Trying to fix arm twist for a UE4 rig

Ruflse

Hey PC,
I have a little problem trying to figure out how to make the bones be able to twist without having the classic "candy wrapper" effect in which the mesh shrinks where bone is being twisted. Until now I've been using Blender's option "Preserve Volume" (which basically activates Dual Quaternion Skinning) for most of my meshes that have this problem, but since Unreal Engine 4 doesn't support this feature, only Linear Blend Skinning, I can't really export this rig without those ugly deformations.

So this is the model:


This is how it looks with Preserve Volume on:



And this is how it looks without it:



As you can see the arms get messed up.
I tried adding more bones to the wrist and arm, but this only moves the shrinking from one point to another, so I'm not sure about what's the usual solution for this kind of problem if the rig is going to be exported to a game engine.
Any halp?

Obscura

Check out the mannequin mesh that comes with Unreal. You can export it from there, bring it into blender, and see how they've set up the arm rig.

pior

Well, the downside of the Epic mannequin is that it is a ball-and socket kind of character hence these issues mostly don't show up on it. It does have helper bones for the shoulders and wrists ... but the animation packs on the marketplace don't use them, therefore they are better left ignored.

Bottom line : if anything one thing you can do is to move the joint a bit closer to the elbow/surface,  and also use harsher skin weights over the transition - which will cause limb sections to interpenetrate each other more, but it will preserve the strength of the silhouette somehow. Quite similarly to an old-school segmented character - which is not too far from the style you're using here anyways.

Obscura

Alright. Then maybe one of the Paragon examples.

Ruflse

So I looked up one the Paragon examples, but there is not anything that useful I can get out of it without the controls for the rig.
They used a Hand, a Forearm Twist (with the hand not parented to it)  and a Forearm Bone and they probably made a control to rotate each of them a bit to balance the shrinking along the arm so it's not that noticeable.





For a normal arm and with a decent amount of loops it works, but for a pretty lowpoly and short arm I supose it's not so easy.
Guess I'll have to fiddle around with constraints

Ruflse

I think I've found a pretty decent solution.
Since the arm is so short I ended up turning the bones for what would be the forearm and the arm into helper bones that rotate a bit when the hand bone rotates in a certain direction and also added an stretch to constraint to make them still be connected to each other.




So to avoid having conflicts between parents and constraints I had to remove the hand being parented to the forearm, and instead created a new control bone and parented the hand and the first helper bone to it, so you can still rotate and move around the whole arm.



It's a weird solution but I think it works, the only problem is that the helper bones sometimes suffer from Gimbal lock or something and start making sudden rotations.