How can I record an ARKit scene but exclude UI elements?

I'm using ARKit with Scenekit for rendering. I'd like to let users capture videos of the AR session so that they can save it to their photos or share it.

Currently I'm using ARVideoKit for this, but the performance leaves something to be desired and I've run into some difficult to workaround bugs. Other libraries I've found haven't been any better.

ReplayKit seems like ideal solution but it records my entire app, including the user interface. Is there a way to get ReplayKit to record just the AR content while excluding the user interface?

You can use ReplayKit for this but it isn't very well documented. The key is that you render all of your UI elements in a separate UIWindow that is overlaid on top of a primary UIWindow that contains the AR content. ReplayKit only records the primary window, so with this structure the user interface elements will not show up in the recording.

While there may be a better way to do this, here's an example of how I setup this window structure for my SwiftUI based app. Here I use the UIWindow.level property to mark the AR content as the main window, while putting the UI into its own secondary window at a higher level:

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var arWindow: UIWindow?    
    var uiWindow: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options 
        if let windowScene = scene as? UIWindowScene {
            // Create a window for the AR content.
            // This is the main window. 
            let arWindow = UIWindow(windowScene: windowScene)
            self.arWindow = arWindow
            arWindow.windowLevel = .normal
            
            // Add your AR view controller here or set the view controller lazily
            // when you actually need to show AR content
            let vc = UIViewController() 
            arWindow.rootViewController = vc
            

            // Now create a window for the UI
            let uiWindow = UIWindow(windowScene: windowScene)
            self.uiWindow = uiWindow
            // Setting the level makes this window's content be excluded from replaykit
            uiWindow.windowLevel = UIWindow.Level(UIWindow.Level.normal.rawValue + 1)
            uiWindow.isOpaque = false
            
            // Render your SwiftUI based user interface
            let content = MyUserInterfaceView()
                .background(Color.clear)
            
            let hosting = UIHostingController(rootView: content)
            hosting.view.backgroundColor = .clear
            hosting.view.isOpaque = false
            uiWindow.rootViewController = hosting
            
            uiWindow.makeKeyAndVisible()
        }
    }
}

My app initializes the AR content lazily, so I simply update arWindow.viewController when I need to show it.

A few notes:

• This approach requires that you separate your AR view controller from the rest of your user interface. This is a simple change in many cases but could be quite involved for a more complex app.

• Keep in mind that any user event handlers and gesture recognizers you have on your AR view controller may no longer work as expected after you split the AR content and layout into their own windows.

  I work around this by having a transparent proxy view controller in my main layout that forwards user events to the real AR view controller

• This approach isn't ARKit specific. It should also work for normal SceneKit apps, Metal apps, and conventional apps.

• Unlike some third party recording libraries, ReplayKit prompts the user when recording the screen.

  I actually like this feature as it makes it easy for the user to pick if they want the microphone recorded or not. You may find it gets in the way though, so there is still a use case for third party recording libraries.

• ReplayKit also provides a nice built-in user interface for trimming and saving the recorded video.