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motion-bricks.cpp

A C++23/GGML port of NVIDIA MotionBricks for CPU and Vulkan inference, with a stable C ABI suitable for PureGo.

The released batch-one G1 inference path is implemented end to end: strict GGUF loading, root/duration planning, pose-token prediction, VQ decoding, 418/414/413 feature conversion, style alignment, and skeletal animation output. CPU and Vulkan use the same public API and preserve the same duration and pose-token decisions in the reference suite.

Build

The normal build uses CMake and does not depend on Nix:

cmake --preset debug
cmake --build --preset debug
ctest --preset debug

Configuration downloads and SHA-256-verifies the published 0.73 GB G1 F32 GGUF and style bundles into generated/ when they are not already present. The same operation can be run explicitly:

python scripts/download_gguf_weights.py

For an offline or source-only build, preserve an existing local bundle or use cmake --preset debug -DMOTIONBRICKS_DOWNLOAD_MODELS=OFF. The repository and revision are configurable with MOTIONBRICKS_MODEL_REPOSITORY and MOTIONBRICKS_MODEL_REVISION.

On NixOS, enter the reproducible development shell first:

nix develop
cmake --preset debug
cmake --build --preset debug
ctest --preset debug

When the pinned ggml/ submodule is present, it is included automatically. The non-neural ABI and validation subset can also be built without GGML:

cmake -S . -B build/debug -G Ninja -DMOTIONBRICKS_ENABLE_GGML=OFF

The Go binding and demo use PureGo to load libmotionbricks at runtime; they do not use cgo or import "C". Once the native shared library has been built, the Go components therefore need no C compiler and can be built with cgo explicitly disabled:

cd demo
CGO_ENABLED=0 go build -o ../build/debug/bin/motionbricks-demo .

CGO_ENABLED=0 is optional but recommended for making this property explicit in builds and CI. It affects only the Go build—the native C++ library is still built separately with CMake.

The sanitizer lane is:

cmake --preset asan-ubsan
cmake --build --preset asan-ubsan
ctest --preset asan-ubsan

Current ABI

The installed C API uses only fixed-width scalars, pointers, and opaque heap handles. Callers never reproduce a C or C++ structure layout. All constructors have matching free functions, and no C++ exception crosses the ABI boundary.

The current CLI can report ABI information:

./build/debug/bin/motionbricks-cli abi

After producing the trusted safetensors intermediates described in reference/README.md, build and inspect an F32 runtime bundle with:

python scripts/convert_to_gguf.py \
  --safe-directory generated/safe \
  --output generated/g1-f32
./build/debug/bin/motionbricks-cli inspect generated/g1-f32

The released G1 inference path contains exactly 183,148,382 learned F32 parameters. The bundle loader validates the upstream revision, source checkpoint identities, component roles, tensor counts, parameter counts, anchor shapes, and the 34-joint parent topology before accepting a model.

Convert the 15 original demo styles with:

python scripts/convert_styles.py \
  --safe-directory generated/safe \
  --output generated/styles

At runtime the high-level flow is: load one immutable model, load one or more .mbstyle assets, create an agent, reset it from an initial style (or supply at least four frames of G1 context), set movement/facing/style on a command, then call mb_agent_plan. The returned motion owns row-major F32 root translations [frames,3] and local XYZW rotations [frames,34,4]. Call mb_agent_advance as playback progresses so replanning uses the generated motion as its next context.

The current implementation covers original preprocessed G1 styles. Direct Kimodo GLB-to-.mbstyle conversion remains subsequent integration work.

Weights

Ready-to-run native weights and all 15 upstream style primitives are published as MotionBricks-G1-GGML under the Hugging Face LocalAI-io organisation. NVIDIA currently distributes MotionBricks checkpoints through Git LFS in NVlabs/GR00T-WholeBodyControl, not a separate Hugging Face model repository, so the model card links to that pinned upstream revision. The downloader verifies the version-controlled distribution manifest before accepting any file.

The default build is pinned to Hugging Face commit cc2a47603dbc203a4f18f35dd06ed3611833f506 rather than the mutable main branch.

Interactive demo

The initial Go/Three.js demo renders the model’s 34-joint skeleton alongside the four actual placed target-keyframe ghosts. It lets you steer with W/A/S/D, turn facing with the arrow keys, orbit/zoom the camera, and switch among the converted upstream styles. It uses the reusable PureGo binding and the same stateful native agent as other applications. See the demo guide for build, run, architecture, and headless-Chromium test instructions.

Design

• Human-led design
• Implementation sketch and plan
• Versioned formats
• Go/Three.js demo
• Pinned upstream reference

License

motion-bricks.cpp source code is licensed under the Apache License 2.0. NVIDIA’s original model weights and converted GGUF/style distributions remain under the NVIDIA Open Model License reproduced with the published model. It permits derivative models and redistribution with conditions including retention of the agreement and attribution, Trustworthy AI terms, and trade compliance. Bundled third-party components retain their own licenses; the vendored Three.js files are covered by demo/web/vendor/THREE-LICENSE.txt.