The SL Skeleton

Creating the Avatar Workbench and Avastar has given us great insight on the SL avatar. The goal of this article is to give you detailed information about the construction of the Second Life avatar and what you can do with it.

We will also talk about some areas where we simply do not know what happens, or where there are inconsistencies and the techniques are highly experimental. We’ll update the page as we learn more.

Basic Bones

• 26 Basic bones
• 32 Basic Attachment Points
• 26 Collision Volumes

Extended Bones

• 104 Extended Bones
• 13 Extended Attachment Points

The Base Skeleton

The skeleton uses 26 bones for driving

• the body (split into two meshes upper and lower),
• the head and hair meshes,
• the skirt mesh,
• the eye meshes.

Note: 5 Bones are not used for weighting of the System Avatar. The bone names are displayed in Cyan blue in the Listings below.

The 26 Base Bones

• mEyeLeft
• mCollarLeft
• mShoulderLeft
• mElbowLeft
• mWristLeft
• mHipLeft
• mKneeLeft
• mAnkleLeft
• mFootLeft
• mToeLeft

• mSkull
• mHead
• mNeck
• mChest
• mTorso
• mPelvis

• mEyeRight
• mCollarRight
• mShoulderRight
• mElbowRight
• mWristRight
• mHipRight
• mKneeRight
• mAnkleRight
• mFootRight
• mToeRight

The face rig is connected to the SL Appearance Sliders. That is the Face bones move along with the Appearance sliders. So if your mesh characters are weighted to the Face bones, then many of the face appearance sliders now also affect your character look.

But note that the shape sliders affect bone location (bone translation). So the face shape may be affected by custom animations as well (see below for details).

• mFaceEyeAltLeft
• mFaceForeheadLeft
• mFaceEyebrowOuterLeft
• mFaceEyebrowCenterLeft
• mFaceEyebrowInnerLeft
• mFaceEyeLidUpperLeft
• mFaceEyeLidLowerLeft
• mFaceEyecornerInnerLeft
• mFaceEar1Left
• mFaceEar2Left
• mFaceNoseLeft
• mFaceCheekLowerLeft
• mFaceCheekUpperLeft
• mFaceLipUpperLeft
• mFaceLipCornerLeft
• mFaceLipLowerLeft

• mFaceRoot
• mFaceForeheadCenter
• mFaceNoseCenter
• mFaceNoseBridge
• mFaceNoseBase
• mFaceLipLowerCenter
• mFaceLipUpperCenter
• mFaceJaw
• mFaceJawShaper
• mFaceChin
• mFaceTeethLower
• mFaceTeethUpper
• mFaceTongueBase
• mFaceTongueTip

• mFaceEyeAltRight
• mFaceForeheadRight
• mFaceEyebrowOuterRight
• mFaceEyebrowCenterRight
• mFaceEyebrowInnerRight
• mFaceEyeLidUpperRight
• mFaceEyeLidLowerRight
• mFaceEyecornerInnerRight
• mFaceEar1Right
• mFaceEar2Right
• mFaceNoseRight
• mFaceCheekLowerRight
• mFaceCheekUpperRight
• mFaceLipUpperRight
• mFaceLipCornerRight
• mFaceLipLowerRight

The hand bones are very closely adjusted to the System character hands. This will allow to create animations for hand gestures very similar to what the System character can do. But since animations are completely free, you can animate the hands to do whatever you like.

• mHandMiddle1Left
• mHandMiddle2Left
• mHandMiddle3Left
• mHandIndex1Left
• mHandIndex2Left
• mHandIndex3Left
• mHandRing1Left
• mHandRing2Left
• mHandRing3Left
• mHandPinky1Left
• mHandPinky2Left
• mHandPinky3Left
• mHandThumb1Left
• mHandThumb2Left
• mHandThumb3Left

• mHandMiddle1Right
• mHandMiddle2Right
• mHandMiddle3Right
• mHandIndex1Right
• mHandIndex2Right
• mHandIndex3Right
• mHandRing1Right
• mHandRing2Right
• mHandRing3Right
• mHandPinky1Right
• mHandPinky2Right
• mHandPinky3Right
• mHandThumb1Right
• mHandThumb2Right
• mHandThumb3Right

Not much to say here.

