Data Model
The 21-Joint Skeleton
The product's coordinate system: a 21-joint, 20-bone topology from pelvis to ears, with a documented mapping from COCO-17 pose output.
Twenty-one joints, twenty bones
Everything the platform produces — reconstructions, embeddings, features, evidence — is expressed in one coordinate system: a 21-joint, 20-bone skeleton reaching from the pelvis to the ears, with heels and toes included deliberately. Ground contact is where movement quality lives; a topology that stops at the ankle cannot see it.
Joint ledger
The canonical serialization order, region, and how each joint is sourced from a COCO-17 pose estimate. Row color encodes provenance: direct keypoints, derived midpoints (violet), and extended-estimator joints (amber).
| Idx | Joint | Region | COCO-17 source |
|---|---|---|---|
| 0 | pelvis | core | midpoint(left_hip, right_hip) |
| 1 | neck | core | midpoint(left_shoulder, right_shoulder) |
| 2 | nose | head | nose |
| 3 | hip_left | left leg | left_hip |
| 4 | knee_left | left leg | left_knee |
| 5 | ankle_left | left leg | left_ankle |
| 6 | heel_left | left leg | extended estimator or model inpainting |
| 7 | toe_left | left leg | extended estimator or model inpainting |
| 8 | hip_right | right leg | right_hip |
| 9 | knee_right | right leg | right_knee |
| 10 | ankle_right | right leg | right_ankle |
| 11 | heel_right | right leg | extended estimator or model inpainting |
| 12 | toe_right | right leg | extended estimator or model inpainting |
| 13 | shoulder_left | left arm | left_shoulder |
| 14 | elbow_left | left arm | left_elbow |
| 15 | wrist_left | left arm | left_wrist |
| 16 | shoulder_right | right arm | right_shoulder |
| 17 | elbow_right | right arm | right_elbow |
| 18 | wrist_right | right arm | right_wrist |
| 19 | ear_left | head | left_ear |
| 20 | ear_right | head | right_ear |
Mapping from COCO-17
Most pose estimators emit the 17 COCO keypoints. Fifteen of the twenty-one joints correspond directly; the pelvis and neck are derived as midpoints; heels and toes come from extended estimators that add foot keypoints — or, where a source lacks them, from the model itself, which treats missing joints as one more mask to reconstruct. One consequence is worth stating plainly: when WithoutBall materials mention 17-keypoint pose output, that refers to the upstream computer-vision stage — the product’s own coordinate system is, and remains, these 21 joints.
Why these twenty-one
Each addition to the minimal pose earns its place. Heels and toes expose ground contact, stride events, and push-off mechanics. The pelvis anchors the kinematic tree and separates trunk motion from leg motion. The neck completes the trunk axis, making lean and counter-rotation measurable. Ears stabilize head orientation where broadcast angles hide the face.
The result is the smallest topology that still supports the biomechanical questions the platform exists to answer — asked through the model documented in the architecture reference.