neon-sprawl/docs/plans/NEO-15-implementation-plan.md

# NEO-15 — Implementation plan

## Story reference

| Field | Value |
|--------|--------|
| **Key** | NEO-15 |
| **Title** | E1.M2: Isometric follow camera (fixed yaw prototype; CameraState seam) |
| **Linear** | [NEO-15](https://linear.app/neon-sprawl/issue/NEO-15) |
| **Parent** | [Epic 1 — Core Player Runtime](https://linear.app/neon-sprawl/project/epic-1-core-player-runtime-client-controls-character-loop-66bd590cd016) |
| **Module** | [E1.M2 — IsometricCameraController](../decomposition/modules/E1_M2_IsometricCameraController.md); umbrella [E1.M2](https://linear.app/neon-sprawl/project/e1m2-isometriccameracontroller-deac8ef10395) |

## Goal, scope, and out-of-scope

**Goal:** Client-local isometric follow camera that tracks the E1.M1 player anchor with fixed framing for prototype play, while **modeling yaw in state** for future consumers and optional orbit.

**In scope**

- Godot: `Camera3D` (on a small rig) for isometric presentation; follow the player / follow target each frame per **locked defaults** below (smooth follow + teleport snap).
- **Player-facing controls:** no camera rotate / orbit; **yaw stays at default** (`0` rad) for normal play.
- **`CameraState`** (dedicated script type): follow target reference, **yaw** (always `0` in prototype UX), default distance or zoom index (**single band** / scalar distance until NEO-16).
- **Seam:** `@export` or config fields (e.g. `allow_yaw`, `max_yaw_deg`) default off; short comment where orbit input would apply.
- **Semantics:** movement and picking remain **world-anchored** (existing `ground_pick` + nav); camera does not redefine aim axes.
- Authority: camera pose stays **client-only** (see `docs/decomposition/modules/client_server_authority.md`).

**Out of scope**

- Binding rotate input or shipping orbit UX.
- Multiple zoom bands (NEO-16).
- Occlusion policy (NEO-17).
- Server use of camera pose.

## Acceptance criteria checklist

- [x] While the player moves via E1.M1, the avatar stays **readably framed** without manual camera control.
- [x] **Pitch/roll** do not change from user input; **yaw** remains at configured default (`0`) for prototype UX — document how enforced.
- [x] `CameraState` exposes yaw (even when always zero) so dependents do not assume yaw is impossible.
- [x] **Seam** for optional yaw (config/flags) documented in code or module implementation snapshot.
- [x] Short note in client or `E1_M2_IsometricCameraController.md` implementation snapshot when merged.

## Locked defaults (agreed before implementation)

| Topic | Choice |
|--------|--------|
| **Follow motion** | **Damped / lerped** toward the desired rig position; **teleport snap** when error exceeds a threshold (e.g. after `snap_to_server` or huge separation) so the camera never eases across the map. |
| **Framing** | **Match** the current static `World/Camera3D` framing in `main.tscn` as the starting **distance / pitch / FOV**; fine-tune in the inspector after follow is wired. |
| **`CameraState` refresh** | Rig **owns one instance** and **updates it every frame** after computing pose (simplest contract for future readers). |
| **Follow target wiring** | **`@export NodePath`** (resolve to `Node3D` each tick); tests may assign path or set target via code as needed. |
| **Update phase** | **`_process`** (not `_physics_process`) for the rig. |