• mTail1
• mTail2
• mTail3
• mTail4
• mTail5
• mTail6

Not sure how we will support this within Avastar (We need to do some experiments first, maybe add IK Targets for the hind feet?)

• mHindLimb1Left
• mHindLimb2Left
• mHindLimb3Left
• mHindLimb4Left

• mHindLimbsRoot

• mHindLimb1Right
• mHindLimb2Right
• mHindLimb3Right
• mHindLimb4Right

Not much to say here.

• mWing1Left
• mWing2Left
• mWing3Left
• mWing4Left
• mWing4FanLeft

• mWingsRoot

• mWing1Right
• mWing2Right
• mWing3Right
• mWing4Right
• mWing4FanRight

Note: The Spine bones are folded into the legacy Skeleton and can be unfolded for custom rigs (you need to upload with Joint Positions enabled in that case)

• mGroin
• mSpine1
• mspine2
• mSpine3
• mSpine4

The character can be shaped to a large extent by using the SL appearance editor. The shape parameters can be changed by use of shape sliders.

Within the appearance editor you find 3 sections with 23 shape parameters for the body, torso, and legs. Another 6 sections with 55 shape parameters for eyes, ears, and mouth, as well as for the nose, chin, and the head form. In addition there are 42 controls for the clothes morphs.

Body	3	Shirt	7
Torso	12	Pants	6
Legs	8	Socks	1
		Shoes	7
Head	11	Jacket	6
Eyes	11	Skirt	7
Ears	4	Gloves	2
Nose	11	Undershirt	4
Mouth	9	Underpants	2
Chin	9

When you just work with the default avatar, then you can create small and fat avatars as well as large and slim characters only by adjusting the shape parameters according to your taste. You can do all of that in your Viewer without need to use an external 3D tool.

However, when you use Custom Meshes then some different rules apply…

Shape and Skeleton matching

It is important to always remind that your Avatar Shape settings in your viewer effectively change your skeleton’s joint positions and Bone Scales. This is controlled via 20 of the available shape parameters (appearance sliders) for the Legacy Skeleton and 40 additional parameters for the Extended Skeleton (mostly for the Face expressions and Hands).

So if you want to create a specific mesh attachment for a specific shape then you need to know exactly how the shape values are related to the joint positions (and bone scales) of your particular shape. But this information is normally not available outside of Second Life.

Fortunately you can get around this problem by using the SL Restpose for your work in your 3D Editor. However this means you create your content by using the Second Life Restpose which does never match the actual Shape that you use in Second Life.

Actually we want to create attachments which can be adjusted (fitted) to any shape. This is where the Fitted Mesh addition steps in. The basic idea is to let more Appearance sliders control the mesh. This is done by using the Fitted Mesh bones, or more precisely the Collision Volume Bones to the mesh.

Shape parameters influencing the Base skeleton

The list of bone length changing Shape keys. Please note, the shape keys marked in orange do also influence the Collision Volumes (see next chapter) Section Shape parameters Body Height, Body thickness Head Head Size, Head Shape, Head Length, Face Shear Eyes Eye Size, Eye Spacing, Eye depth Torso Neck Thickness, Neck Length, Shoulders, Arm Length, Hand Size, Torso Length Legs Leg Length, Hip Width, Hip Length Shoes Heel Height, Platform Height

The default avatar uses 24 additional so called Collision Volumes. These are simplified mesh volumes for calculating when an object or another avatar is bumping into a character. In fact the Collision Volumes are a set of 24 Octahedrons which get influenced by various shape sliders.

