# Phase 3c: Terrain Texture Blending

**Status:** Ready for review
**Date:** 2026-04-10
**Prerequisites:** Phases 1-3a/3b merged (lighting + per-vertex terrain normals)
**Primary reference:** `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/LandSurfaceManager.cs` + `Shaders/Landscape.{vert,frag}`
**Deferred out:** Chunking refactor (moved to a hypothetical Phase 3d when streaming actually matters)

---

## 1. Goal

Replace acdream's current "one terrain texture type per vertex, pick an atlas layer" with AC's real texture-merge blending: per-cell palette codes + alpha masks + up to 3 terrain overlays + up to 2 road overlays composited in the fragment shader. End result: no more jagged grass/dirt/sand seams, terrain looks like real AC.

**Explicitly NOT in scope:**
- Chunking / global VBO slot pools (zero visual benefit until we stream more landblocks)
- Streaming, frustum culling, multi-draw indirect
- Minimap / grid overlay / brush visualization (editor features)
- Collision queries

Keep the existing "one VBO per landblock" render structure. All the changes are inside how each landblock is meshed and shaded.

---

## 2. What changes architecturally

### Current (post-Phase 3b)
```
LandblockMesh.Build → 81 vertices (9×9 grid)
  each vertex: Position, Normal, TexCoord, TerrainLayer(uint)
TerrainRenderer: one VBO per landblock
Fragment shader: single sampler2DArray lookup
```

### After Phase 3c
```
LandblockMesh.Build → 384 vertices (64 cells × 6 vertices)
  each vertex: Position, Normal, TexCoord, Data0, Data1, Data2, Data3
  where Data0..3 encode: base + 3 overlays + 2 roads + rotations + split direction
TerrainRenderer: one VBO per landblock  (unchanged)
Fragment shader: two sampler2DArray lookups (terrain atlas + alpha atlas),
                 layered alpha-weighted composite
```

**Per-cell, not per-vertex:** every 6 vertices sharing a cell carry identical `Data0..3`. The vertex shader uses them to compute UVs + rotations for the fragment shader, which does the actual blend.

---

## 3. Core algorithm port

All of these are **pure functions** lifted (with attribution) from WorldBuilder's `LandSurfaceManager.cs`. Live in a new file `src/AcDream.Core/Terrain/TerrainBlending.cs`.

### 3.1 Palette code
```csharp
// Encodes 4 corner terrain types (5 bits each) + 4 road flags (2 bits each)
// into a 32-bit hash. Same corners → same palCode → same blending choices.
public static uint GetPalCode(int r1, int r2, int r3, int r4,
                              int t1, int t2, int t3, int t4)
{
    uint terrainBits = (uint)((t1 << 15) | (t2 << 10) | (t3 << 5) | t4);
    uint roadBits    = (uint)((r1 << 26) | (r2 << 24) | (r3 << 22) | (r4 << 20));
    uint sizeBits    = 1u << 28;
    return sizeBits | roadBits | terrainBits;
}
```

### 3.2 SurfaceInfo (derived from palCode via `BuildSurface`)
```csharp
public readonly record struct SurfaceInfo(
    byte BaseTexIdx,                                       // 0..35, terrain atlas layer
    byte Ovl0TexIdx, byte Ovl0AlphaIdx, byte Ovl0Rotation, // 255 idx = unused; rot 0..3
    byte Ovl1TexIdx, byte Ovl1AlphaIdx, byte Ovl1Rotation,
    byte Ovl2TexIdx, byte Ovl2AlphaIdx, byte Ovl2Rotation,
    byte Road0TexIdx, byte Road0AlphaIdx, byte Road0Rotation,
    byte Road1TexIdx, byte Road1AlphaIdx, byte Road1Rotation);

// Ports WorldBuilder's BuildTexture + FindTerrainAlpha + FindRoadAlpha
public static SurfaceInfo BuildSurface(
    uint palCode,
    TexMergeInfo texMerge,               // from Region.TerrainInfo.LandSurfaces.TexMerge
    IReadOnlyDictionary<uint, byte> terrainTypeToLayer,
    IReadOnlyList<byte> cornerAlphaLayers,  // atlas layers 0..3
    IReadOnlyList<byte> sideAlphaLayers,    // atlas layers 4..7
    IReadOnlyList<byte> roadAlphaLayers);   // atlas layers 5..14
```

