Rigging and weight painting

Window > Rigging turns a still model into a rigged one that a Skinned Mesh Renderer can animate, the way Mixamo does: place a few markers on the model, and the skeleton and skin weights are made for you. It also works for four-legged animals (cats, dogs, horses…) and for anything else, with bones you place yourself.

A person gets Mixamo’s bone names (Hips, Spine, LeftArm, LeftForeArm, LeftHand, …), so animations made for Mixamo characters play on it straight away, and it is recognised as a humanoid for retargeting (docs/retargeting.md).

Rigging a model

1. Select an object that shows the model: a Mesh Renderer with a .glb or .gltf, or an editable model from the Model window (written out as Models/<name>.glb first). Rigging an object that already plays a _Rigged.glb made here picks up its still model again.
2. Open the Rigging tab (beside the Inspector; Window > Rigging brings it back if closed, and the editor reopens whichever windows were open last time) and press Rig. The model is shown on its own (untick Show the model on its own to see the scene round it), and the camera turns to face it: a person from the front, an animal from its side. Front View / Side View turns it back.
3. Work through the four steps at the top of the window.

1. Markers

The template is guessed from the model’s shape (longer than it is tall: a four-legged animal). Change it if the guess is wrong:

Drag the markers onto the model in the Scene view; the skeleton follows as you drag. Hover a marker for a hint about where it goes (for example “the back of the knee, where the leg bends”). With Symmetry on, moving a left marker moves the right one too. With Centre markers in the body when let go on, a marker you let go is moved to the middle of the part it is on, along the way the camera looks, so markers placed on a flat front view end up inside the limb, as Mixamo’s do. Paws, the nose, the tail tip, ear tips and fingertips stay on the surface.

Click a marker’s name in the list to bring the camera close to it. Fingertip markers are drawn small and named only when you point at one; zoom in on a hand to place them. Each finger’s bones run from its knuckle to its tip marker (the thumb’s knuckle near the wrist), so moving a tip moves the whole finger. The fingertips are guessed from the hand’s shape beyond the wrist: the thumb is the point sticking out furthest to the front, and the four fingers are the ends of four strips across the hand, from the first finger at the front to the little finger at the back.

An animal’s head turns about its Head marker, so a long snout or a nose moved far out only makes the head longer; the ears and jaw hang from it. The neck’s base is guessed over the front legs and the tail’s base where the thin tail meets the rump, and the tail tip is followed to its end, even when the tail curls up over the back.

Faces says which way the model’s front is (glTF’s front is -Z). It is guessed from the feet for a person and from the longest side for an animal. Guess Markers Again starts over.

Build Skeleton and Weights goes on to step 3 with weights made.

2. Bones

Click a joint to pick its bone, and move it with the gizmo. The list shows every bone under its parent.

• Symmetry moves the bone on the other side too. Move children brings the bones below along.
• Name, Joint (its place) and Bends the mesh (bones that only hold others, like the ends of fingers, do not take weights).
• Add Child Bone and Delete Bone (its weight goes to its parent, and its children join the parent). Shift+click the model to add a bone there, joined to the picked one.
• Mirror to the Other Side copies this side’s bones to the other side, making any that are missing (Left... to Right..., _L to _R, .L to .R).

Moving bones after the weights were made says so; Make Weights Again brings them up to date.

3. Weights

Automatic weights use “bone heat”, as Blender and Mixamo do: each bone warms the parts of the model it is nearest to and can see (no part of the model in between, so an arm does not move the side it hangs beside), and the warmth spreads over the surface, so weights fall off smoothly across joints. Vertices in the same place (seams where a texture is cut) are weighed as one, so seams never tear. Each vertex keeps its four strongest bones, adding up to 1.

Then paint them. The model is coloured by the picked bone’s weight, from blue (none) through green and yellow to red (all). Show can colour every bone in its own colour instead.

Size and Strength set the brush. Ctrl+click the model picks the bone that moves that spot most. Mirror paints the other side too (the window says how much of the model has a mirror image). Flex while painting swings the bones while you paint, to see the result at once. Smooth All, Clean Up (four bones each, none under 1%), Mirror Left to Right / Right to Left and Make Weights Again work on the whole model. Ctrl+Z undoes each stroke and change.

4. Pose

Click a joint and turn it with the rotate gizmo (or type its Turn) to see how the model bends. Flex every bone swings them all. Poses are only for trying and are not saved; Reset Pose puts the model back.

Saving

• Export Rigged Model writes <model>_Rigged.glb beside the model: the model with its materials and textures, the skeleton, and the weights. Every skeleton made here faces the same way at rest (its top bone turns it to face the way the model does), so a clip made on one rigged person or animal plays the same on any other made here: people through their Mixamo bone names (and any other humanoid through retargeting, docs/retargeting.md), animals by their shared bone names (bones one animal has and another lacks, such as extra tail bones, stay at rest). Change the name in Rigged model to write it somewhere else.
• Export and Use on This Object also gives the object a Skinned Mesh Renderer playing it and an Animator, with Expose Bones on, so the bones are objects under it in the Hierarchy and Inspector, ready to pose, key in the Animation window, curl with Hand Posing (the Animation window’s Hands) and drive with IK. Give that a Clip, or a Clip Library of animations made for the same bones (Mixamo’s, for a person), to see it move.

The rig itself (template, settings, markers, bones and painted weights) is kept beside the model as <model>.trig, saved when you stop rigging or close the editor, so it can be picked up again. If the model changes, the rig keeps its markers and bones and says to make the weights again.

From code

The rigger is in the engine (Engine/src/Rigging/), so tools and tests can use it without the editor:

let rig = scope Rig();
let problem = scope String();
if (!rig.LoadSource("Models/Wolf.glb", problem))
	Log.Error(problem);
rig.template = RigTemplates.GuessTemplate(rig);
RigTemplates.GuessMarkers(rig);
let triangles = scope List<uint32>();
rig.Triangles(triangles);
let rays = scope RigRays(rig.positions, triangles);
RigTemplates.CentreMarkers(rig, rays);
RigTemplates.Build(rig);
AutoWeights.Make(rig);
RigExport.Write(rig, "Models/Wolf_Rigged.glb", problem);
rig.Save("Models/Wolf.trig");

WeightPainter paints (Dab, SmoothAll, MirrorWeights), WeightTools normalises and limits weights, and Rig.Posed skins the model in a pose. The heat solve runs in the native library (native/tallow_rig.cpp), on every core.

Checks