OpenXR Hand Skeleton Bone Mapping Bug: Hand_IndexTip Issue

by SLV Team 59 views
OpenXR Hand Skeleton Bone Mapping Bug: Hand_IndexTip Issue

Hey guys! Today, we're diving deep into a tricky bug that's been causing some headaches for developers working with OpenXR hand skeletons, specifically the incorrect bone mapping for Hand_IndexTip. This issue can lead to some seriously wonky behavior in your VR applications, so let's break it down and see how to fix it.

The Problem: Incorrect Bone Mapping

When you're using OpenXR hand tracking with SkeletonType.XRHandLeft or SkeletonType.XRHandRight, the BoneLabelFromBoneId method in OVRSkeleton.cs is supposed to map bone IDs to their corresponding bone names. However, it incorrectly maps BoneId.Hand_IndexTip to "XRHand_RingTip" instead of the correct index finger tip bone. This means that any code trying to access the index finger tip using Hand_IndexTip will end up getting the ring finger tip transform instead. Ouch!

Digging Deeper into the Issue

This bug stems from an attempt to reuse OVR hand bone IDs (values 0-24) for OpenXR hand tracking. While seemingly convenient, the fundamental structural differences between the two skeleton systems make this problematic:

  • OVR Hand skeleton: Comprises 24 bones, with the tip bones neatly located at IDs 19-23.
  • OpenXR Hand skeleton: Consists of 26 bones, exhibiting a completely different layout.

The BoneLabelFromBoneId method tries to bridge this gap by creating a "universal" mapping. It reuses the Hand_* bone ID enum values to map to XRHand_* bone names within a switch statement (lines 1054-1077 in the decompiled code). Unfortunately, this approach results in incorrect mappings due to the inherent structural discrepancies.

To illustrate, consider this:

  • The correct OpenXR index tip bone is BoneId.XRHand_IndexTip = 10, which maps perfectly to "XRHand_IndexTip".
  • However, developers intuitively try to use BoneId.Hand_IndexTip = 20, which incorrectly maps to "XRHand_RingTip".

This seemingly small error can cascade into significant issues within your VR applications.

Reproducing the Bug

Want to see this in action? Here's how you can reproduce the bug in Unity:

  1. Create a new Unity scene and enable OpenXR hand tracking.

  2. Add an OVRSkeleton component to your scene and set the skeleton type to "OpenXR Hand (Left)" or "OpenXR Hand (Right)".

  3. Write a simple script to retrieve the index finger tip using BoneId.Hand_IndexTip:

    Transform indexTip = FindTip(skeleton, OVRSkeleton.BoneId.Hand_IndexTip);
Debug.Log(indexTip.name); // Output: "XRHand_RingTip" (WRONG!)

  4. Run the scene and observe that the returned bone transform is actually the ring finger tip. Doh!

Real-World Impact

This bug can have a significant impact on the functionality of your VR applications. Imagine these scenarios:

  • Incorrect finger poke detection: Your virtual marker appears on the wrong finger, leading to frustrating user experiences.
  • Misaligned UI interactions: Users struggle to interact with UI elements because their finger positions are misread.
  • Wrong finger tracking visualizations: The virtual hands in your application don't accurately reflect the user's real hand movements.
  • Confusion for developers: Especially for those porting projects from OVR to OpenXR hands, this inconsistency can lead to wasted time and frustration.

Analyzing the Code

Let's take a peek at the code to understand what's going on under the hood. Looking at line 1073 in the decompiled OVRSkeleton.cs, we see the culprit:

BoneId.Hand_IndexTip => "XRHand_RingTip",  // INCORRECT MAPPING

This line clearly shows the incorrect mapping of Hand_IndexTip to XRHand_RingTip. Meanwhile, the correct index tip mapping is on line 1066, but it's using the wrong enum:

BoneId.Hand_Middle2 => "XRHand_IndexTip",  // Correct, but uses wrong enum

It seems all the tip bones are incorrectly shifted for OpenXR:

  • Hand_IndexTip (20) maps to XRHand_RingTip (should be unused/invalid)
  • Hand_MiddleTip (21) maps to XRHand_LittleMetacarpal (wrong!)
  • Hand_RingTip (22) maps to XRHand_LittleProximal (wrong!)
  • Hand_PinkyTip (23) maps to XRHand_LittleIntermediate (wrong!)

Workarounds: Getting Things Back on Track

Don't despair! There are a couple of workarounds you can use to sidestep this bug:

1. Detect Skeleton Type and Use Appropriate Bone IDs

The most reliable workaround is to detect the skeleton type and use the correct bone ID enum accordingly:

var skelType = skeleton.GetSkeletonType();
var indexTipBoneId = (skelType == OVRSkeleton.SkeletonType.XRHandLeft || 
                      skelType == OVRSkeleton.SkeletonType.XRHandRight)
    ? OVRSkeleton.BoneId.XRHand_IndexTip  // Use for OpenXR (value = 10)
    : OVRSkeleton.BoneId.Hand_IndexTip;    // Use for OVR hands (value = 20)

Transform indexTip = FindTip(skeleton, indexTipBoneId);

This code snippet checks if the skeleton type is OpenXR and then uses OVRSkeleton.BoneId.XRHand_IndexTip if it is, or OVRSkeleton.BoneId.Hand_IndexTip if it's an OVR hand.

2. Search by Bone Name

Alternatively, you can search for the bone by name instead of relying on the ID:

foreach (var bone in skeleton.Bones)
{
    if (bone.Transform.name.Contains("IndexTip"))
    {
        indexTip = bone.Transform;
        break;
    }
}

This approach iterates through the bones and checks if the bone's name contains "IndexTip". While slightly less efficient, it avoids the incorrect mapping issue altogether.

Suggested Fixes: How Meta Can Solve This

There are a few ways Meta could address this bug directly:

Option 1 (Non-Breaking - Recommended)

The most straightforward solution is to fix the mapping in BoneLabelFromBoneId so that Hand_IndexTip correctly returns "XRHand_IndexTip" for OpenXR skeletons:

// Line 1073 - Fix the mapping
BoneId.Hand_IndexTip => "XRHand_IndexTip",  // Fixed!
BoneId.Hand_MiddleTip => "XRHand_MiddleTip",
BoneId.Hand_RingTip => "XRHand_RingTip",
BoneId.Hand_PinkyTip => "XRHand_LittleTip",

This approach is non-breaking, meaning it won't require developers to change their existing code.

Option 2 (Breaking Change - Most Correct)

A more radical approach would be to remap the Hand_*Tip bone IDs to return BoneId.Invalid for OpenXR skeletons. This would force developers to use the correct XRHand_* bone IDs, but it could break existing projects.

Option 3 (Documentation)

At a minimum, Meta should clearly document that Hand_* bone IDs should not be used with OpenXR skeletons. They could also provide a helper method like this:

public static BoneId GetFingerTipBoneId(SkeletonType type, FingerType finger)
{
    if (type.IsOpenXRHandSkeleton())
    {
        return finger switch
        {
            FingerType.Thumb => BoneId.XRHand_ThumbTip,
            FingerType.Index => BoneId.XRHand_IndexTip,
            FingerType.Middle => BoneId.XRHand_MiddleTip,
            FingerType.Ring => BoneId.XRHand_RingTip,
            FingerType.Pinky => BoneId.XRHand_LittleTip,
            _ => BoneId.Invalid
        };
    }
    // Return Hand_* IDs for OVR hands
}

This helper method would encapsulate the logic for getting the correct bone ID based on the skeleton type.

Conclusion: Let's Get Those Fingers Pointing Right!

The incorrect bone mapping for Hand_IndexTip in OpenXR hand skeletons is a frustrating issue, but hopefully, this article has shed some light on the problem and provided you with the tools to work around it. Whether you choose to detect the skeleton type, search by bone name, or wait for a fix from Meta, you can get your VR applications back on track. Keep coding, and let's build some awesome VR experiences together!