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Introduction to Custom Control Rigs for FBX Export

What is a Control Rig?

Limitations in Rigging in Maya for Unity

Preparing Geometry for Rigging

Creating a Clean Skeleton

Creating a Clean Rig Hierarchy

Joint Scaling in Maya

Bind Skin Settings

Use of Blend Shapes

Rig Cleanup

Afterword

Intro to Working with Control Rigs

Animators use control rigs to control assets used to create animations. In this workflow, you’ll learn guidelines and best practices for creating control rigs in Maya. These guidelines will help you create custom control rigs that can be cleanly exported as FBX files for use in Unity.

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This tutorial has been verified using Unity 2019.4 LTS

Animators use control rigs to control assets used to create animations. In this workflow, you’ll learn guidelines and best practices for creating control rigs in Maya. These guidelines will help you create custom control rigs that can be cleanly exported as FBX files for use in Unity. 

Baking Animation onto Joints and Blendshapes

This workflow assumes you have experience using Maya and are familiar with the following terms/concepts:

A control rig drives the underlying skeleton and blend shapes that in turn drive the Mesh. Often the skeleton is templated or hidden so that animators don’t interact with the joints directly. Instead, Non-Uniform Rational Basis spline (NURBS) curves are generally used as controls for the animators to animate the asset. By using a control rig, you can create complex rig setups that can be animated using simplified controls.  

It’s important to understand the limitations when creating control rigs for the purpose of exporting as FBX to Unity. By working within these limitations, you’ll ensure that the work you do in Maya will carry through correctly to Unity.

The only deformers that can be exported as FBX files are skinclusters and blend shapes.

Unsupported deformers include lattices, clusters, wraps, and nonlinear deformers.



Only static blend shapes can be used. This means you can’t connect a Mesh that’s  being driven by a skincluster, or other deformers as a blend shape into the final Mesh.



Classic Linear is the only supported skinning method (Dual Quaternion and weight blended are not supported).



In Unity, four joints per vertex is the max number of influences supported for skinning. (Note: this limitation is only in Unity versions 18.3 and older.)

After your model is complete, but before you start rigging, you should do some quick cleanups to prevent issues during the rigging process.


Check Scale of Model: Make sure your asset is scaled to the correct size relative to other assets. This prevents having to scale the asset after it’s rigged, which can be a problem if the rig isn’t set up to do so. 

Check Placement of Model: Make sure your asset is centered at origin and sitting on the ground plane.

Delete History on Geometry: Make sure to delete history on all geometry (Figure 01). Leaving history on a Mesh can cause issues, such as sliding textures, and can also affect rig performance. 

1. To delete history, go to Edit > Delete by Type > History. Or you can use Alt + Shift + D.
 

Freeze Scale Transforms on Geometry: Unfrozen translate and rotate values on geometry can be useful for rigging. However, scale values can cause issues later on. (Figure 02)

1. To freeze scale, right-click on the channel box and select Freeze > Scale; or, from the main menu, go to the Modify > Freeze Transformations option box and toggle scale only. 

Note: When freezing scale with negative values, you may end up with flipped normals. To correct this, use [Modeling] Mesh Display > Reverse. 

Figure 01: Channel Box before and after deleting history

Figure 02: Channel Box before and after freezing scale

Once you’ve placed your skeleton joints, you’ll need to make sure there are no rotation or scale values, that orientations are correct, preferred angles are set, and joints are mirrored. 

It’s important to clean up rotation and scale values on a skeleton before skinning to Mesh or attaching a control rig. (Note: translations cannot be frozen on joints.)

1. With the top joint in the Hierarchy selected, go to Modify > Freeze Transformations.
 

Orienting joints in a skeleton is an important step. The main tool for this is the Orient Joint tool. (Figure 03) 

2. Open the Orient Tool option box: [Rigging] Skeleton > Orient Joint. 

3. Use the Orient Tool to orient a primary axis down the bone and a secondary axis as an up vector.

4. The axis you choose as a main axis depends on the rig and the standards you choose. 

Note: A good rule is to have the main axis match the direction of the joint chain in World Space.

5. If you want to use an Inverse Kinematics (IK) setup on a joint chain that’s not bent (like a straight arm or leg), you’ll need to set the preferred angle. This lets the IK setup know which way to bend. In the image below (Figure 04), the elbow is rotated and then the preferred angle is set.

6. To set the preferred angle, rotate the joint in the direction it should bend. Right-click and hold on the joint and select Set Preferred Angle, then rotate the joint back to its default position.

7. If you are working with a symmetrical asset like a biped character, use the Mirror Joint tool (Figure 05) to mirror from one side to the other. 

