Skinning Basics

Skinning Basics

This chapter is a very brief but hopefully complete introduction to the fundamentals of skinning and how this applies to Blender. Please be prepared to read this chapter more than once. It is not an easy document and i have put a lot of effort into describing the topic as simple as possible.

This is a theory chapter…

I am aware that this text only scratches the surface. But i believe that you will find your way through this topic over time. Just read this document in little chunks, once, twice, multiple times. Get back here when you got stuck in your project and just don’t see why things happen as they do. Maybe this document is then good for finding the problem. The more you understand in this document the easier things will become for you in the later chapters!

In the image you see an SL Avatar and the skirt is adjusted to its anticipated location (but not attached). I have also posed the Avatar into a walking pose. However, you see the upper leg pokes through the skirt fabric.

This is because the skirt does not know anything yet about the avatar, neither does it know where the avatar is, nor does it know how the avatar moves. Hence it stays where it is.

So the very first thing we have to do is to attach the skirt to the avatar. In SL you would probably attach it to the pelvis or the Torso. In Blender we will attach it to the Avatar’s rig (the Armature)…

The model skirt (not rigged yet).

Note: You could import this skirt as it is to SL (or OpenSim or similar) and attach it to the Hips or the pelvis attachment point. But when the legs move you will see how they poke through the mesh like in the image above.

Bones and More

The SL Avatar Mesh

Let us first take a more detailed look on the SL Avatar. Actually the SL Avatar consists of a mesh made out of about 7500 triangles. When it comes to posing and animation all of these triangles have to be moved around somehow. However, it would not be very practical if we had to define the locations of all Mesh faces one by one for every possible pose. And luckily there is a better way to do this.

Bones

The SL Avatar has a set of (159) bones which can be moved separately. And we will see in a moment that each bone controls the deformation (bending and stretching) of a specific area of the mesh. These bones are also named Deform Bones. But one step at a time…

Bone Relations

How do the vertices know how to move ?

Bone Weight Maps

A Bone Weight Map defines how much influence a specific bone has on each vertex of a specific mesh. The influence is typically defined in a range from 0.0 (no influence) to 1.0 (maximum possible influence). So if a vertex shows up in a Bone Weight Map, then it will be controlled by that bone. The amount of weight defines if the vertex only slightly follows the bone (weight ~0.0) or moves as if it where glued to the bone (weight ~1.0)

Note: Bone weight Maps are defined per Mesh (as a set of vertex groups, see next chapter)

Visualizing Weight Maps

Weight maps are often visualized directly on their associated mesh by using a color code (where each weight value between 0 and 1 is mapped to a color in a color band. Blender uses by default a color band that maps a weight of 0.0 to the color blue, and a weight of 1.0 to the color red. In between weight values are mapped to the rainbow colors:

Here is an example for the visualization of the Weight Map of the SL Avatar’s mHead bone:

Note: a vertex can be animated by more than one bone in parallel. So it is possible that a vertex can be referenced in more than one Bone Weight Map (providing the potential for very complex movements) This happens typically where 2 bones touch each other. In that case the influence is split between the 2 adjacent bones.

The green and yellow vertices indicate the weights are split between mHead and mNeck in this region

SL Avatar Weight maps

Since a Bone Weight Map is always related to a specific bone, that means for the SL Avatar: We have to provide 159 weight maps (one Map per bone) in each mesh that we want to rig.

So, lets see…

• A weight map is always related to one bone.
• And it contains one weight value for each vertex.
• the SL system character has 7500 vertices
• 7500 Vertices * 159 weightmaps =  1.2 million weights.. huh?

Well, this is insane. And fortunately it is not exactly what happens. Actually there is a rule:

The next image shows areas with different numbers of “weights per vertex” for the SL Avatar. Indeed most areas of the SL Avatar are assigned to only one bone (areas displayed in light grey), while near the joints we always find 2 bone weights per vertex (areas displayed in light green).

Note: The above rule helps to avoid unnecessary clutter in the Weight Maps. Please note that the SL Avatar uses 2 weights per vertex at most. So actually the number of weights is at least 7500 for the SL Character, but no more than 2*7500.

 

Weights per vertex.
Grey:1 weight
Green:2 weights

Note: Remember that each vertex has to show up in at least one weight map, otherwise you get an error “unweighted verts in mesh” when you try to import your mesh to SL or compatible worlds.

