Child object on rigidbody hands lag behind the hands

[adam.wilko]

In Unity, if you create an object directly in the transform hierarchy as a child of one of the rigidbody bones, eg, a tiny point attached to bone3 at the fingertip for added precision with small buttons, or a wristwatch attached as a child of the forearm, the child transforms don't get the same anti-latency treatment that the hand bones do, so they end up lagging a frame behind the hands, graphically as well as their transform info accessed in any scripts.

Just wondering if there is a simple fix for this before I go digging too deeply through the leap motion code to try and find out exactly how it reduces latency on the bones.

edit: I have tried changing script execution order, and also reducing physics step time. The latter worked to remove "judder" in the child transforms quite well, but they are still constantly a frame behind the parent bone.

[Joe_Ward]

The physics hands are updated during the fixed update, but I'm not sure why child objects would lag behind their parent. I don't think we do any special anti-latency treatment to individual bones; the time warping code selects the "best" frame to use for each fixed update. We update the position and orientation of the bones using the selected frame, beyond that, the movement of any child objects is under control of Unity. We did find some odd behavior with rigid body interpolation causing lag. It worked best to set interpolation to none.

[adam.wilko]

Thanks Joe,
OK, that is odd. The physics interpolation is set to none in my project, it's among the things I tired changing to fix the problem. Also it doesn't seem to make a difference if there is a rigidbody on the child or not.

It is pretty easy to set up and see though, especially with the capsule hands. If you put something between the wrist bones, under the forearm in the hierarchy, the lag should be easy to see - no code required. I can send an example project if you like?

I have one more idea that I will try today, which is to make sure the child is exactly like one of the bones in terms of rigidbody and collider settings, but the lag was there even when there is no rigidbody or colliders attached to the child.

Edit: Changing the child object to have identical properties (ie maxed out mass, zero drag, isKinematic etc) didnt work, this led me to test adding a MeshFilter and MeshRenderer on the bones themselves. Sure enough the same effect happens. Meshfilter / MeshRenderer components on an Object with a rigidbody seem to lag one frame behind a skinnedMesh using the same rigidbody.

I guess this is an more an issue with Unity itself, rather than leap motion specifically, I would have to do some isolated tests to find out.