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docs/user/main-feature.md

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+# Main Feature
+
+This document explains the main interactive feature currently being prototyped in La-Fabrik: grabbing and moving 3D objects with hand tracking.
+
+## What It Does
+
+In debug mode, the player can use their webcam to control object grabbing in the physics scene.
+
+The intended user flow is:
+
+1. Open the app with `?debug`.
+2. Switch the scene to `Physics` in the debug panel.
+3. Move close to a grabbable object.
+4. Show a hand to the camera.
+5. Close the hand into a fist near the object.
+6. Move the hand to move the object.
+7. Open the hand to release the object.
+
+## Why It Matters
+
+This prototype tests whether La-Fabrik interactions can feel more physical and embodied than a classic mouse or keyboard interaction.
+
+For the final experience, this can support low-tech repair gestures, object manipulation, and more expressive interaction sequences.
+
+## Current Behavior
+
+The feature works with one or more detected hands. A hand is considered active for grabbing when the backend detects a closed fist.
+
+When the fist starts close enough to a grabbable object, the object attaches to the hand target. The object then follows the hand center in screen space and also reacts to relative hand depth.
+
+Moving the hand left, right, up, or down moves the object in that direction. Moving the hand closer or farther from the camera changes the object's distance from the camera.
+
+## Debug Requirements
+
+Hand tracking currently requires:
+
+- Chrome or another browser that allows `getUserMedia()` reliably
+- the local Python backend running
+- the local MediaPipe model file available in `backend/hand_landmarker.task`
+- the app opened with `?debug`
+- the debug scene set to `Physics`
+
+Backend command:
+
+```bash
+source backend/.venv/bin/activate
+python -m backend.main
+```
+
+Frontend command:
+
+```bash
+npm run dev
+```
+
+Debug URL:
+
+```txt
+http://localhost:5173/?debug
+```
+
+## On-Screen Feedback
+
+The debug build shows several helpers:
+
+- a hand tracking status panel
+- a hand landmark wireframe
+- the `lil-gui` debug panel
+- the `r3f-perf` performance panel
+- optional interaction spheres
+
+The wireframe turns yellow when the detected hand is a fist.
+
+## Current Limitations
+
+- The feature is still a prototype.
+- It is enabled only in the debug physics scene.
+- The SVG hand wireframe is temporary.
+- Depth movement depends on relative webcam tracking and may need tuning.
+- The system has not yet been integrated into final mission gameplay.
+
+## Expected Next Improvements
+
+- Smooth the hand position and depth signal.
+- Add a better 3D hand representation.
+- Add calibration controls for grab radius and depth sensitivity.
+- Connect hand gestures to final repair or transformation tasks.