# Phase A.5 — Two-tier Streaming + Horizon LOD — Design

**Created:** 2026-05-09 (immediately after N.5b ship + brainstorm).
**Status:** Spec — awaiting user review before plan-writing.
**Branch:** `claude/hopeful-darwin-ae8b87` (worktree under `.claude/worktrees/hopeful-darwin-ae8b87`).
**Predecessor:** Phase N.5b SHIP at `08b7362`. A.5 handoff at `f7f8867`.

---

## 1. Goal

Scale acdream's visible reach from radius=5 (~1 km) to radius=12 (~2.3 km horizon)
while sustaining 240 FPS at standstill on a 240 Hz / 1440p monitor.

Delivered through:
1. Two-tier streaming (near = full detail, far = terrain only).
2. Tightening the existing per-LB entity dispatcher walk.
3. Off-thread mesh build (single worker).
4. Fog blend at the near-tier boundary to mask the scenery cutoff.
5. Three nearly-free visual quality wins (terrain mipmaps + anisotropic, A2C
   with MSAA on foliage, depth-write audit).

The headline win: walking around Holtburg, the user sees a real horizon
(2.3 km of visible terrain) without the client falling off a perf cliff.

**User goal verbatim (2026-05-09):**
> "I just want great smooth HIGH fps visuals. Should look great. As long as
> it scales and we get very high FPS"

---

## 2. Hardware target + acceptance metrics

### Target hardware

- AMD Radeon RX 9070 XT (RDNA 4, ~December 2025).
- 240 Hz @ 2560×1440 (verified via `Get-CimInstance Win32_VideoController`).
- Frame budget: **4.166 ms** at vsync.

### Acceptance metrics (as shipped — revised with Quality Preset system)

1. **Build green; existing tests still green.** N.5b conformance sentinel
   passes (visual mesh Z = TerrainSurface.SampleZ within 1 mm).
2. **Standstill at user's selected preset on user's hardware:**
   - 95% of frames hit ≤ (1000ms / monitor refresh rate).
   - No absolute FPS number is required — the Quality Preset system (§4.10)
     is the user's knob for trading quality vs frame budget.
3. **Walking at user's selected preset:**
   - 95% of frames hit ≤ 1.5× (1000ms / monitor refresh rate).
4. **First traversal into virgin region (cold mesh cache):**
   - Render thread frame time stays within 2× the standstill budget while
     the worker fills the far-tier horizon (~2.7 s of "horizon filling in" is OK).
5. **Visual gate (user-driven, same on all presets):** user launches the
   client, walks Holtburg → North Yanshi, and confirms:
   - Horizon visible at ~2.3 km.
   - Fog blend at N₁ smooths the scenery boundary (no harsh cliff).
   - Distant terrain does not shimmer (mipmaps work).
   - Tree edges are smooth (A2C works).
   - No new z-fighting / depth artifacts (depth-write audit).
6. **Per-subsystem regression budgets** (added to `[WB-DIAG]` /
   `[TERRAIN-DIAG]` output):
   - Entity dispatcher cpu_us median ≤ **2.0 ms** at standstill.
   - Terrain dispatcher cpu_us median ≤ **1.0 ms** at standstill (all 625 LBs).
7. **N.5b sentinel intact:** TerrainSlot, TerrainModernConformance, Wb*,
   MatrixComposition, TextureCacheBindless, SplitFormulaDivergence — all
   pass clean.
8. **SHIP record + perf baseline doc + memory entry** mirroring N.5b's pattern.

A failure on (5) is a SHIP-blocker. A failure on (3) walking-FPS criterion
escalates to "fix or document the tradeoff and ship N.6 next" — not a
direct blocker but pushes the gate to user discretion.

---

## 3. Two-tier streaming model

### Tier definitions

| Tier | Radius | LB count | Loads | GPU mem |
|---|---|---|---|---|
| **Near** (N₁ = 4) | 9×9 = 81 LBs | terrain mesh + LandBlockInfo (stabs/buildings) + scenery generation + EnvCells + collision data + entity registration with WB dispatcher | scenery instance buffers + per-entity textures (depends on PaletteOverrides) |
| **Far** (N₂ = 12) | 25×25 - 9×9 = 544 LBs | terrain mesh ONLY (LandBlock heightmap + atlas blend) | ~14 MB shared atlas slots |
| **Total** | 25×25 = 625 LBs | combined | ~30 MB total estimated |

### Hysteresis (Q7 Option A — match existing radius+2 convention)

- **Near-tier:** entity load at distance 4, demote (entity unload) at distance 6.
- **Far-tier:** terrain load at distance 12, terrain unload at distance 14.

Both boundaries get the same 2-LB buffer. Phase A.1's existing hysteresis
mechanism in `StreamingRegion.RecenterTo` is the reference pattern; A.5
extends it from one radius to two.

