Skeletons

A Skeleton is a hierarchy that is used to define Bones (sometimes called joints) in a Skeletal Mesh. In some ways, these Bones mimic a real biological skeleton due to their position and control over how characters deform.

In Unreal Engine, Skeletons are used to store and associate animation data, the overall skeletal hierarchy, and Animation Sequences. Skeleton Assets can also be shared through a variety of methods, enabling for additional animations and data to be shared between different Skeletons.

This document provides an overview of how to create and use Skeletons.

Prerequisites

• Your project contains a Skeletal Mesh Actor, or you have a skinned FBX character to import into Unreal Engine.

Creating Skeletons

The primary way to create a Skeleton is to import a skinned character FBX, which then converts to a Skeletal Mesh in Unreal Engine. When importing a Skeletal Mesh, in the FBX Import Options window, leaving the Skeleton field empty will automatically create a Skeleton Asset based on the skinned character being imported.

After importing your character, the Skeleton Asset will be created along with other Skeletal Mesh Assets.

Double-click the Skeleton Asset to open the Skeleton Editor.

Skeleton Tree Information

Bones and other items displayed in the Skeleton Tree can appear different depending on several factors.

Animation Data Storage

In addition to controlling animation, Skeletons in Unreal Engine also store animation-specific data. When data is created from those sources, such as creating an Animation Notify in an Animation Sequence, it gets added to the Skeleton as shared data.

Skeletons store the following types of animation data:

• Animation Notifies.

• Animation Curves.

• Slots.

• Retarget Sources.

• Blend Profiles and Blend Masks.

This data can be viewed in dedicated tool panels by clicking Window in the Skeleton Editor menu, then enabling one or more of these panels.

Sharing Skeletons

An important feature of Skeleton Assets is that a single Skeleton Asset can be used by multiple Skeletal Meshes, so long as the Skeletal Meshes use the same general rig hierarchy. This means that the names and hierarchical order of your Bones must be consistent in order for sharing to work correctly.

For example, consider a limb with three Bones in a Skeletal Mesh. The bones are named 1, 2, and 3:

If you want to share this Skeleton with another Skeletal Mesh, you will need to keep these Bones in the same order and with the same names. The second Skeletal Mesh, however, can contain Bones that are additions or peripheral to the hierarchy. Any time animation data is received for a Bone that is not included in the Skeletal Mesh, that animation data will be ignored.

In that case, your new hierarchy could look like the image below. Here, the second Skeletal Mesh has extra Bones, while still retaining and not interfering with the original hierarchy from the first Skeletal Mesh.

However, in order for both Skeletal Meshes to use the same Skeleton Asset, you cannot change the hierarchy order or rename the Bones. If a second Skeletal Mesh uses a different Bone hierarchy and naming structure, you will need to create a new Skeleton Asset.

If you insert a bone into the hierarchy without changing the order, you will be able to share successfully. However in most cases the extra bone may cause unintended transform offsets in your skeleton. It is recommended that you avoid this if possible.

Taking these sharing rules into account, there are several ways you can share Skeletons between Skeletal Meshes in Unreal Engine. These are detailed below.

Merging during Import

The first method for sharing Skeletons is done during the FBX import process. When importing your new Skeletal Mesh (with additional and peripheral Bones compliant with the rules above), you can select a Skeleton from a Skeletal Mesh that already exists in your project. Unreal Engine will then merge the Skeletons, appending any new Bones into the hierarchy. Additionally, your Skeleton's proportions will be defined by the original Skeletal Mesh from which it was created.

When viewing your merged Skeleton, you will see these additional Bones listed in your hierarchy, but they will only be visible and active for the Skeletal Mesh they are intended for.

Compatible Skeletons

You can also share Skeletons in a non-destructive manner by using the Compatible Skeletons property in the Skeleton Editor. This allows for different skeletons to act similar to merging, but still maintains separate Skeleton Assets.

To make a Skeleton compatible with another, open the Skeleton Editor, then open the Asset Details panel by selecting Window > Asset Details from the main menu. In the Asset Details panel, locate the Compatible Skeletons property.

Click Add (+) to add a Skeleton entry to the list, and then select a different Skeleton to use.

Now, animations can be shared from the Skeleton that was added to the compatible list.

Skeleton Features

Skeletons in Unreal Engine support a variety of features for attaching, blending, and other settings. Refer to the following pages to learn more about these features: