Skin a mesh with Mannequin armature

The article explains how to skin a humanoid character with a Mannequin-compatible armature (skeleton). That will allow reuse of Mannequin animations for the character via animation retargeting in Unreal Engine 4.

Intro

In this article we will skin a character mesh with a mannequin-compatible armature.

The mesh proportions are not matching the Mannequin, so we will change the bones and later use retargeting manager in Unreal Engine 4 to make the animation work. The mesh does not have armature, vertex groups and weight paints.

Here is the character we will be skinning

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1. Add Armature

The first step is to add the armature we'll be working with. To do that, find the menu and the armature to the scene. It will be easier to work if we make it show up "In Front", so let's do it straight away.

1. Add -> Armature -> Chiro (UE4 Mannequin) -> A-Pose
2. Armature sidebar -> Viewport Display -> In Front

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2. Scale Armature

Now we scale the armature so its spine length following the torso size accordingly.
Here are a couple guidelines you could follow to make it right

1. pelvis root should be at the top of the groin
2. clavicle bone tails should be just in the middle of the character shoulders

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3. Adjust pelvis position

Now we shall adjust the bone positions. However, pelvis is the root for all the other bones and it is important that we begin with it first.
To find the correct position for pelvis, you could follow these guidelines.

• front view: top of the groin, middle of the mesh
• side view: middle of the mesh, middle of glutes
• rear view: middle of glutes, middle of the mesh

To make it easier, in this step we will move pelvis into a separate layer, so the other bones don't obscure the view. To do that, we are switching to the Edit Mode. Then, we return to the Object Mode and adjust the pelvis position by dragging the armature around.

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4. Adjust pelvis tail

The rest of the bones we will adjust in Pose mode.
Now we are correcting spine_01 position by following these guidelines

• front view: just below navel
• side view: middle of the mesh
• rear view: just above the pelvis

Since the bones are connected, to adjust spine_01 position we will update the pelvis bone instead. In Pose Mode we will rotate pelvis and increase its Scale Y.

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5. Adjust spine_01

Same here. To adjust spine_02 we will update spine_01 in Pose Mode. Rotate the bone and update its Scale Y to change the bone length.
Here are the guidelines for spine_02 adjustments

• front view: celiac plexus (aka solar plexus), just below the ribcage, top of the abdomen
• side view: middle of the mesh

To do this, we will move the spine_01 bone to the current layer via Edit Mode

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6. Adjust spine_02

Repeating the same proceduce to adjust spine_03 position.
The guidelines

• front view: middle of the chest
• side view: middle of the mesh, below the bottom of armpit, following the shape of the mesh

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7. Adjust neck and head

Repeating the procedure for neck and head bones.

neck root guidelines:

• front view: root of the neck, above clavicles
• side view: root of the neck, just above the shoulders

neck tail guidelines:

• side view: behind the jaw, below the ear

head guidelines:

• parallel to face

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8. Adjust clavicles

The clavicle bones are the most error prone in retargeting Mannequin animations. Most likely you won't be able to find the correct position for your character from the first try. The shoulders may look broken, or positioned weirdly. To mitigate that effect, find a correct spot for clavicle bone roots. Although, for now, we are going to leave it "as is".

The guidelines

• turn on X-mirror
• do not change clavicle root position
• clavicle tail should be placed in the middle of the shoulder joint

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9. Left Arm

Following the procedure, we're leaving X-mirror turned on and update the left arm bones.
Here are some simple guidelines for that

• all bones should be placed in the middle of the mesh
• upperarm_l tail should be placed exactly at the elbow joint
• lowerarm_l tail should be placed exactly at the wrist joint
• hand_l tail should be placed at the root of the middle finger

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10. Fingers

Skin the fingers right is tricky. Although, if you're careful enough, the result should be good.
Here are the guidelines for you:

• roll hand_l bone to find the correct thumb to pinky position
• drag roots of gingers, place them to match knuckles
• rotate the rest finger bones to find correct finger positions
• scale the bones to match finger phalanges
• make sure X axis of every bone except thumb looks the same way as the palm

We are still using X-mirror and only dealing with the left hand.

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11. Left leg

Now we're ready to proceed with legs.
The guidelines are:

• thigh_l root should be already in a good position, so we don't touch it
• thigh_l tail should be placed in the knee joint
• calf_l tail should be placed at the ankle joint
• foot_l tail should be placed at the beginning of the toes
• ball_l should be aligned straight forward

We're still using X-mirror and only adjust left leg bones.
In the process we're also using Select Bone Group: Twist Bones to select all twist bones and move them into a different layer, so they don't obscure the view.

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12. Adjust the right side

Ideally the mesh will by symmetrical along the X axis. However, the world is not ideal, so we should account for the differences between the left and right sides of the mesh.

1. Turn OFF X-mirror
2. Correct right arm, palm, fingers and leg

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13. Apply the pose as Rest Pose

We have the armature in the correct pose precisely following the mesh posture and proportions.
Now we need to apply all our changes to the armature by applying the pose as Rest Pose.

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14. Parent the armature

It is time to parent the mesh with the armature. Ideally you may like to amend the weight paints for the vertex groups, but for the brevity of this tutorial we will use auto-weights and leave it as is. This will give us good enough result for now.

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15. T-Pose

A requirement for the armature to be mannequin-compatible is correct bone rolls.
The Chiro add-on has built-in functionality to correct the rolls. However, to do so properly it needs the character to be in T-Pose. Thus, we will put our mesh into T-Pose now.

Here are the guidelines

• Face: looking straight forward, head is aligned by Z axis
• Feet: looking straight forward, parallel to the floor
• Arms: aligned by X axis, parallel to the floor
• Upper arms: parallel to the floor
• Elbows: looking straight back (towards Y axis)
• Palms: parallel to the floor

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16. Apply the pose to mesh

Before we move on, we need to put the mesh into the new T pose at rest. To do so, we will use Apply Pose to Mesh & as Rest feature. Without it, the mesh will keep its original form and not the T-Pose we've just made for it.

Pose -> Apply -> Apply Pose to Mesh & as Rest (Chiro)

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16.5. (Optional) Snap clavicles

This step is optional, because it may have different results depending on the character. Sometimes it may help with shoulders, sometimes it would make things worse. You should try both ways and choose the best option that suits your character anatomy.

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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17. Fix bone rolls

Now we should be able to correct the bone rolls via the Bone Roll T-Pose transformation.

1. Switch to Edit Mode
2. Armature -> Chiro (UE4) -> Mannequin --> Bone Roll T-Pose

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18. Fix feet rotation

The Bone Roll T-Pose transformation will try to detect the best 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, the roll may be incorrect.

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

• Make sure X-Mirror is turned OFF
• fix the left foot roll so X axis looks down
• fix the right foot roll so X axis looks up

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19. A-Pose

The final step before it's ready for exporting. The add-on Export FBX operator relies on the armature to be in A-Pose to correct the bone rotations and make it compatible with the Mannequin armature. Thus, the last step before exporting is to pose it into A-Pose and apply it to the mesh. Similar to what we did with T-Pose before.

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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20. 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 rarely want to do manually. For more 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 use in Unreal Engine 4 with default import settings.
Although, most likely it will need animation retargeting in Unreal Engine 4. If you are not sure how to do that, read HowTo: UE4 Retargeting.

Results

Third person

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Advanced Locomotion System v4