Skin Weighting Limit

Modified on Tue, 17 Sep at 3:31 PM

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6 bones per vertex

When creating an avatar, clothing or attachment product a process called skinning is applied to the mesh geometry whereby the mesh can be deformed by a skeleton. The skeleton is a hierarchy of connected bones. Skinning assigns the bones of the skeleton to each of the vertices of the mesh.

When bones are associated with a vertex, they are given a number value between 0.0 and 1.0 of how much influence they have on the vertex. A value of 0.0 is no influence and a value 1.0 is 100% influence. This is the weight of that bone for the vertex. When many bones are assigned to a vertex, the weights must be evenly distributed between the bones and add up to a total of 1.0. This is called normalization.

 

 

pelvisNode

lfHip

lfThigh

rtHip

rtThigh

Vertex #1

0.5

0.2

0.05

0.2

0.05


This process of assigning bone weights to each of the vertices of a mesh is called skinning, the result being the skin weighting of the mesh.


Limiting bones per vertex

3D modeling tools such as Blender and 3ds Max allow for skinned meshes to have a large number of bones per vertex (up to 100 bones per vertex in the case of 3ds Max). Each bone adds more data that needs to be stored and processed. Having more bones per vertex comes at a cost in performance during data upload/download and engine rendering.

Historically, in game development, there has been a standard of 4 bones per vertex. Both Unity and Unreal Engine historically used 4, although now support more. The IMVU Northstar engine used by all Next platforms (Studio, Web, Desktop, iOS, and Android) supports up to 6 bones per vertex.


When a vertex is assigned more than 6 bones, only the 6 greatest weight values will be used, and the rest will be ignored.
 

What happens if there are more than 6 bones per vertex?

Here are a couple of examples with more than 6 bones per vertex demonstrating what happens when the number of bones gets limited and what the results would look like.


Example 1

In this example, the weight values are not all the same. The 6 greatest weight values will be used and normalized, and the rest will be ignored.


Original from 3D editor

 

pelvisNode

lfHip

lfThigh

lfCalf

rtHip

rtThigh

rtCalf

Vertex #1

0.4

0.15

0.1

0.05

0.15

0.1

0.05


Northstar engine result

 

pelvisNode

lfHip 

lfThigh

lfCalf

rtHip

rtThigh

rtCalf

Vertex #1 

0.7

0.1

0.045

0.01

0.1

0.045

0.05


Example 2

In this example the weight values are all the same. Only 6 weight values will be used and normalized and the rest ignored.


Original from 3D editor

 

lfHip

lfThigh

lfCalf

lfFoot

rtHip

rtThigh

rtCalf

rtFoot

Vertex #1

0.125

0.125

0.125

0.125

0.125

0.125

0.125

0.125


Northstar engine result

 

lfHip

lfThigh

lfCalf

lfFoot

rtHip

rtThigh

rtCalf

rtFoot

Vertex #1

0.166

0.166

0.166

0.166

0.166

0.166

0.125

0.125

 

Workflow

Fortunately, there is a way to prevent issues right from the start in your 3D editing tool. Most 3D editors have controls for limiting the number of bones per vertex. Here are the workflows for a couple of the most popular 3D editors used by IMVU creators.
 

3ds Max

Set the ‘Bone Affect Limit’  to 6 in the Skin Modifier panel before you begin weighing your mesh.


User-added image

3ds Max Skin Modifier panel


Blender (version 2.83)

Bone weight limits are set by running a tool. The best workflow would be to run the Limit Total tool as you work and not after you have completed all of your weighting. More information about the tool can be found here Weight Paint – Limit Total


User-added image

Switch to Weight Paint mode
 

User-added image

Run the Limit Total tool
 

User-added image

Set the Subset to ‘All Groups’ and the Limit to 6

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