Simple CIFilter Passthru with CGImage conversion returns black pixels - ios

Question

The following code:
let skView = SKView()
let scene = SKScene()
override func viewDidLoad() {
super.viewDidLoad()
self.scene.scaleMode = .resizeFill
self.skView.presentScene(self.scene)
self.scene.backgroundColor = UIColor.black
self.view.addSubview(skView)
self.scene.shouldEnableEffects = true
let sprite = SKSpriteNode(imageNamed: "NAME_THAT_PIC")
sprite.position = CGPoint(x: 300, y: 400)
let effectNode = SKEffectNode()
effectNode.filter = MyFilter()
effectNode.addChild(sprite)
will call this custom filter that does nothing but create a CGImage from a CIImage, correctly invoking context.createCGImage() as reported by many people (CIImages are not pixel buffered.)
MyFilter is reduced to a simple repro test:
class MyFilter: CIFilter {
var inputImage: CIImage?
var inputImageRect: CGRect? {
guard let image = self.inputImage else {
return nil
}
return image.extent
}
public override init() {
super.init()
}
required public init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
override open var outputImage: CIImage? {
guard let inputImage = self.inputImage else {
return nil
}
let context = CIContext(options:nil)
let cgImage = context.createCGImage(inputImage, from: inputImageRect!)
// ... DO SOMETHING WITH CGIMAGE DATA ...
return CIImage(cgImage: cgImage!)
}
}
If I replace MyFilter() by another built-in filter, it works and will show the altered image so the viewcontroller code works. If instead, I return inputImage directly from the filter output call, it works and the image passed in will display.
When I dump the CGImage, the dimensions of the image are correct but every pixels are set to black.
I tried creating a UIImage using UIImage(cgImage: cgImage!) but the same happens.
What is causing pixels not to be loaded in the cgImage I generated from the inputImage?