CondMDI is the unified diffusion model from Flexible Motion In-betweening with Diffusion Models (Cohan et al., SIGGRAPH 2024). It accepts text together with arbitrary observed frames or joint subsets. The Motius release packages the official randomly sampled frames-and-joints checkpoint behind one pipeline for text-to-motion, keyframe in-betweening, trajectory control, and partial-body control.

Preview

HumanML3D Sample	Input Text	SMPL Preview
`014457`	the person swings a golf club.	CondMDI HumanML3D 014457 SMPL demo
`001840`	hands in fighting position while the left foot kicks aggressively up and over.	CondMDI HumanML3D 001840 SMPL demo
`006944`	the person who does arms straight out and then it's doing something with their right hand in front of their face.	CondMDI HumanML3D 006944 SMPL demo

512px / 30fps GIF previews rendered from released HumanML3D test outputs.

Release Snapshot

Item	Value
Method	Conditional Motion Diffusion In-betweening (CondMDI)
Tasks	T2M, Motion Control
Venue	SIGGRAPH 2024
Training data	HumanML3D
Native representation	HumanML3D-263 with absolute root rotation and translation, 20 fps
Public I/O representation	Standard HumanML3D-263, physical scale, 20 fps
Text encoder	OpenAI CLIP ViT-B/32, frozen
Default sampler	DDIM, 100 steps, classifier-free guidance 2.5
Checkpoint	`ZeyuLing/motius-condmdi-humanml3d`
Pipeline	`motius.pipelines.condmdi.CondMDIPipeline`

The Hugging Face artifact is self-contained apart from the frozen OpenAI CLIP text encoder. It contains SafeTensors weights, the exact network and diffusion configuration, and the official absolute-root normalization statistics. No upstream source checkout or dataset directory is needed at runtime.

For offline inference, set MOTIUS_CLIP_PATH to a local OpenAI CLIP ViT-B/32 checkpoint. MOTIUS_CLIP_CACHE can instead redirect the normal CLIP download cache.

Usage

Install the method-specific dependencies:

pip install -e ".[condmdi]"

Text-to-motion generation:

python

from motius.pipelines.condmdi import CondMDIPipeline

pipe = CondMDIPipeline.from_pretrained(
    "ZeyuLing/motius-condmdi-humanml3d",
    bundle_kwargs={"respacing": "ddim100"},
    device="cuda",
)

motions = pipe.infer_t2m(
    ["a person walks forward and waves with the right hand"],
    [120],
    seed=42,
)

First-and-last-frame in-betweening uses a standard HML263 reference motion:

python

controlled = pipe.infer_control(
    ["a person turns around and walks away"],
    [reference_hml263],
    control_mode="first_last",
    transition_length=10,
    seed=42,
)

Other built-in control modes include start, sparse, prefix, suffix, middle, trajectory, lower_body, pelvis_feet, pelvis_vr, and joints. For arbitrary controls, pass an (B, 263, 1, T) Boolean observation_mask or provide keyframe_indices. All returned arrays have shape (T, 263) in the standard, denormalized HumanML3D representation.

Evaluation Results

Text-to-Motion

Protocol: all 4,042 motions are generated from the HumanML3D selected-caption test manifest. The official evaluator consumes 3,970 valid HumanML3D clips; the MotionStreamer retrieval evaluator consumes 4,032 complete batch entries; the Motius evaluator pairs 4,034 SMPL-22 motions. Results use one deterministic generation per caption and one metric repeat. For FID and MM-Dist, lower is better.

Evaluator	Samples	R@1	R@2	R@3	FID	MM-Dist	Diversity
HumanML3D Official	3,970	0.449	0.642	0.749	0.294	3.218	9.795
MotionStreamer Evaluator	4,032	0.453	0.611	0.702	121.837	19.970	25.464
Motius Joint-Position Evaluator	4,034	0.430	0.604	0.702	349.987	39.127	55.795

The Motius row reports raw embedding-space FID for consistency with the public T2M leaderboard; its L2-normalized FID is 0.1919. MotionStreamer and Motius evaluation first convert every output through the same SMPL-22 skeleton bridge.