Interestingly these volumes are also counted as full bones, and they can be animated and weighted just like the standard bones. The most interesting property of these bones is that they get moved and scaled by some of the shape keys. Weighting mesh clothes or avatars to these bones means they will respond to the Shape keys, for example Torso muscles.

But note that only a very small fraction of the Shape sliders actually does influence the Collision Volumes. thus you can not expect to get a full control over your mesh object by using the Collision volumes. for example changing the breast shape is not at all mapped to collision volumes. As well as facial expressions are not supported.

PELVIS, BELLY, BUTT, LEFT_PEC, RIGHT_PEC, CHEST, LEFT_HANDLE, RIGHT_HANDLE, NECK, HEAD, L_CLAVICLE, L_UPPER_ARM, L_LOWER_ARM, L_HAND, R_CLAVICLE, R_UPPER_ARM, R_LOWER_ARM, R_HAND, L_UPPER_LEG, L_LOWER_LEG, L_FOOT, R_UPPER_LEG, R_LOWER_LEG, L_FOOT

Shape keys and the Collision Volumes

This is the list of Shape keys which do affect the Collision Volumes. Please note, the shape keys marked in orange do also influence the Bone length (see previous chapter)

Body	Height, Body Thickness, Body fat
Head	Head Size, Head Stretch, Head Length
Torso	Torso Muscles, Neck Thickness, Neck Length, Shoulders, Arm Length, Hand Size, Torso Length, Love handles, Belly Size
Legs	Leg Muscles, Leg Length, Hip width, Hip Length, Butt Size, Saddle Bags, Knee Angle, Foot Size

We already know that we can modify the Second Life default Skeleton by using the Appearance Sliders. But this is not all we can do. In fact it is possible to rearrange the Skeleton in any way we want. There are only 3 basic rules:

• We can only use the existing set of Joints (bones).
  We can not add new joints to the Skeleton.
• We must keep the hierarchical Joint structure as it is.
  We can not rearrange the joint hierarchy of the Skeleton.
• We are free to move the existing Joints wherever we want.

As mentioned before we only can rearrange the location of Joints but not their relationship. Because of this we always can define a modified skeleton just by recording how far each Joint has been moved away from its original location. This distance is the Joint Offset

So one way to define the modifications of the Skeleton is by recording the Joint offsets for all Joints (the small dots in the image on the right side).

The image shows the default SL Skeleton in its T-Restpose (the orange skeleton). And a modified Horse Skeleton (the grey Skeleton). The blue lines mark the offsets from original joints to the modified joints.

The good news is: You only need to move the joints of your Skeleton into place. For Blender users: You can do this by selecting your Skeleton (the Armature) in EDIT mode and then move the bones one by one.

So, how does Second Life know if a Skeleton uses joint offsets? The Second Life importer decides this for each joint by checking if its location is close to the default joint location of the SL Skeleton in Rest pose. As soon as a joint is offset by more than 0.1 millimeter it is counted as a joint with offset.

Note: The joint offsets are only checked when you enable “with joint positions” in the SL Importer’s Option tab.

The SL Viewer allows you to display the Skeleton Bones. You enable the bone display in the Develop menu under:

Develop -> Avatar -> Show Bones

• Joints are displayed as small dots (pink in the image for better visibility)
• Weighted Joints without Joint offset are displayed in Cyan
• Weighted Joints With Joint offset are displayed in Red

The model shown in the image above has only its right arm edited into an A Rest Pose. You can see the joint locations are displayed where we expect them to be. But the Bone orientation (the lines) are not rotated as you might expect.

If you take a closer look into this then you find the Bones orientation is always taken from the Restpose orientation. Because of this the bones of the right arm do not line up as expected but point to the right (left from your point of view).

The Appearance slider behavior depends on whether a joint has offsets defined or not:

• For regular joints (without offsets) the appearance sliders can influence the joint location (Translation) and the Scale
• As soon as a joint has an offset, the Translation part is disabled

But you also must know that the orientation of the Joint scaling does not change. This is important as you can see in the image aside. In this example the right arm has been edited to point straight downwards. While the left arm remains unchanged.

