NDK Guidance on Controller Arm Model Daydream - google-vr

Question

I need some guidance on how to implement an Arm model for the Daydream controller as required by: https://developers.google.com/vr/distribute/daydream/design-requirements#UX-C1.
Background / Problem:
I'm trying to develop a daydream application with the Android NDK with controller interactions for the Pixel, using ControllerApi from references like [1]. However, there doesn't seem to be any GVR Arm Model helper class we can use in the Android Daydream SDK / NDK, nor is there any documentation guidance on Daydream's SDK documentation site.
As such, my questions are:
1) Is there a GVR Arm Model helper class, or is this something that developers would implement themselves individually? If the latter, is there documentation on how this can be done?
2) If we do implement it ourselves, could we simplify the problem by assuming:
A fixed point for the Elbow joint in absolute space (an assumed Vector3f position)
Forearm and Hand Length
So as to then calculate the controller location / rotation based on the rotation around the wrist + Elbow? Or is there a separate recommended approach?
[1] - https://developers.google.com/vr/android/ndk/reference/group/controller#gvr_controller_state_create

Answer 1

1) Is there a GVR Arm Model helper class, or is this something that developers would implement themselves individually? If the latter, is there documentation on how this can be done?
There is no GVR Arm Model helper class included in the Android NDK. However, both the Unreal and Unity Daydream integrations have Arm Model code built-in that you can utilize. I would recommend using the C++ version of the of the arm model that is included as part of Unreal. The arm model has no dependencies on Unreal code, so you should be able to integrate it into your app without too much refactoring.
The C++ Arm Model in Unreal behaves just like the arm model in Daydream Home. It exposes a Wrist Position and Wrist Rotation that are relative to the user's head, as well as a recommended alpha to render the controller at so that it doesn't clip uncomfortably into the user's head. It also provides an angle that the the laser should be tilted downward from the wrist for ergonomic comfort when pointing at objects. Some more information about the Arm Model can be found here: https://developers.google.com/vr/unity/controller-support
2) If we do implement it ourselves, could we simplify the problem by assuming:
I would strongly recommend you use the Unreal C++ Arm Model as a starting point, however you should definitely feel free to modify it and tune it so that it suites the needs of your application.