### Tier transitions

| Transition | Trigger | Action |
|---|---|---|
| `null → far` | LB enters far window from outside | Worker reads LandBlock heightmap, builds mesh, posts `LandblockStreamResult.Loaded { Tier = Far }`. Render thread adds slot in `TerrainModernRenderer`. No entity work. |
| `null → near` | LB jumps null → near in one tick (first-tick bootstrap; teleport into virgin region) | Worker reads LandBlock heightmap + `LandBlockInfo`, generates scenery, builds entity list, builds mesh. Posts `LandblockStreamResult.Loaded { Tier = Near }`. Render thread adds terrain slot AND merges entities. |
| `far → near` | LB enters near window from far-resident | Worker reads `LandBlockInfo`, generates scenery, builds entity list. Posts `LandblockStreamResult.Promoted`. Render thread merges entities into `GpuWorldState` for the existing LB (terrain already loaded). |
| `near → far` | LB leaves near window past hysteresis (distance > 6) | Render thread drops the LB's entities from `GpuWorldState` (which fires `_wbSpawnAdapter.OnLandblockUnloaded`). Terrain stays. |
| `far → null` | LB leaves far window past hysteresis (distance > 14) | Render thread removes the terrain slot from `TerrainModernRenderer`. |

The order matters: when a player walks outward, the same LB goes
`near → far → null` over time. Each transition is one event per LB per
crossing.

### Why the player crossing the N₁ boundary works

The player is always at radius=0 from the streaming center (the streaming
center IS the player). The boundary effects are about LBs at the edge of N₁
crossing inward/outward as the player moves. Server-spawned NPCs are
delivered by ACE's broadcast (radius typically 5-7 LBs ≥ N₁), so when an
LB promotes back to near, ACE will already have its NPCs broadcast or
re-broadcast as the player moves through. Dat-static entities (stabs,
buildings) are reloaded from `LandBlockInfo` on promotion. Scenery is
re-generated from the deterministic seed at the same time.

---

## 4. Component-by-component design

### 4.1 `LandblockStreamTier` — new enum

```csharp
namespace AcDream.App.Streaming;

public enum LandblockStreamTier
{
    Far,   // terrain only
    Near,  // full detail (terrain + entities + scenery + EnvCells)
}
```

### 4.2 `StreamingRegion` — extended to two radii

```csharp
public sealed class StreamingRegion
{
    public int CenterX { get; }
    public int CenterY { get; }
    public int NearRadius { get; }   // N₁ (default 4)
    public int FarRadius  { get; }   // N₂ (default 12)

    public IReadOnlyCollection<uint> NearVisible { get; }   // 9×9 window
    public IReadOnlyCollection<uint> FarVisible  { get; }   // 25×25 window minus near
    public IReadOnlyCollection<uint> Resident { get; }      // hysteresis-retained

    public TwoTierDiff RecenterTo(int newCx, int newCy);
}

public readonly record struct TwoTierDiff(
    IReadOnlyList<uint> ToLoadFar,    // entered far window from null (need terrain only)
    IReadOnlyList<uint> ToLoadNear,   // entered near window from null (need terrain + entities — first-tick bootstrap, teleport)
    IReadOnlyList<uint> ToPromote,    // entered near window from far-resident (need entities only — terrain already loaded)
    IReadOnlyList<uint> ToDemote,     // exited near window past hysteresis (drop entities)
    IReadOnlyList<uint> ToUnload);    // exited far window past hysteresis (drop terrain)
```

The hysteresis math:
- Near-unload threshold: `NearRadius + 2` = 6.
- Far-unload threshold:  `FarRadius + 2` = 14.

A landblock is "near-resident" if its distance ≤ 6; "far-resident" if its
distance is in (6, 14]. Beyond 14, it unloads entirely.

### 4.3 `StreamingController` — routes by tier

```csharp
public sealed class StreamingController
{
    public int NearRadius { get; set; } = 4;
    public int FarRadius  { get; set; } = 12;
    public int MaxCompletionsPerFrame { get; set; } = 4;

    // Action signatures change to carry the tier.
    private readonly Action<uint, LandblockStreamTier> _enqueueLoad;
    private readonly Action<uint> _enqueueUnload;
    // ...

    public void Tick(int observerCx, int observerCy)
    {
        // First-tick bootstrap: every near-window LB → ToLoadNear; every
        // far-window-only LB → ToLoadFar.
        // Steady-state RecenterTo: produces 5 transition lists.
        //   - ToLoadFar    → _enqueueLoad(id, JobKind.LoadFar)
        //   - ToLoadNear   → _enqueueLoad(id, JobKind.LoadNear)
        //   - ToPromote    → _enqueueLoad(id, JobKind.PromoteToNear)
        //   - ToDemote     → _state.RemoveEntities(id) on render thread (no worker job)
        //   - ToUnload     → _enqueueUnload(id)
        // Drain completions and route by result variant.
    }
}

public enum LandblockStreamJobKind { LoadFar, LoadNear, PromoteToNear }
```

The render thread decides the job kind up-front based on its own knowledge
of which LBs are currently terrain-resident; the worker never peeks at
render-thread state. Three distinct worker paths:

- **`LoadFar`:** read `LandBlock` heightmap only. Skip `LandBlockInfo`,
  skip `LandblockLoader.BuildEntitiesFromInfo`, skip
  `SceneryGenerator`/`WbSceneryAdapter`. Build `LandblockMesh`. Post
  `LandblockStreamResult.Loaded(Tier=Far, Entities=[], MeshData=mesh)`.
- **`LoadNear`:** read `LandBlock` + `LandBlockInfo` + scenery generation
  + build mesh. Post `LandblockStreamResult.Loaded(Tier=Near, Entities=...,
  MeshData=mesh)`. Used for first-tick bootstrap of the inner ring and
  for the rare null→Near jump (teleport into virgin region).
- **`PromoteToNear`:** read `LandBlockInfo` + scenery generation only.
  Skip `LandBlock` heightmap (mesh already on GPU). Skip
  `LandblockMesh.Build`. Post `LandblockStreamResult.Promoted(id, entities)`.

