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NullBackbone

NullBackbone is a reference Backbone that returns the rest pose for every request. It exists so DCC plugin authors can develop against a real MMCP server with zero ML or GPU dependencies.

Run it​

The package ships a CLI entry point:

python -m motionmcp.null_backbone
# → MMCP server on :8000, model id "null"

Or programmatically:

from motionmcp import serve
from motionmcp.null_backbone import NullBackbone

serve(NullBackbone())

Hit it like any MMCP server:

curl -s http://localhost:8000/capabilities | jq
curl -s http://localhost:8000/generate \
  -H 'Content-Type: application/json' \
  -d '{ "protocol_version":"1.0", "model":"null",
        "skeleton":{"joints":[{"name":"Hips","parent":null,
                               "rest_translation":[0,0.95,0],
                               "rest_rotation":[0,0,0,1]}]},
        "segments":[{"type":"text","prompt":"stand","duration_frames":5}] }'

Customising​

from motionmcp.null_backbone import NullBackbone
from motionmcp import Skeleton, Joint, serve

serve(NullBackbone(
    model_id="my-fake-model",      # what shows up in /capabilities
    fps=24.0,                      # claim a different fps
    canonical_skeleton=Skeleton(joints=[
        Joint(name="Hips", parent=None,
              rest_translation=(0.0, 0.95, 0.0),
              rest_rotation=(0.0, 0.0, 0.0, 1.0)),
        # …your studio's actual skeleton…
    ]),
))

This is useful when:

  • Your DCC plugin expects a specific skeleton naming.
  • You want to demo your real model id without booting the real model.
  • You're stress-testing transport with num_samples > 1 or large durations.

CLI flags​

python -m motionmcp.null_backbone --help

usage: motionmcp-null [-h] [--host HOST] [--port PORT] [--model-id MODEL_ID] [--fps FPS]

  --host HOST          default 0.0.0.0
  --port PORT          default 8000
  --model-id MODEL_ID  default "null"
  --fps FPS            default 30.0

What it returns​

For any request, NullBackbone.generate() builds:

  • rotations — identity quaternion (0, 0, 0, 1) per joint per frame per sample.
  • root_translations — the root joint's rest_translation, held constant across all frames.

Multiple samples (options.num_samples > 1) all return the same rest pose; the SDK still encodes them as separate animations so client code that expects N samples works.

When to graduate​

NullBackbone is a development aid, not a real motion model. As soon as you have actual model weights, swap to your own Backbone subclass — see Backbone → for the full surface.

You can keep NullBackbone registered alongside your real model for smoke tests:

serve({
    "soma":      MyRealBackbone("weights.pt"),
    "null":      NullBackbone(model_id="null"),   # cheap probe target
})