Custom Model Creation

Custom Model Creation combines models. It has two sections: Merge Armatures, to join two rigged models into one, and Attach Mesh, to glue an unrigged object to a single bone.

Merge Armatures

Use it to put a rigged outfit, hair or body part onto an avatar. The section needs at least two armatures in the scene; with fewer, it only shows Two armatures are required!

Before you merge

• Run Fix MMD Model on both models if they are raw MMD or VRM imports, so their bones share the same standard names.
• Move only the mesh of the model you are adding into place. Cats moves its bones to match the base armature.
• Neither armature may share its data with another object. If one does, run Object > Relations > Make Single User first.

Merge

1. In Base, pick the armature that stays: usually the avatar.

2. In To Merge, pick the armature you are adding.

3. If Cats shows Armatures can be merged automatically!, the two share their main bone names and you can go on. Otherwise pick the bone of the base armature the new part should hang from in Attach to.

4. Set the options, then click Merge Armatures.

Cats reports Armatures successfully joined. Bones with the same name are merged, and their weights are added together.

Option	Default	What it does
Merge All Bones	Off	Merges every bone with the same name, not only the main bones (Hips, Spine and so on). Rename the bones you want merged so they match. Check the result afterwards: this can have side effects.
Apply Transforms	Off	Tick this if both models are already exactly where they should be. They stay in place while merging.
Join Meshes	On	Joins all meshes into one after merging.
Remove Zero Weight Bones	On	Deletes bones that move no vertices after the merge.
Cleanup Shape Keys	On	Removes shape keys that do nothing. Untick it if shape keys you want disappear.

Transform errors

Merging expects both armatures at the same scale, and the model you add with no rotation. If that is not the case, Cats stops with Transform Issue Detected! Either tick Apply Transforms, or select both objects and apply their transforms with Ctrl + A > All Transforms.

If an armature has a parent object, that parent must be at location 0, rotation 0 and scale 1, or have its transforms applied.

Attach Mesh

Use it for rigid accessories: a hat, glasses, a weapon. The mesh must have no parent yet.

1. In Base, pick the armature.

2. In Mesh, pick the unparented mesh.

3. In Attach to, pick the bone it should follow, such as Head for a hat.

4. Click Attach Mesh.

Cats parents the mesh to the armature, creates a new bone named after the mesh under the chosen bone, and weights the whole mesh to it. It reports Mesh successfully attached to armature.

Caution

Attach Mesh deletes the mesh’s existing vertex groups and assigns every vertex to one bone. That suits rigid objects only: a jacket or a skirt needs several bones and will not deform correctly. Apply the mesh’s scale first if it is not uniform.

The How to use button at the top opens the upstream documentation for this panel.