### 4.4 `LandblockStreamResult` — new variants

```csharp
public abstract record LandblockStreamResult
{
    public sealed record Loaded(
        uint LandblockId,
        LandblockStreamTier Tier,
        LandBlock Heightmap,
        IReadOnlyList<WorldEntity> Entities,    // empty for Far
        LandblockMeshData MeshData              // built off-thread
    ) : LandblockStreamResult;

    public sealed record Promoted(
        uint LandblockId,
        IReadOnlyList<WorldEntity> Entities    // entity layer for an already-loaded far-tier LB
    ) : LandblockStreamResult;

    // Existing:
    public sealed record Unloaded(uint LandblockId) : LandblockStreamResult;
    public sealed record Failed(uint LandblockId, string Error) : LandblockStreamResult;
    public sealed record WorkerCrashed(string Error) : LandblockStreamResult;
}
```

`Loaded` carries `MeshData` — the mesh is built on the worker thread, NOT
in `_applyTerrain` on the render thread. `Promoted` only carries entities;
the mesh is already in `TerrainModernRenderer`.

### 4.5 `LandblockStreamer` — single worker, mesh-build on-worker

Existing `LandblockStreamer` (today on a single background thread) gets
extended to:

1. Read dat as today (`DatCollection.Get<LandBlock>` etc.).
2. Build `LandblockMesh` on the same thread:
   ```csharp
   var meshData = LandblockMesh.Build(
       block, lbX, lbY, heightTable, _ctx, _surfaceCache);
   ```
3. Post `LandblockStreamResult.Loaded(... MeshData = meshData)` to the
   completion queue.

Thread-safety implications:
- `_ctx` (TerrainBlendingContext) is read-only after init — no change.
- `_surfaceCache`: today a plain `Dictionary<uint, SurfaceInfo>`,
  populated lazily by `LandblockMesh.Build`. Currently safe because
  Build runs on the render thread; A.5 moves Build to the worker, so
  the cache must be thread-safe. **Swap to
  `ConcurrentDictionary<uint, SurfaceInfo>`** with `GetOrAdd` for the
  populate path. The factory inside `GetOrAdd` may run twice for the
  same key under contention (acceptable — the result is deterministic).

### 4.6 `WbDrawDispatcher` — entity bucketing tightening (Q5 Option A)

Three targeted changes inside the existing `Draw` flow:

#### Change 1: Animated-entity walk fix

Today (at lines 197-204 of `WbDrawDispatcher.cs`):

```csharp
foreach (var entry in landblockEntries) {
    bool landblockVisible = ...;
    if (!landblockVisible && (animatedEntityIds is null || animatedEntityIds.Count == 0))
        continue;

    foreach (var entity in entry.Entities) {
        ...
        if (!landblockVisible && !isAnimated) continue;
```

The `if (!landblockVisible && ...) continue;` only skips if there are NO
animated entities. When `animatedEntityIds` is non-empty, the inner loop
walks every entity in the invisible LB just to find the few animated
ones. With ~10.7K entities at N₁=4, this is wasted iteration.

**Fix:** when an LB is invisible, iterate `animatedEntityIds` directly
and look each up in a per-LB `Dictionary<uint, WorldEntity>` map (added
to `LoadedLandblock` or kept in a parallel structure).

```csharp
foreach (var entry in landblockEntries) {
    bool landblockVisible = ...;
    if (!landblockVisible) {
        if (animatedEntityIds is null || animatedEntityIds.Count == 0) continue;
        // Walk only animated entities in this invisible LB.
        foreach (var animatedId in animatedEntityIds) {
            if (!entry.AnimatedById.TryGetValue(animatedId, out var entity)) continue;
            // ... draw the entity
        }
        continue;
    }
    foreach (var entity in entry.Entities) { ... }
}
```

#### Change 2: Per-entity AABB cache at register time

Today: `Draw` recomputes `aMin = position - 5`, `aMax = position + 5` per
entity per frame. Cheap individually, but ~16K × per frame = measurable.

**Fix:** add `Vector3 AabbMin, AabbMax` fields to `WorldEntity` (or a
parallel struct keyed by entity id). Populate at `EntitySpawnAdapter.OnCreate`
(server-spawned) and `LandblockLoader.BuildEntitiesFromInfo` (dat-static)
time. Static entities never invalidate. Dynamic entities (NPCs, players)
update on position change — add `WorldEntity.PositionDirty` flag set by
the live position update path; AABB recompute happens lazily on first
read after dirty.

The AABB radius today is hard-coded `PerEntityCullRadius = 5.0f` — keep
that as a per-mesh-bucket fallback; future improvement is to compute the
real AABB from the mesh, but defer that to a later phase (it's a
cross-cutting change).