Physical diagnostics use all 4,042 converted SMPL motions. Lower is better for all metrics; PoseQ is the MBench NRDF pose-quality score.

Slide	Float	Jitter	Dynamic	Penetration	PoseQ
4.222	18.689	6.937	21.509	0.000	1.830

Motion Control

Control results use 4,012 HumanML3D test motions. Start 1f observes the first frame, Both 1f observes the first and last frames, Prefix 20 observes the first 20 frames, and Middle 80 observes a centered 80-frame interval.

Setting	Evaluator	R@1	R@2	R@3	FID	MM-Dist	Diversity
Start 1f	MotionStreamer	0.529	0.688	0.766	64.106	18.672	26.462
Start 1f	Motius Joint-Position	0.492	0.661	0.751	107.142	34.124	55.393
Both 1f	MotionStreamer	0.568	0.730	0.801	54.043	18.186	26.787
Both 1f	Motius Joint-Position	0.561	0.734	0.814	56.623	31.615	54.927
Prefix 20	MotionStreamer	0.402	0.536	0.596	166.292	21.075	24.323
Prefix 20	Motius Joint-Position	0.374	0.518	0.600	428.528	40.855	51.799
Middle 80	MotionStreamer	0.484	0.628	0.707	123.567	19.812	25.010
Middle 80	Motius Joint-Position	0.466	0.622	0.706	269.269	36.836	52.746

The following reconstruction and physical diagnostics are computed on the same 4,012 cases after conversion to the shared SMPL-22 skeleton. MPJPE and P-MPJPE are in meters; lower is better for every column.

Setting	Full MPJPE	Generated-region MPJPE	P-MPJPE	Jitter	Foot skating
Start 1f	0.1339	0.1345	0.0126	46.206	0.1601
Both 1f	0.1134	0.1144	0.0206	49.102	0.1829
Prefix 20	0.1007	0.1235	0.0105	25.850	0.0726
Middle 80	0.0945	0.1138	0.0189	34.526	0.1240

Motion Representation

The official CondMDI model changes the four root channels of HumanML3D-263 from root-relative velocities to absolute yaw and horizontal translation. All remaining joint, rotation, velocity, and contact channels keep their original HumanML3D layout.

Motius performs this conversion inside the pipeline:

Standard HML263 input is integrated into the official absolute-root form.
The official normalization statistics are applied before diffusion.
The generated root trajectory is converted back to standard relative HML263 before it is returned.

This keeps public CondMDI outputs compatible with the representation toolkit, SMPL renderer, and all three T2M evaluators. The conversion round-trip matches the official formulation to floating-point precision for every recoverable frame; as with standard HML263, the final forward root delta is not encoded.

Motius Components

Component	Path
Pipeline	`motius.pipelines.condmdi.CondMDIPipeline`
Bundle	`motius.models.condmdi.CondMDIBundle`
UNet and diffusion runtime	`motius.models.condmdi.network`
HumanML3D selected-caption runner	`tools/eval_condmdi_humanml3d.py`
Official checkpoint exporter	`tools/export_condmdi_hf.py`

The vendored method runtime retains the upstream MIT license in motius/models/condmdi/LICENSE.

Reproduction Check

The migrated network was checked against the official implementation using the same checkpoint, text embedding, input tensor, and diffusion timestep. A single UNet forward pass differs by at most 1.41e-5 (6.45e-7 mean absolute error). For a complete 100-step fp16 sample, accumulated mean absolute error is 8.62e-4 (1.59e-2 maximum).

Citation

bibtex

@inproceedings{cohan2024flexible,
  title={Flexible Motion In-betweening with Diffusion Models},
  author={Cohan, Setareh and Tevet, Guy and Reda, Daniele and Peng, Xue Bin and van de Panne, Michiel},
  booktitle={ACM SIGGRAPH 2024 Conference Proceedings},
  year={2024}
}

Direct Loading

python

from motius import Pipeline

pipeline = Pipeline.from_pretrained("ZeyuLing/motius-condmdi-humanml3d")