Clarify jointParents value for root bones in XR_FB_hand_tracking_mesh

[flashk]

I'm adding support for the XR_FB_hand_tracking_mesh extension to my application, and it wasn't clear to me what the jointParents value of the XrHandTrackingMeshFB struct would be for root bones. Based on my testing it appears the joint parent value for root bones is XR_HAND_JOINT_MAX_ENUM_EXT. I think it would be useful to clarify that in the specification.

[rpavlik]

Hmm, MAX_ENUM shouldn't actually be used anywhere, it solely exists to force the size of an enum type due to annoying limitations of C. I'll see if I can ping somebody who can answer this.

[rpavlik-bot]

An issue (number 2038) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2038 ), to facilitate working group processes.

This GitHub issue will continue to be the main site of discussion.

[RobertC-Meta]

I'm adding support for the XR_FB_hand_tracking_mesh extension to my application, and it wasn't clear to me what the jointParents value of the XrHandTrackingMeshFB struct would be for root bones. Based on my testing it appears the joint parent value for root bones is XR_HAND_JOINT_MAX_ENUM_EXT. I think it would be useful to clarify that in the specification.

Sorry for the delay on this. I have confirmed that the this is the way that the extension is intended to work (returning XR_HAND_JOINT_MAX_ENUM_EXT for the parent of the wrist joint). I will update the documentation to reflect this. It would likely cause more problems to change it.

[flashk]

Great, thanks for the clarification.