## Technical approach

1. **Introduce `CameraState`** as a small **RefCounted** (or equivalent) script under `res://scripts/` with explicit fields: follow target path (or resolved node identity in state for debug), **yaw** (`float`, default `0`), **distance** or zoom index placeholder for NEO-16, and room for future flags. Avoid `class_name` so headless CI matches [godot-client-script-organization](../../.cursor/rules/godot-client-script-organization.md) guidance; tests can `preload` the script.
2. **Isometric rig** — add a **`Node3D`** (e.g. `IsometricFollowCamera` or `CameraRig`) with script that:
   - Owns or references the active **`Camera3D`** (`current = true`).
   - Each **`_process`**: resolve **`NodePath`** follow target, read **`global_position`**, compute desired eye position from **fixed pitch** + **effective yaw** (forced to `0` when `allow_yaw` is false), **lerp** rig toward target+offset; **snap** rig/camera when separation exceeds a configured threshold.
   - Updates the owned **`CameraState`** instance **every frame** after pose is finalized.
3. **Scene wiring** — replace the static **`World/Camera3D`** transform in `main.tscn` with a rig under `World` (camera as child), **`@export`** follow **`NodePath`** → `Player`, initial exports chosen to **match** the previous static camera readability.
4. **`main.gd`** — keep thin: obtain rig/camera reference for **`ground_pick.fallback_camera`** (unchanged contract); no camera math in `main.gd`.
5. **Yaw seam** — when `allow_yaw` is false: ignore any future input hook and set state yaw to `0`; when true (dev-only): clamp by `max_yaw_deg` and apply to basis (implementation stub can leave input unbound).
6. **Docs** — add a short **Implementation snapshot** subsection to `E1_M2_IsometricCameraController.md` after merge (or README pointer + module snapshot — satisfy AC).

## Files to add

| Path | Purpose |
|------|---------|
| `client/scripts/camera_state.gd` | RefCounted (or similar) holding follow target ref/path, **yaw**, distance/zoom index placeholder, future flags; no `class_name`. |
| `client/scripts/isometric_follow_camera.gd` | `Node3D` rig: positions/orients `Camera3D`, updates `CameraState`, exports distance/pitch/yaw seam. |

Godot may add companion `.uid` files when scripts are first touched in the editor; commit them if the toolchain creates them.

## Files to modify

| Path | Rationale |
|------|-----------|
| `client/scenes/main.tscn` | Replace fixed `Camera3D` transform with rig + follow target wiring; keep `current` camera and layers consistent with picking. |
| `client/scripts/main.gd` | Point `_camera` (and `ground_pick.fallback_camera`) at the rig’s **`Camera3D`**; minimal diff. |
| `client/README.md` | One short paragraph: follow camera behavior, fixed yaw prototype, where to tune distance/pitch. |
| `docs/decomposition/modules/E1_M2_IsometricCameraController.md` | **Implementation snapshot** (date + NEO-15): rig script names, yaw seam, single-band distance until NEO-16. |

## Tests

| File | What to cover |
|------|----------------|
| `client/test/camera_state_test.gd` | **New** — default **yaw** is `0`; fields survive assign/read; optional: follow target path assignment (if exposed). |
| `client/test/isometric_follow_camera_test.gd` | **New** — if pure helpers are extracted (e.g. static offset from pitch/distance/yaw), unit-test them; otherwise minimal suite: instantiate rig + dummy `Node3D` target, advance one frame, assert camera global position moves toward expected quadrant (tolerance-based). If full follow is too scene-heavy, document **manual** verification as primary and keep **camera_state** tests as the automated baseline. |

**Manual (required):** Run main scene with server per README: click-move across floor and **NEO-10** bumps; confirm avatar stays framed, no user-driven rotation, ground pick still hits walkable surfaces from the moving camera.

## Open questions / risks

- **Smoothing tuning:** If follow feels floaty or laggy after the locked defaults, adjust lerp speed or snap threshold; document chosen constants in script or README.
- **`player.gd` `_snap_capsule_upright`:** Identity basis today; camera yaw `0` matches world-aligned capsule. Future facing yaw must stay consistent with E1.M2 policy (not this story’s implementation unless trivial).
- **gdUnit rig tests:** May require small refactor (static math helpers) for stable headless asserts; acceptable follow-up within the same story if the first PR ships manual AC + `camera_state` tests.