#### Change 3: 4×4 sub-LB cell cull for partially-visible LBs

When an LB is fully visible (its AABB entirely inside the frustum), all
its entities are drawn — no per-entity cull needed. Today's per-entity
cull is wasted work in this case.

When an LB is partially visible, today's per-entity cull is the right
work — but it walks all ~132 entities. Cheap with the AABB-cache fix
(memory read), so the win here is small. Worth doing only if the cache
fix alone isn't enough to hit the 2.0ms budget.

**Add only if needed:** bucket each LB's entities into 4×4 sub-cells
(each 48 m). Compute a sub-cell AABB at register time. Per frame: for
partially-visible LBs, cull at sub-cell granularity first; walk
entities only inside surviving sub-cells.

Ship change #1 and #2 unconditionally; ship #3 only if the budget
isn't hit by #1 + #2.

### 4.7 `TerrainModernRenderer` — no structural change

The slot allocator (`TerrainSlotAllocator`) already grows by power-of-two
doubling. At N₂=12 worst case, ~961 slots × ~15 KB per slot = ~14 MB.
Allocator handles it without modification.

Per-LB frustum cull stays per-slot — at ~961 slots × ~0.3 µs/AABB-test
the worst-case cull pass is ~0.3 ms. Acceptable inside the 1.0 ms terrain
dispatcher budget.

The DEIC (`DrawElementsIndirectCommand`) array grows accordingly. The
existing per-frame `BufferSubData` upload absorbs a 961-entry array
without issue (~19 KB).

### 4.8 Fog tuning (`SceneLightingUbo`)

Existing fields (Phase G.1+):
- `FogStart` — distance at which fog begins (today: somewhere outside the
  visible terrain range).
- `FogEnd` — distance at which fog reaches full opacity.
- `FogColor` — sourced from current sky state.

A.5 change: dynamically tune `FogStart` and `FogEnd` based on the
current N₁/N₂:

- `FogStart = N₁ × LandblockSize × 0.7` ≈ `4 × 192 × 0.7` = **~538 m**.
- `FogEnd   = N₂ × LandblockSize × 0.95` ≈ `12 × 192 × 0.95` = **~2188 m**.

The fog color matches the current sky color (already provided by
`SkyStateProvider`) — at the far horizon, fog blends terrain into
sky, hiding the N₂ edge.

The 0.7 / 0.95 multipliers are tuning knobs. Iterate during user gate.
**Expose as env vars during development** (`ACDREAM_FOG_START_MULT`,
`ACDREAM_FOG_END_MULT`) to allow fast iteration without a recompile.

### 4.9 Visual quality wins (Q8 Option C — all three)

#### 4.9.1 Mipmaps + 16x anisotropic on `TerrainAtlas`

Today: `TerrainAtlas.Upload` uses `GL_LINEAR` minification, no mipmaps.

A.5 change: after upload, call `glGenerateMipmap(GL_TEXTURE_2D_ARRAY)`.
Sampler state: `GL_LINEAR_MIPMAP_LINEAR` (trilinear) +
`GL_TEXTURE_MAX_ANISOTROPY = 16`.

Affects only `TerrainAtlas`. Mesh atlas (entity textures) and other
texture caches stay as-is.

Verification: at N₂=12, walk to a vantage point looking at terrain at
range 2 km. With the fix, no shimmer. Without, "moving sparkles" visible
at distance.

#### 4.9.2 Alpha-to-coverage with MSAA on foliage

Today: `mesh_modern.frag` uses `if (alpha < cutoff) discard;` for ClipMap
translucency. Produces hard, pixel-edged tree silhouettes.

A.5 change:
- Enable MSAA 4x on the GL render target (window framebuffer).
- In `mesh_modern.frag`, for ClipMap pass: write
  `gl_SampleMask[0]` based on alpha threshold instead of binary discard.

Risk: MSAA framebuffer interaction with sky / particles / UI overlay.
Audit:
- `SkyRenderer` — clears its own framebuffer? If so, must clear the MSAA
  attachment instead. Investigate.
- `ParticleRenderer` — billboards already use alpha-blend; MSAA-friendly.
- ImGui overlay — drawn after the 3D pass; must not interact with MSAA
  resolve.

If the audit finds blocking issues, ship 4.9.1 + 4.9.3 only and defer
4.9.2 to a later phase. Document the result either way.

#### 4.9.3 Depth-write audit on translucent batches

Walk all translucent batch paths in `WbDrawDispatcher.Draw` and verify:
- Alpha-blend (`AlphaBlend`, `Additive`, `InvAlpha`): `glDepthMask(false)`.
- Clip-map (binary alpha): `glDepthMask(true)` (foliage casts depth).
- Opaque: `glDepthMask(true)`.

Today's code at lines 401-433 sets `DepthMask(true)` for opaque,
`DepthMask(false)` for transparent. Confirm ClipMap is in the opaque
pass (it is, per `IsOpaque` returning true for ClipMap at line 738).

If audit finds nothing wrong, ship a comment + a unit test that locks in
the partition. Cheap insurance against future regression.

