renderer.image { (ctx) in
        desiredView.drawHierarchy(in: CGRect(origin: .zero, size: size), afterScreenUpdates: true)
    }
    return image
}

Rendering image of the different size

But what to do, when you want a specific size for your exported image? So from my use-case I wanted to render image of final size (1080 x 1920), but a view I wanted to capture had a smaller size (in my case 275 x 487). If you do such a rendering without anything, there must be a loss in quality.

If you want to avoid that and preserve sharp labels and other subviews, you need to try to scale the view ideally to the desired size. In my case, make it from 275 x 487 to 1080 x 1920.

func makeImageFrom(_ desiredView: MyView) -> UIImage {
    let format = UIGraphicsImageRendererFormat()
    // We need to divide desired size with renderer scale, otherwise you get output size larger @2x or @3x
    let size = CGSize(width: 1080 / format.scale, height: 1920 / format.scale)

    let renderer = UIGraphicsImageRenderer(size: size, format: format)
    let image = renderer.image { (ctx) in
    // remake constraints or change size of desiredView to 1080 x 1920
    // handle it's subviews (update font size etc.)
    // ...
    desiredView.drawHierarchy(in: CGRect(origin: .zero, size: size), afterScreenUpdates: true)
    // undo the size changes
    // ...
    }
    return image
}

My approach

But because I didn't want to mess with a size of a view displayed to the user, I took a different way and used second view which isn't shown to the user. That means that just before I want to capture image, I prepare "duplicated" view with the same content but bigger size. I don't add it to the view controller's view hierarchy, so it's not visible.

Important note!

You really need to take care of subviews. That means, that you have to increase the font size, update position of moved subviews (for example their center) etc.! Here is just a few lines to illustrate that:

        // 1. Create bigger view
        let hdView = MyView()
        hdView.frame = CGRect(x: 0, y: 0, width: 1080, height: 1920)

        // 2. Load content according to the original view (desiredView)
        // set text, images...

        // 3. Scale subviews
        // Find out what scale we need
        let scaleMultiplier: CGFloat = 1080 / desiredView.bounds.width // 1080 / 275 = 3.927 ...

        // Scale everything, for examples label's font size
        [label1, label2].forEach { $0.font =  UIFont.systemFont(ofSize: $0.font.pointSize * scaleMultiplier, weight: .bold) }
        // or subview's center
        subview.center = subview.center.applying(.init(scaleX: scaleMultiplier, y: scaleMultiplier))

        // 4. Render image from hdView
        let hdImage = makeImageFrom(hdView)

Difference in quality from real usage – zoomed to the label: