From Keypoints to Measurements: Why Landmarks Alone Are Useless

Every hand-tracking demo shows you 21 dots. The interesting part is what nobody shows: turning dots into numbers someone can act on. Dots are a capability, not a product Run any modern hand-tracking model and you get 21 beautifully stable landmarks per hand at 30 FPS. Impressive — and by itself, worthless. No client has ever paid for dots. They pay for measurements : is this clearance compliant, is this part aligned, did this patient's range of motion improve. I learned this on utility infrastructure work, where the deliverable was never "we detected the wire" — it was the attachment height of that wire, and whether it violates clearance rules . Keypoints were step one of three. The demo: live metrics, not just a skeleton My portfolio's keypoint demo derives three measurements per hand, every frame: const wrist = lm [ 0 ]; const palm = distance ( wrist , lm [ 9 ]); // scale reference const pinch = distance ( lm [ 4 ], lm [ 8 ]) / palm ; // thumb tip ↔ index tip The crucial line is the scale reference . Pixel distances are meaningless — they change as you move toward the camera. Dividing by palm length (wrist to middle knuckle) gives a relative measurement that's stable under distance, and multiplying by the average adult palm length (~8.5 cm) converts it into an approximate real-world gap — the demo shows "≈ 3.2 cm" floating on the pinch line. In infrastructure work the same role is played by a known object dimension — a standard crossarm, a pole class height. Every measurement-from-pixels system needs its ruler. Finger counting is a geometric test (is each fingertip farther from the wrist than its middle joint?), and "hand openness" averages fingertip extension — three lines of geometry each, but they convert a model output into a readout a human understands instantly. Honest layering The landmarks come from MediaPipe's pretrained pipeline (palm detector → landmark regressor → gesture classifier, float16, WASM + GPU delegate) — Google's models, credited on the page