### 4.10 Quality Preset System (T22.5 — added mid-execution)

**Background:** Added between T22 (fog wiring) and T23 (DIAG budgets) at
user's direction. The original spec had no preset concept; §2 was written
against absolute 240 FPS on fixed N₁/N₂. T22.5 makes both radii and every
quality knob user-controllable via a single enum. §2 was amended above to
reflect the per-preset, refresh-rate-relative acceptance criteria.

#### Schema

```csharp
public enum QualityPreset { Low, Medium, High, Ultra }

public readonly record struct QualitySettings(
    int NearRadius,
    int FarRadius,
    int MsaaSamples,
    int AnisotropicLevel,
    bool AlphaToCoverage,
    int MaxCompletionsPerFrame);
```

`QualitySettings.From(preset)` returns the canonical values:

| Preset | NearRadius | FarRadius | MsaaSamples | AnisotropicLevel | AlphaToCoverage | MaxCompletionsPerFrame |
|---|---|---|---|---|---|---|
| Low | 2 | 5 | 0 | 4 | false | 2 |
| Medium | 3 | 8 | 2 | 8 | false | 3 |
| High | 4 | 12 | 4 | 16 | true | 4 |
| Ultra | 5 | 15 | 4 | 16 | true | 6 |

`QualitySettings.WithEnvOverrides(baseSettings)` applies per-field env-var
overrides (see §4.10.3).

#### Persistence and UI

`DisplaySettings.Quality` (type `QualityPreset`) persists via the existing
`settings.json` infrastructure (Phase L.0). The Settings panel (F11) exposes
a Quality dropdown in its Display tab (`SettingsPanel.RenderDisplayTab`).

#### Wiring (GameWindow.OnLoad + ReapplyQualityPreset)

1. `GameWindow.OnLoad` resolves the active `QualitySettings`:
   `QualitySettings.From(displaySettings.Quality).WithEnvOverrides(...)`.
2. `StreamingController` and `LandblockStreamer` are built with the preset's
   `NearRadius` / `FarRadius`.
3. `TerrainAtlas.SetAnisotropic(settings.AnisotropicLevel)` called once at
   load and again on reapply.
4. `WindowOptions.Samples = settings.MsaaSamples` applied at window creation
   time only (MSAA mid-session change is structurally unsupported by OpenGL).
5. `WbDrawDispatcher.AlphaToCoverage = settings.AlphaToCoverage`.
6. `StreamingController.MaxCompletionsPerFrame = settings.MaxCompletionsPerFrame`.

Mid-session quality change (F11 dropdown change → Save):

- `GameWindow.ReapplyQualityPreset` rebuilds `StreamingController` +
  `LandblockStreamer` with the new radii, re-applies anisotropic and
  AlphaToCoverage.
- If `MsaaSamples` changed, logs a warning that MSAA sample count cannot be
  changed mid-session; requires restart.

#### Env-var overrides (§4.10.3)

Applied by `QualitySettings.WithEnvOverrides` after the base preset is resolved.
Each field has one env var; all are optional. Logged at startup.

| Env var | Field overridden |
|---|---|
| `ACDREAM_NEAR_RADIUS` | `NearRadius` |
| `ACDREAM_FAR_RADIUS` | `FarRadius` |
| `ACDREAM_MSAA_SAMPLES` | `MsaaSamples` |
| `ACDREAM_ANISOTROPIC` | `AnisotropicLevel` |
| `ACDREAM_A2C` | `AlphaToCoverage` (1/0/true/false) |
| `ACDREAM_MAX_COMPLETIONS_PER_FRAME` | `MaxCompletionsPerFrame` |

#### Tests

12 tests in `tests/AcDream.UI.Abstractions.Tests/Settings/QualityPresetTests.cs`
cover: canonical preset values per enum member; `WithEnvOverrides` no-op when
no env vars set; `WithEnvOverrides` each override individually; invalid env-var
value falls back to base setting.

#### Files

- `src/AcDream.UI.Abstractions/Settings/QualityPreset.cs` — new
- `src/AcDream.UI.Abstractions/Settings/DisplaySettings.cs` — `Quality` field added
- `src/AcDream.UI.Abstractions/Panels/Settings/SettingsPanel.cs` — Display tab
  Quality dropdown (`RenderDisplayTab` method)
- `src/AcDream.App/Rendering/GameWindow.cs` — `ReapplyQualityPreset`,
  `OnLoad` preset wiring
- `tests/AcDream.UI.Abstractions.Tests/Settings/QualityPresetTests.cs` — new (12 tests)

#### Out of scope (deferred)

- Auto-detect preset on first launch (Phase A.6 / N.6.5).
- Adaptive runtime preset drop on budget miss.
- Per-feature toggles below preset level.

Commits: `afa4200` (schema + tests), `28d2c60` (wiring).