### 3.3 FillCellData (pack into 4 uints)
```csharp
// Per-cell shared-among-6-vertices data. Bit layout matches WorldBuilder's
// Landscape.vert unpacking so the shader stays as a straight port.
public static (uint d0, uint d1, uint d2, uint d3) FillCellData(
    SurfaceInfo s, CellSplitDirection split)
{
    uint d0 = (uint)(s.BaseTexIdx | (0 /*base alpha unused*/ << 8)
                   | ((s.Ovl0TexIdx | (s.Ovl0AlphaIdx << 8)) << 16));
    uint d1 = (uint)( (s.Ovl1TexIdx | (s.Ovl1AlphaIdx << 8))
                   | ((s.Ovl2TexIdx | (s.Ovl2AlphaIdx << 8)) << 16));
    uint d2 = (uint)( (s.Road0TexIdx | (s.Road0AlphaIdx << 8))
                   | ((s.Road1TexIdx | (s.Road1AlphaIdx << 8)) << 16));
    uint d3 = (uint)(/*base rot=0*/
                   | (s.Ovl0Rotation << 2) | (s.Ovl1Rotation << 4) | (s.Ovl2Rotation << 6)
                   | (s.Road0Rotation << 8) | (s.Road1Rotation << 10)
                   | (((int)split & 1) << 12));
    return (d0, d1, d2, d3);
}
```

### 3.4 Cell split direction (deterministic hash)
```csharp
public enum CellSplitDirection { SwToNe = 0, NwToSe = 1 }

// Ports WorldBuilder TerrainUtils.GetCellSplitDirection — the magic constants
// must match exactly or our splits won't line up with server collision physics.
public static CellSplitDirection GetCellSplitDirection(
    int landblockX, int landblockY, int cellX, int cellY);
```

### 3.5 PseudoRandomIndex
```csharp
// Used by BuildSurface to pick which of several alpha variants to use for a
// given palCode, so visually similar cells don't all use identical blend masks.
public static int PseudoRandomIndex(uint palCode, int max);
```

Every function above is **trivially unit-testable** with canned inputs. We'll seed golden values from an instrumented WorldBuilder run.

---

## 4. Per-cell mesh generation

Rewrite `LandblockMesh.Build` to the new per-cell layout. Still one landblock → one `LandblockMeshData` → one VBO (so `TerrainRenderer` stays unchanged at the draw-call level).

```csharp
public static LandblockMeshData Build(
    LandBlock block,
    int landblockX, int landblockY,          // for split-direction hashing
    float[] heightTable,
    TexMergeInfo texMerge,
    IReadOnlyDictionary<uint, byte> terrainTypeToLayer,
    IReadOnlyList<byte> cornerAlphaLayers,
    IReadOnlyList<byte> sideAlphaLayers,
    IReadOnlyList<byte> roadAlphaLayers,
    Dictionary<uint, SurfaceInfo> surfaceCache)  // palCode → SurfaceInfo, shared across landblocks
{
    // Pre-sample the 9×9 heightmap (existing Phase 3b logic, unchanged)
    var heights = new float[9, 9];
    // ...

    var vertices = new List<Vertex>(64 * 6);
    var indices  = new List<uint>(64 * 6);

    for (int cy = 0; cy < 8; cy++)
    {
        for (int cx = 0; cx < 8; cx++)
        {
            // 1. Gather 4 corner TerrainInfos (x-major, block.Terrain[x*9+y])
            var tBL = block.Terrain[cx * 9 + cy];
            var tBR = block.Terrain[(cx + 1) * 9 + cy];
            var tTR = block.Terrain[(cx + 1) * 9 + (cy + 1)];
            var tTL = block.Terrain[cx * 9 + (cy + 1)];

            // 2. palCode + SurfaceInfo (cached)
            uint palCode = TerrainBlending.GetPalCode(
                tBL.Road, tBR.Road, tTR.Road, tTL.Road,
                (int)tBL.Type, (int)tBR.Type, (int)tTR.Type, (int)tTL.Type);
            if (!surfaceCache.TryGetValue(palCode, out var surf))
            {
                surf = TerrainBlending.BuildSurface(
                    palCode, texMerge, terrainTypeToLayer,
                    cornerAlphaLayers, sideAlphaLayers, roadAlphaLayers);
                surfaceCache[palCode] = surf;
            }

            // 3. Cell data (shared across all 6 vertices of the cell)
            var split = TerrainBlending.GetCellSplitDirection(landblockX, landblockY, cx, cy);
            var (d0, d1, d2, d3) = TerrainBlending.FillCellData(surf, split);

            // 4. 4 corner positions + per-corner central-difference normals
            //    (Phase 3b logic lifted into a helper)
            var posBL = CornerPosition(cx,     cy,     heights);
            var posBR = CornerPosition(cx + 1, cy,     heights);
            var posTR = CornerPosition(cx + 1, cy + 1, heights);
            var posTL = CornerPosition(cx,     cy + 1, heights);
            var nBL = CentralDiffNormal(heights, cx,     cy);
            // ...same for BR, TR, TL

            // 5. Per-corner UVs in cell-local [0,1] space
            var uvBL = new Vector2(0, 0);
            var uvBR = new Vector2(1, 0);
            var uvTR = new Vector2(1, 1);
            var uvTL = new Vector2(0, 1);

            // 6. Two triangles per split direction
            //    SwToNe: (BL,BR,TR), (BL,TR,TL)
            //    NwToSe: (BL,BR,TL), (BR,TR,TL)
            void Emit(Vector3 p, Vector3 n, Vector2 uv)
            {
                uint idx = (uint)vertices.Count;
                vertices.Add(new Vertex(p, n, uv, d0, d1, d2, d3));
                indices.Add(idx);
            }

            if (split == CellSplitDirection.SwToNe)
            {
                Emit(posBL, nBL, uvBL); Emit(posBR, nBR, uvBR); Emit(posTR, nTR, uvTR);
                Emit(posBL, nBL, uvBL); Emit(posTR, nTR, uvTR); Emit(posTL, nTL, uvTL);
            }
            else
            {
                Emit(posBL, nBL, uvBL); Emit(posBR, nBR, uvBR); Emit(posTL, nTL, uvTL);
                Emit(posBR, nBR, uvBR); Emit(posTR, nTR, uvTR); Emit(posTL, nTL, uvTL);
            }
        }
    }

    return new LandblockMeshData(vertices.ToArray(), indices.ToArray());
}
```

