Save and align ARKit created and rendered geometry later in SceneKit

I am exploring ARKit and SceneKit, but I am not sure if what I want to do is possible.

In App1:

  1. Run an ARKit session using configuration.sceneReconstruction = .mesh
  2. I am rending the mesh in func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) and func renderer(_ renderer: SCNSceneRenderer, didUpdate node: SCNNode, for anchor: ARAnchor) and the mesh appears correct
  3. I have set up a button that does the following:
  • Capture the mesh to an .obj file (based off the excellent answer in here)
  • Capture a snapshot (SCNView snapshot)
  • Store the AR Camera Transform
  • Store the AR SCN View Point Of View Projection Transform

Storing the transforms in a CSV, and taking care to ensure I restore them in column major order.

In App2:

  1. Load the geometry from the .obj file into SCNNode, do not apply any transform to it, apply a wireframe to it so I can visualise it
  2. Add a camera and apply the 2 saved transforms (AR Camera Transform and then AR SCN View Point Of View Projection Transform Set the background of the scene to be the image from the snapshot.

I was expecting that the mesh, as visualised by the wireframe in App2 would match the rendering as captured in a point in time from App1 - however I cannot get it to match.

I have two specific questions:

  • Have I missed something fundamental in my understanding of what is possible? Should this be possible and could I be missing some step? The codebase is large but I have put the basic outline here
  • What is the difference between ScnCamera Projection Transform and AR Camera Project Matrix - they appear different in App1 which is not expected to me.

Thanks very much

Save and align ARKit created and rendered geometry later in SceneKit
 
 
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