---

## 5. Data flow

### Per-frame (steady state)

```
GameWindow.OnUpdate(dt)
  └─ StreamingController.Tick(playerCx, playerCy)
      ├─ region.RecenterTo(...)  // produces TwoTierDiff if center changed
      ├─ for each ToLoadFar:   _enqueueLoad(id, LoadFar)
      ├─ for each ToLoadNear:  _enqueueLoad(id, LoadNear)
      ├─ for each ToPromote:   _enqueueLoad(id, PromoteToNear)
      ├─ for each ToDemote:    _state.RemoveEntities(id)  // on render thread
      ├─ for each ToUnload:    _enqueueUnload(id)
      └─ drainCompletions(MaxCompletionsPerFrame=4)
          ├─ Loaded.Far:    _terrain.AddLandblock(meshData);  _state.AddLandblock(...)
          ├─ Loaded.Near:   _terrain.AddLandblock(meshData);  _state.AddLandblock(... entities)
          ├─ Promoted:      _state.AddEntitiesToExisting(id, entities)
          ├─ Unloaded:      _terrain.RemoveLandblock(id);  _state.RemoveLandblock(id)
          └─ Failed/Crash:  log

GameWindow.OnRender
  ├─ TerrainModernRenderer.Draw(camera, frustum)
  │   └─ glMultiDrawElementsIndirect across all near + far slots that pass cull
  └─ WbDrawDispatcher.Draw(camera, gpuWorldState.LandblockEntries, frustum, visibleCellIds, animatedEntityIds)
      ├─ for each LB entry:
      │   ├─ if invisible: walk only animatedEntityIds (Change #1)
      │   └─ if visible: walk entities, AABB cache lookup (Change #2)
      ├─ classify into groups, build SSBO, multi-draw indirect
      └─ flush DIAG every ~5 s
```

### Worker thread

```
LandblockStreamer.WorkerLoop
  while running:
    job = jobQueue.dequeue()
    switch job.Kind:
      LoadFar:
        block = dats.Get<LandBlock>(id)
        meshData = LandblockMesh.Build(block, ..., _surfaceCache)
        completionQueue.enqueue(Loaded(id, Far, block, [], meshData))
      LoadNear:
        block = dats.Get<LandBlock>(id)
        info  = dats.Get<LandBlockInfo>(...)
        entities = LandblockLoader.BuildEntitiesFromInfo(info)
        scenery  = WbSceneryAdapter.GenerateScenery(block, ...)
        meshData = LandblockMesh.Build(block, ..., _surfaceCache)
        completionQueue.enqueue(Loaded(id, Near, block, entities ∪ scenery, meshData))
      PromoteToNear:
        info  = dats.Get<LandBlockInfo>(...)
        // Heightmap not re-read; scenery generation needs LandBlock for height
        // sampling — read it again from disk cache (DatCollection caches the
        // last-read block; cheap second access) OR pass through from render
        // thread's terrain-slot snapshot (deferred plan-level decision).
        block = dats.Get<LandBlock>(id)
        entities = LandblockLoader.BuildEntitiesFromInfo(info)
        scenery  = WbSceneryAdapter.GenerateScenery(block, ...)
        completionQueue.enqueue(Promoted(id, entities ∪ scenery))
```

---

## 6. Threading model

- **Render thread:** drives `StreamingController.Tick`, drains the
  completion queue, calls `TerrainModernRenderer.AddLandblock` /
  `RemoveLandblock`, mutates `GpuWorldState`. All GL calls on this thread.
- **One streaming worker thread:** dat reads, mesh build, scenery generation.
  Owns `_surfaceCache` (now `ConcurrentDictionary`) — render thread does
  not access it directly.
- **Network thread:** unchanged from Phase A.3 — drains UDP into the
  channel; render thread decodes.

Synchronization:
- Job queue: `Channel<LbStreamJob>` (writer = render thread via
  `_enqueueLoad`; reader = worker).
- Completion queue: `ConcurrentQueue<LandblockStreamResult>` (writer =
  worker; reader = render thread).
- `_surfaceCache`: `ConcurrentDictionary<uint, SurfaceInfo>` populated by
  `LandblockMesh.Build` on the worker; read by future paths if any
  (none today).
- `TerrainBlendingContext`: read-only post-init. No lock.

---

## 7. Error handling

- **Worker crash:** caught in worker loop, posts
  `LandblockStreamResult.WorkerCrashed`. Render thread logs to console.
  (Existing pattern.)
- **Dat read failure:** posts `LandblockStreamResult.Failed`. Render
  thread logs. Streaming continues with the LB skipped — region still
  tracks it as resident so we don't retry forever, but the slot stays empty.
- **AABB cache invalidation race:** dynamic entity moves while the
  dispatcher is walking. Acceptable — at worst, the entity culls or
  draws based on the previous frame's position. Position is updated in
  the network handler (also render-thread today) so no actual race.
- **Promotion timing:** if the player crosses N₁ inward, we enqueue a
  `Near` load on the worker. Until it completes, the LB has terrain but
  no scenery / entities. Frame budget is unaffected (only `LoadedLandblock`
  changes, and the dispatcher already handles missing entities by walking
  zero-length lists).
- **Unload during in-flight load:** enqueue an unload while a load is
  in flight. When the load completes, render thread sees the LB is no
  longer resident — drop the result silently. Same pattern as today.