Skeletons

From a technical point of view each Bone that has a weight map in a given mesh, also can take part in the animation (deformation) of the mesh. We call the set of Deform Bones also a Skeleton. However, in Blender a Bone actually has to be enabled as Deform Bone otherwise its corresponding weight map is ignored:

A bone must be enabled as deform bone in the Bone data properties.

In Avastar all Basic skeleton Bones are displayed in Blue and all additional Bento bones are displayed in purple :

The System character Skeleton (Bones displayed as sticks)

Armatures

The Avastar Rig has many more bones. In detail we find:

• Animation Control Bones (green).
• IK Bones (yellow).
• SL Basic Bones (Deform Bones, blue).
• SL Extended Bones (Deform Bones, Purple)
• Attachment Bones (red).(1)
• Collision Volumes (orange).(2)
• Structure Bones (steel blue).

All Bones together are the “Armature”.

Note: From all these bones, only the SL Base Bones, the Attachment Bones and the Collision Volumes should be used for skeletal animation. Attachment bones can also be used as deform bones, but it is unclear if this is just tolerated or officially accepted by LindenLab.

All Bones of the Avastar system character at a glance.

(1): The use of the Attachment Bones for skeletal animation is discouraged by Linden Lab. Furthermore some of the attachment Bones have spaces in their name which results in issues during uploading. Firestorm Viewer has a patch for this and can also upload weighted bones with spaces in their name.

(2): The Collision Volumes have originally been used only for defining the click surface of the Avatar (when a click is detected as “avatar clicked”. But they can now officially be used as Deform Bones (This is Fitted Mesh, we get to this in more detail later).

Rigs

In a Nutshell:

• Bone: Animates other Bones and/or Meshes.
• Control Bone: Animates other Bones.
• Deform Bone: Animates areas of meshes(1)
• Skeleton: Set of Deform Bones plus constraints.
• Armature: Skeleton plus Control Bones.
• Rig: Animation Control System for a Model (Armatures are Rigs based on Bones).

(1): Deform Bones can also be control bones, for example in a bone hierarchy the parent bones control the child bones.

Armature modifiers

There is one last ingredient missing here. That is we have to tell a mesh that it actually has to be animated by an armature. And this is done with the aid of an Armature Modifier:

• An armature Modifier is always created in the Context of a mesh.
• Within the Modifier you specify which Armature Object shall control the mesh. You do this in the Armature modifier’s Object field.
• Finally you specify if the modifier shall use the mesh’s Vertex Groups or the Armature’s Bone Envelopes.

Note: In the context of Second Life animation with Bone envelopes is not supported, so we always use Vertex Groups here.

Armatures and rigged Attachments

So far we have been talking about Armatures in relation to Avatars. And it has become clear how the Avatar Meshes get animated in principle (by animating the Avatar Skeleton). But what about attachments ? How do they fit into the overall picture ? Well, it could best be described as:

Attachments are specialized characters

What? That sounds weird… but actually it is a correct picture. See this:

• Any rigged attachment needs a rig.
• The Rig must be compatible to the SL Avatar’s Armature,
• The rig can only contain the Bones which are also defined for the SL Skeleton.
• And any mesh attachment can be rigged to a complete skeleton as well, however it is sufficient to provide just the needed bone chains (all chains start at the pelvis and end in a leaf bone on a limb (or the skull)
• So a Mesh attachment is technically a (partially) rigged character.

Example: Boots rigged to the SL Avatar Rig

Rigged Mesh & Attachment Points

So, every rigged attachment has to be rigged to the same Skeleton as the SL Avatar. So in fact a mesh attachment is handled in exactly the same way as a mesh character. And technically there is no difference between the SL Avatar, a complete Mesh character and a Mesh attachment. All of them get controlled by the same rig in the exact same way.

While the SL Avatar just “exists”, the Mesh attachments and Mesh based Avatars have to be worn “on top of the SL Avatar” by attaching them to an attachment point. Yes, even a mesh character has to be “worn” on an attachment point of the SL Avatar!

The difference of Prims and Rigged mesh

And for SL and compatible worlds it is as easy as that: A rigged mesh always uses the pre defined Avatar Rig.

Putting all together

So what we have to do next is to combine what Avastar provides (the SL Armature) with your mesh attachment (in our case the demo skirt). In particular we have 2 tasks:

• Associate our Mesh to the Avastar Rig so that both know each other.
• Create/Modify/Optimize Bone weight maps which define the relation between the skeleton bones and particular areas of your mesh.

This process is named : Skinning And this tutorial is actually about skinning.