Wrist twist IK issue

[shiftyscales]

Following up from #1120:

@Frooxius had noted:

That's more about using the right bend/twist vectors. The way the IK system works is that it doesn't (and pretty much cannot) rely on constraints like that - the constraints come naturally from your own body. It sounds like a bit more obscure bug though, but when I was originally implementing and fixing issues, I did test a lot of crazy positions like that and it worked fine.

It might be something avatar-specific, so I'd probably need some samples and replication steps. But it's better for another topic separate from the hips.

There is a very replicable issue that happens across a number of tested models, e.g. narcacuga, yeen, human male, and others from the Neos Essentials avatars folder.

Steps to replicate:

• Hold your hand such that your wrist/palm is upright
• Curl your wrist inward

This issue is demonstrated here:
https://youtu.be/Z5trBCE6Elk

[H3BO3]

Can confirm this has happened across many many avatars, ones I've made myself as well. The way I replicate it is by holding my palm up and my hand slightly forward and out, and raising my hand to near my head level.

[Frooxius]

This is likely because of the angles / reference axes used to break the twist being weird. I'd need some avatars to test. It might help to check what are the axes for the bones for the forearm and wrist and how they differ in avatar where it works normally and where it twists at weird angles.

Problem is, that there always be some angle at which the wrist/forearm twists weirdly, arms can't twist 360°, but normally you want that to be at the point which your arm can't reach physically.

[shiftyscales]

As noted, some avatars that can be tested included many found in the Neos Essentials including:

Neos Essentials > Avatars > Human Male
Neos Essentials > Avatars > Anthro > StripedYeenCy.fbx
Neos Essentials > Avatars > Anthro > Nargacuga.fbx

The common issue appears to be that the y-axis of the bone on the wrists of those models is pointing towards the fingertips. The one avatar of those I've tested so far that works differently is the rexouium and nargacuga- in the case of the rexouium and nargacuga, it's even easier to replicate. All you need to do is turn your wrist upward.