This results in drastic differences how sliders influence the arm length.

Now lets turn back to the horse from our initial example (see above). Lets say the mChest Bone of this horse has not been used for weighting the mesh (just as an example). The mChest bone lies in the middle of the Bone chain that starts at the mPelvis and ends at the mHead

Since Bento it is possible to only import partial skeletons. Especially you can decide to only export the weighted bones of your character (because unweighted bones are not used for deforming your mesh). So, when you import this horse with the Bento Viewer, then the mChest bone is not imported. But…

During importing your character all “missing” bones are taken from the original skeleton. But the SL Importer has no way to find out where the mChest bone has been placed in the original model. So it simply keeps the mChest bone at its default location . And this results in a skeleton where no joint position is defined for the mChest.

Bone
mCollarRight
mShoulderRight
mElbowRight
mWristRig

 Pivot-Rig location offsets
-0.020927 -0.085000  0.165396
 0.000000 -0.079418 -0.000000
 0.000000 -0.248000 -0.000000
-0.000000 -0.205000 -0.00000

 Pos-Rig location offsets
-0.021 -0.085 0.165
 0.000 -0.079 -0.000
 0.000 -0.248 -0.000
 0.000 -0.205 -0.00

For example the mElbowRight joint is exactly 0.248 m (24.8 cm ) away from the mShoulderRight joint in both Rigs (see image)

But  the joints mCollarRight and mShoulderRight are placed at slightly different locations in both rigs (see the numbers in the table)

I have marked the differing numbers in orange boldface.

POS Rigs

Are only used by the Secondlife System character. The POS Rig is declared as the one Rig that has no Joint Offsets.

So when you import a mesh that was rigged to a clean POS Skeleton then even if you Import with Joint Positions the SL Import willl correctly see that there are no Joint offsets at all defined in the rig.

PIVOT Rigs

Are only used by the Custom Mesh attachments and Custom Mesh characters.

So when you import a mesh that was rigged to a clean PIVOT Skeleton then the SL Importer detects falsely that some bones have Joint offsets.

When you use a clean POS Rig for your mesh character then an import with Joint offsets works just fine and correctly detects there are no joint offsets in the rig. But the Mesh gets bound to a PIVOT Rig and so you will see mismatches when you compare to the System Character.

When you use a clean PIVOT Rig for your mesh character then an import with Joint offsets uncorrectly detects joint offsets for some bones. But the Mesh gets bound to a PIVOT Rig and so it will match better when you compare it to the System Character. But there are still mismatches… why?

Besides the 26 standard bones and the 19 Collision Volumes we have attachment points. An attachment point is simply a location where you can attach (wear) other objects. The locations of the attachment points are fixed relative to a particular bone in the skeleton. Thus you might believe that there is not much to say about these points, but technically the attachment points are just another set of bones in the skeleton.Indeed it is possible to animate attachment points in the same way as you can animate the skeleton bones, and anything you’ve attached to those points will go along for the ride with the animation itself. These bones can also be used for weighting custom meshes just like the standard set of bones.

What we have so far

So we have effectively 77 bones available for animation and rigging. Furthermore 20 Shape keys do influence bone lengths, and 23 shape keys do influence the Collision Volumes. All together (when combining the bone shape keys and collision volume shape keys) we find that 30 Shape keys do influence the mesh and its skeleton provided it is correctly rigged and weighted.So far this is what we found by inspection and experimentation and this reflects our current knowledge about the Skeleton. In the following chapter we will take a closer look on practical issues.

Summary

The Skeleton and the associated Shape key system provide:

• 26 standard bones
• 32 attachment points
• 24 collision volumes
• 120 Shape keys

30 of the shape keys influence the Mesh and/or the Skeleton and thus they can be used for limited customization of custom meshes.