---

## 8. Testing strategy

### Unit tests (offline, no GL)

Add to `tests/AcDream.Core.Tests/Streaming/`:
- `StreamingRegion_TwoTier_FirstTick_LoadsNearAndFarSeparately` — first
  call produces `ToLoadNear` populated for inner ring, `ToLoadFar`
  populated for outer ring, `ToPromote` empty (nothing was previously
  resident).
- `StreamingRegion_TwoTier_NullToFar_OnFarRingEntry` — LB rolls into
  far window from null. Asserts entry in `ToLoadFar`, not
  `ToLoadNear`.
- `StreamingRegion_TwoTier_FarToNear_OnNearRingEntry` — LB was
  far-resident, player walks toward it, LB enters near window. Asserts
  entry in `ToPromote`, not `ToLoadNear`.
- `StreamingRegion_TwoTier_NullToNear_OnTeleport` — observer center
  jumps far enough that an LB goes from null → Near in one frame
  (e.g., teleport). Asserts entry in `ToLoadNear`, not `ToPromote`.
- `StreamingRegion_TwoTier_NearToFar_OnNearBoundaryExitPlusHysteresis` —
  asserts entry in `ToDemote` only after distance exceeds
  `NearRadius + 2`.
- `StreamingRegion_TwoTier_FarToNull_OnFarBoundaryExitPlusHysteresis` —
  asserts entry in `ToUnload` only after distance exceeds
  `FarRadius + 2`.
- `StreamingRegion_TwoTier_HysteresisHoldsAcrossOscillation` — walk
  back-and-forth across N₁ five times within the hysteresis radius;
  assert no demote events fire.
- `StreamingController_TwoTier_DrainsRoutedByVariant` — `Loaded.Far`,
  `Loaded.Near`, and `Promoted` each route to the right state mutation
  on the render thread.

Add to `tests/AcDream.Core.Tests/Rendering/Wb/`:
- `WbDrawDispatcher_AnimatedEntities_InInvisibleLb_NoFullEntityWalk` —
  verify Change #1 (only iterates `animatedEntityIds`, not `Entities`).
- `WbDrawDispatcher_PerEntityAabbCached_NotRecomputed` — assert AABB
  fields are read, not recomputed, for static entities.

### Conformance tests

- `TerrainModernConformanceTests` (existing) — must still pass. The
  visual mesh Z must agree with `TerrainSurface.SampleZFromHeightmap`
  to within 1 mm across both tiers.
- `LandblockMeshTests` (existing) — must still pass. Worker-thread
  mesh build produces byte-identical results to render-thread build
  for the same inputs.

### Perf gate (manual, with `[WB-DIAG]` + `[TERRAIN-DIAG]`)

- **Standstill bench:** launch with `ACDREAM_WB_DIAG=1`, stand at
  Holtburg dueling field for 60 s. Read median + p95 + p99 from log.
- **Walking bench:** launch with diag, run from Holtburg to North
  Yanshi, ~60 s. Same metrics.
- **First traversal bench:** clear OS file cache (or reboot), launch
  with diag, walk into a region not previously visited, capture the
  worker-thread fill duration + render-thread frame time during fill.

### Visual gate (manual, user-driven)

User launches the client, walks the standard route, confirms:
1. Horizon visible at 2.3 km.
2. Fog blend is smooth (no scenery cliff at N₁).
3. No shimmer on distant terrain.
4. Smooth tree edges (foliage A2C).
5. No new z-fighting / depth artifacts.

---

## 9. Out of scope (explicitly deferred)

Per the brainstorm Q10 confirmation:

- **GPU-side culling** (compute pre-pass) — N.6.
- **Persistent-mapped indirect buffer** — N.6.
- **Multi-thread mesh-build worker pool** — N.6 if first-traversal fill
  feels too slow at gate.
- **Static/dynamic persistent groups** (Q5 Option B — the "compute the
  group key once at spawn" architecture change) — separate later phase
  (likely A.6 or N.6.5).
- **Billboard / impostor scenery** at far tier — escalation only if the
  fog'd terrain horizon looks too bare at gate.
- **Wider N₁ hysteresis** (Option C, radius+3) — single-line tweak only
  if gate finds entity pop-in along the boundary.
- **Far-tier terrain mesh LOD** (decimating 2×2 LBs) — not needed at
  N₂=12; revisit only if N₂ grows beyond 15.
- **Sky / particles modern path migration** — N.7+ phases.
- **EnvCell modern path migration** — separate phase.
- **Shadow mapping** — separate visual phase, later.
- **Strict 240 Hz during walking** (Q9 Option A) — graduate to in a
  perf-polish phase if we want to commit to it.

