Motion Retargeting - Bind many animations onto your characters

Motion retargeting can be a very trivial task. In reality, however, characters and rigs have all sorts of unconventional conventions.

Your mocap corpus may use one naming convention, the character that your studio rigged in-house uses another (or perhaps even none), and the library you licensed uses its own; we all have been there. For in-house things, conventions tend to go out of the window and it's a Wild West there.

If you are luckily working on something anatomically similar, such as a bipedal skeleton with same joints in more or less the same relative places, there still comes a problem such as labels not matching up; the solution will then be to hand-author a bone map for each pair in the hopes that you can map another animation on it. It was easy at first, then comes the problem: You have a number of clips and animations, and character – you are multiplying the two quantities.

Metrixel 2026.3.0 comes to the rescue, whereby we approach it this way – bone names may be arbitrary, but the biped’s shape, for example, remains the same. We can bridge the gap.

The Three Gates

When you run Motion Retargeting on Metrixel, each of the character files will be resolved on their own. The run’s result will also tell you which gate caught it:

• matched:<convention> - Joint names matched a known convention.
• inferred - Names do not mean much here. The skeleton’s structure did the identification.
• custom - A user-supplied JSON map was applied instead.
• unresolved - None of the above. Instead, a fillable starter map was written for the case.

We will be looking at the `inferred` result, which is far more interesting. What it means is that even if the in-house rig has not labelled anything or went against every conceivable naming convention, the structure will still be unmistakable. At the end of the day, it’ll be a bipedal – a pelvis, spine, two legs that connect to each foot, two arms that connect to each hand. It is all more than enough to place all fifteen core slots without needing recognizable name labels.

Why it's all-or-nothing

Inference runs are successful only when all fifteen core slots are resolved, along with the four sanity checks to pass. A partial inference is never used and instead, handed back as a starting file instead.

That restraint is meant as it is, since half of a mapped skeleton doesn't produce half a retarget, it’s either mapped or nothing. Rather than having a broken result in your runs, we figured that it will be much better to show the error or the lack of processing for that one file. 

When it just can't tell

If nothing is resolved, you will not get an error message. Instead, you will get a starter map listing with the rig's own joints in hierarchy order, all ready to be fill in. The tool will tell you what it saw, and hopefully you can work easier from there.

What else does Metrixel do besides naming

• Rest poses are reconciled. The limbs follow the source instead of inheriting the target's binding quirks.
• Root motion transfers. They are scaled to the target's proportions. A travelling clip also gets a follow camera.
• Sources do not need geometry. An animation-only FBX, glTF/GLB or BVH export is valid input, and they are also smaller to work with.
• Batch source folders. They are searched recursively and processed in sorted, deterministic order. The resolution is done per file, so a folder of mixed rigs will need no further organising.
• Everything is .aproj-persisted and CLI-scriptable. A retarget is a pipeline step rather than a GUI chore.

What it does NOT do

With the demo video of ours that may have implied otherwise, I believe that I should mention the following:

• Auto mode needs a recognisable biped. Other bits, such as quadrupeds, partial rigs, or heavily stylised skeletons fall back to a custom map.
• Fingers are all-or-nothing. If both hands aren't laid out conventionally, the hands will hold their rest pose. We believe that having them being animated wrongly is worse than leaving them.
• There are no cloth simulation and no mesh collision. Skinned geometry follows its weights, so for instance, a skirt weighted to a leg will intersect that leg when the leg swings.

Plans

Motion Retargeting is Professional only. It isn't on Standard or the Free tier.

Watch it happen!

There is a relevant video on our YouTube channel that does the demo – first a rig with deliberately meaningless joint names passed through a no-bone-map retarget on Linux, followed by the same process but with a folder of mixed-rig clips. See it here in action! Metrixel Motion Retargeting: Onto a Rig Nothing Recognises.