Re-Skin Mixamo for Unreal Engine 4 Mannequin

The article explains how to swap a Miaxmo armature with a Mannequin-compatible armature via Chiro (UE4 Mannequin) add-on for Blender.

Intro

Mixamo characters may have a variety of armatures. Some of them are more compatible with Mannequin than others. The process described in here will not work for every character you find out there, but should work for some of them.

1. Get the character in T-Pose

The easiest way is to download the character in T-Pose. Otherwise, you may have to pose your character manually into the T-Pose before we proceed.

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2. Convert the armature to Mannequin-compatible

Now we will rename every bone of the armature into its Mannequin counterpart and also delete the redundant leaf bones.

To do that, we will use Skeleton conversion transformation.

1. Select the Armature
2. Go to Edit mode
3. Find and press in the menu Armature -> Chiro (UE4) -> Mixamo --> Skeleton conversion

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3. Connect the bones with children

Some characters may have incorrect bone orientation. The easiest way to make sure all bones are positioned correctly (or at least similar to the Mannequin), is to connect the bone tails to children.

To do that we will use Connect the bones with children transformation.

Find and press in the menu:

• Armature -> Chiro (UE4) -> Mannequin --> Connect the bones with children

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4. Snap clavicles to Mannequin position

Most likely your character T-Pose will have clavicle root positions aligned with the spine bone by the X axis. The Mannequin, however, has its clavicle roots displaced forward, which better replicates human anatomy. Later, when retargeting the Mannequin animation in Unreal Engine 4 for our character, this will make an effect of the shoulders stretched back too far, which looks like if scapulae are brought together. To reduce that effect, we can move the clavicle bone roots forward, similar to the original Mannequin armature.

To do that we will use Snap clavicles to Mannequin position transformation.

Find and press in the menu:

• Armature -> Chiro (UE4) -> Mannequin --> Snap clavicles to Mannequin position

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5. Make Twist bones

Most likely the character does not have the Mannequin Twist bones. Even though there are no Vertex groups nor Weight paints, we still need those Twist bones to exist so that Unreal Engine 4 can work with the skeleton when we retarget Mannequin animation. This is important for the engine that all the bones exist and precisely reproduce the original skeleton hierarchy.

To do that we will use Make twist bones transformation.

• Armature -> Chiro (UE4) -> Mannequin --> Make Twist bones

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6. Roll the bones

The Mixamo characters have the bone heads and tails properly placed out of the box, same as the Weights for the Vertex Groups. However, an important difference with the Mannequin is the bones are rolled differently. Fret not, you don't have to re-roll every bone manually as long as your character is carefully posed in T-Pose. As long as it is the case, the add-on can detect proper rolls for every bone. Then you may correct the small bits manually if you'd like. Although, usually it will have "good enough" result without manual changes.

After we make sure the mesh is in the correct T-Pose, we may reroll the bones.

To do that we will use Bone Roll T-Pose transformation.

• Armature -> Chiro (UE4) -> Mannequin --> Bone Roll T-Pose

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7. Fix feet rotation

The Bone Roll T-Pose transformation will try to detect the best possible rotation. However, depending on the actual mesh pose, it will not be correct for some bones. Particularly, if the feet of the mesh don't look straight forward, then the roll will not be correct.

Not a big problem. We can fix the rolls manually. To do that:

• turn on X-Mirror
• fix the left foot roll so X axis looks down

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8. Fix finger rotation

Same as with feet, finger rolls may need manual update so every X axis of every finger bone (except thumb) looks straight down, following the palm direction.

• turn on X-Mirror
• fix the left arm fingers so that X axes look down

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9. Re-Pose the Mesh to the Mannequin A-Pose

Before we can export the character to FBX, we have to re-pose it into Mannequin A-Pose. This is necessary so that add-on may transform the bone rotations later when exporting to FBX. We will omit the technical details here, but this is necessary for the character to get exported correctly.

To do that

1. Go to Pose mode
2. Find and press in menu Pose -> Re-Pose (Chiro UE4) -> "Mannequin --> A-Pose"
3. Find and press in menu Pose -> Apply -> Apply Pose to Mesh & as Rest (Chiro)

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10. Export FBX

Now it is ready for exporting to FBX. However, the add-on has its own Export FBX operator, that uses correct export settings and takes care about some extra stuff that you usually don't want to do manually. For the details on the Export FBX operator, see its own documentation section.

• File -> Export -> FBX (Chiro UE4 Mannequin) -> Save as ***.fbx

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Conclusion

The resulting FBX file is ready for Unreal Engine 4 with default import settings.
Although, most likely your character will need animation retargeting in Unreal Engine 4. If you are not sure how to do that, see HowTo: UE4 Retargeting.

Here is the result of what we have done in this article, with animation retargeting as per HowTo: UE4 Retargeting.