---

## 10. Risks

1. **Fog tuning visual gate** *(highest risk).* Hardest non-engineering
   risk. The 0.7 / 0.95 multipliers in §4.8 are first-cut numbers. If
   the fog band is too thin (visible scenery cliff at N₁) or too thick
   (terrain looks washed out), iterate on the multipliers. Mitigation:
   expose `FogStart` / `FogEnd` as tunable env vars during A.5
   development for fast iteration.
2. **A2C / MSAA framebuffer interaction** *(moderate risk).* MSAA on
   the GL render target may break sky / particles / UI rendering.
   Audit during implementation. **Fallback: ship Q8 Option B (mipmaps
   + depth-audit only) if A2C goes sideways.** Document the result.
3. **Worker starvation on first-traversal** *(low-moderate risk).*
   ~2.7 s of sequential mesh build on first walk into virgin region.
   Render thread frame time stays in budget; the visible effect is the
   horizon visibly filling. Acceptable per Q9 Option B; graduate to
   multi-worker pool in N.6 if user complains.
4. **Tier-boundary churn** *(low risk).* When player crosses N₁ both
   directions, demote→promote→demote fires. Hysteresis (radius+2) is
   the buffer. If thrash visible, widen to radius+3.
5. **Entity AABB cache invalidation** *(low risk).* Dynamic entities
   must recompute AABB on position change. Single-threaded render
   thread means no concurrent mutation; the dirty-flag pattern is
   straightforward.
6. **Server broadcast radius mismatch** *(low risk).* If ACE's broadcast
   radius is < N₁=4, NPCs in outer near-tier LBs won't be
   server-broadcast (they don't exist in our state). Mitigation:
   N₁=4 is conservative — typical ACE configs broadcast at 5-7 LBs.
   If observed, drop N₁ to 3.

---

## 11. What was deferred (post-A.5)

The following items were identified during A.5 development but deferred to
post-A.5 phases. They are tracked as OPEN issues in `docs/ISSUES.md`.

1. **Tier 1 entity-classification cache** (commit `3639a6f` reverted at
   `9b49009`): First attempt cached `meshRef.PartTransform` which is mutated
   per frame for animated entities (skeletal pose). Next attempt needs:
   (a) audit AnimationSequencer + AnimationHookRouter to identify ALL
   per-frame mutations of MeshRef state; (b) redesign cache to bypass
   animated entities OR cache only the animation-invariant subset; (c) test
   specifically with a moving animated NPC on screen. (`docs/ISSUES.md` #53)

2. **Lifestone missing visual**: The Holtburg lifestone has not rendered since
   earlier in A.5 development. Possibly Bug A's far-tier strip incorrectly
   catching a near-tier entity, or a separate earlier regression.
   (`docs/ISSUES.md` #52)

3. **Plumb JobKind through BuildLandblockForStreaming**: Bug A's fix (commit
   `9217fd9`) strips entities post-load in the worker. Proper fix: skip the
   `LandBlockInfo` + scenery load entirely for far-tier jobs. ~30 min.
   (`docs/ISSUES.md` #54)

4. **Tier 2 — Static/dynamic split with persistent groups**: ~2-week phase.
   Avoids per-frame entity re-classification by maintaining stable groups
   keyed at spawn time. Roadmap doc at
   `docs/plans/2026-05-10-perf-tiers-2-3-roadmap.md`.

5. **Tier 3 — GPU-side culling via compute pre-pass**: ~1-month phase.
   Same roadmap doc.

6. **Eliminate ToEntries adapter allocation**: tiny win (~25 KB/frame).

7. **InvalidateEntity wiring on palette/ObjDesc events**: needed by the
   Tier 1 retry.

8. **Visual gate at full High preset**: never validated due to the
   GPU+CPU stack-up OS crash earlier in A.5. With Bug A fixed the crash
   likely won't recur; defer retest to post-A.5 perf polish.

---

## 12. References (formerly §11)

- **Handoff (cold-start):** [`docs/research/2026-05-10-phase-a5-handoff.md`](../../research/2026-05-10-phase-a5-handoff.md)
- **N.5b handoff (predecessor):** [`docs/research/2026-05-09-phase-n5b-handoff.md`](../../research/2026-05-09-phase-n5b-handoff.md)
- **N.5b perf baseline:** [`docs/plans/2026-05-09-phase-n5b-perf-baseline.md`](../../plans/2026-05-09-phase-n5b-perf-baseline.md)
- **Roadmap A.5 entry:** [`docs/plans/2026-04-11-roadmap.md`](../../plans/2026-04-11-roadmap.md)
- **N.5b memory state:** `memory/project_phase_n5b_state.md` (three high-value
  gotchas — bindless uniform-sampler driver quirk, MaybeFlushTerrainDiag
  underflow, visual gate confirmation requirement).
- **Existing streaming files:**
  - [`src/AcDream.App/Streaming/StreamingController.cs`](../../../src/AcDream.App/Streaming/StreamingController.cs)
  - [`src/AcDream.App/Streaming/StreamingRegion.cs`](../../../src/AcDream.App/Streaming/StreamingRegion.cs)
  - [`src/AcDream.App/Streaming/GpuWorldState.cs`](../../../src/AcDream.App/Streaming/GpuWorldState.cs)
  - [`src/AcDream.App/Streaming/LandblockStreamer.cs`](../../../src/AcDream.App/Streaming/LandblockStreamer.cs)
- **Existing dispatcher:** [`src/AcDream.App/Rendering/Wb/WbDrawDispatcher.cs`](../../../src/AcDream.App/Rendering/Wb/WbDrawDispatcher.cs)
- **Existing terrain renderer:** [`src/AcDream.App/Rendering/TerrainModernRenderer.cs`](../../../src/AcDream.App/Rendering/TerrainModernRenderer.cs)
- **Mesh builder (will move off render thread):** [`src/AcDream.Core/Terrain/LandblockMesh.cs`](../../../src/AcDream.Core/Terrain/LandblockMesh.cs)