8. Open the Mirror Joint Tool box next to the Rigging dropdown menu > Skeleton > Mirror Joints 

Figure 03: Orient Joint tool

Figure 04: Set Preferred Angle

Figure 05: Mirror Joint Tool

Having a clean Hierarchy in the Outliner is key to being able to export your work to FBX. A clean rig Hierarchy is well-named and neatly organized.


Name All Nodes: Make sure all nodes are named and no two nodes have the same name. 

Organize Hierarchy: Keep your control rig in a separate hierarchy from your rig (skeleton and geometry). This will allow you to select only the rig and not the controls for export. (Figure 06)

Connect Control Rig: Use constraints, direct connections, and IK systems from controls in the control rig to drive the skeleton and blend shapes. 

Figure 06: Example of a clean hierarchy in the Outliner with control rig separate from rig

Joint scaling in Maya works differently than scaling of other nodes. Unlike other nodes, when you scale a joint, its children don’t scale with it. This is a useful feature that can be turned on or off with the Segment Scale Compensate attribute.

This attribute is found only on joints and is on by default. It controls how a joint’s scale affects the child joints under it in the hierarchy. When enabled, it prevents the scaling of child joints. This is useful for something like stretchy limbs, where you want to stretch the arms but not the hands. If you want to scale a hand and have the fingers scale with it, you’ll need to disable Segment Scale Compensate on the hand joint.

3. Toggle the Segment Scale Compensate checkbox on or off as needed. (Figure 07) 

Figure 07: Segment Scale Compensate checkbox in Attribute Editor

When you bind your Mesh to your skeleton, there are a few important settings you should use. (Figure 08)


Skinning Method: Use the Classic Linear skinning method. The other options (Dual Quaternion and weight blended) are not supported.

Max Influences: This is the max number of joints that can influence any single vertex on a Mesh. Set this value to 4, which is the max number for Unity versions 18.3 and older.

Maintain Max Influences: Checking this box will ensure you do not unintentionally exceed the max influences while painting skin weights. 

 Note: These settings can also be set after skin binding by selecting the skin cluster and finding these settings in the Attribute Editor. 

Figure 08: Important Bind Skin Options settings

Blend Shapes are useful for creating facial rigs and maintaining volume in areas such as knees, elbows, hips, and shoulders.

Blendshapes can be connected to custom attributes on controls via direct connections or set-driven keys.

They can be connected to Transforms, such as the rotation of an elbow joint to turn on a volume-correcting shape. 

Blend shapes can also be used to create squash and stretch in your rigs. For example, you can have the length of the spine drive fat/thin blend shapes for the body.

After you’ve finished your control rig, you may wish to do some cleanup steps to help prevent animators from accidentally breaking the rig. These steps include locking Transform channels (translate, rotate, scale), hiding/templating the skeleton, and setting limits on control Transforms. 

Any Transform attributes on controls that are not intended to be used by animators should be locked. All groups and geometry should also be locked.

Do not lock rotations on joints driven by an IK system, as this will prevent the IK from evaluating.

It’s a good idea to lock visibility on all nodes so an animator doesn’t intentionally or unintentionally hide part of the rig or model. Any necessary visibility control should be connected to custom attributes on a rig control. 

1. To lock channels, highlight the attributes you wish lock in the channel box and right-click on the attributes.

Hiding the skeleton by turning off its visibility attribute will remove some of the visual clutter in your rig. It will also reduce the chance of an animator interacting directly with the skeleton and potentially causing issues in the scene.

If you still want to see the skeleton but not make it selectable in the scene, then templating it is a good option.

1. To template the skeleton, select the top joint in the skeleton and go to Display > Object Display > Template.

Limits are restrictions on the value range of an attribute. Setting limits on the Transforms of a control in the Attribute Editor can help prevent animators from pushing a rig past its limits. 

To set limits on control Transforms (translate, rotate, scale): 

3. Go to the Limit Information section. (Figure 09)

4. Click the check boxes for the min/max values of the Transforms you want place limits on.

Note: While limits can be very useful, they should be used sparingly and only in instances that will help the animators. For instance, you may want to avoid using limits on areas that rotate in more than one axis, such as a shoulder control. In these cases, limits are likely to cause problems rather than help the animator. 

Figure 9: Limit Information section in Attribute Editor

These guidelines and practices will allow you to create your own custom control rigs in Maya. These control rigs can be animated in Maya and then exported as FBX for use in Unity.  

Intro to Working with Control Rigs

1. Introduction to Custom Control Rigs for FBX Export

2. What is a Control Rig?

3. Limitations in Rigging in Maya for Unity

4. Preparing Geometry for Rigging

5. Creating a Clean Skeleton

6. Creating a Clean Rig Hierarchy

7. Joint Scaling in Maya

8. Bind Skin Settings

9. Use of Blend Shapes

10. Rig Cleanup

11. Afterword

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