384 vertices, 384 indices, 64 cells per landblock.

The `surfaceCache` is passed in from `GameWindow` so all 9 landblocks share the same palette → SurfaceInfo cache. WorldBuilder does this too — palCode hashing is deterministic so two different landblocks with the same corner config hit the same cached SurfaceInfo.

---

## 5. Vertex format change

`src/AcDream.Core/Terrain/Vertex.cs`:
```csharp
public readonly record struct Vertex(
    Vector3 Position,    // 12
    Vector3 Normal,      // 12
    Vector2 TexCoord,    // 8
    uint Data0,          // 4
    uint Data1,          // 4
    uint Data2,          // 4
    uint Data3);         // 4
// Total: 48 bytes (was 36)
```

Drops `TerrainLayer` since the shader now reads layer info from `Data0`.

**Breaking change for `GfxObjMesh`:** static meshes also use `Vertex`. Their normals etc. stay put, but they now emit `0, 0, 0, 0` for `Data0..3`. The `mesh.vert`/`mesh.frag` static-mesh shaders don't reference those attributes so they're ignored — but the VAO binding layout must include them (stride = 48, attribute 3-6 still enabled, they just carry dead bytes).

Alternative: give static meshes their own vertex struct so we don't waste 16 bytes per mesh vertex. Deferred — Phase 2+ mesh memory isn't a bottleneck, and a unified struct is simpler.

---

## 6. TerrainAtlas grows an alpha atlas

`src/AcDream.App/Rendering/TerrainAtlas.cs`:
```csharp
public sealed class TerrainAtlas : IDisposable
{
    // Existing
    public uint GlTerrainTexture { get; }  // 512×512 × 36 layers, RGBA8
    public IReadOnlyDictionary<uint, byte> TerrainTypeToLayer { get; }

    // New
    public uint GlAlphaTexture { get; }    // 512×512 × 16 layers, RGBA8 (R=G=B=A=alpha)
    public IReadOnlyList<byte> CornerAlphaLayers { get; }  // layers 0..3
    public IReadOnlyList<byte> SideAlphaLayers { get; }    // layers 4..7
    public IReadOnlyList<byte> RoadAlphaLayers { get; }    // layers 5..14 (up to 10)
}
```

Loading procedure (`Build` grows):
1. Same as today for the 36 terrain layers.
2. Iterate `Region.TerrainInfo.LandSurfaces.TexMerge.CornerTerrainMaps` — for each, load the referenced `SurfaceTexture` dat, decode to grayscale (single channel), upload as layer N of the alpha atlas where N = insertion order.
3. Same for `SideTerrainMaps` (appended after corners) and `RoadMaps` (appended after sides; layers may overlap per WorldBuilder's `5 + index` convention).

---

## 7. TerrainRenderer VAO update

`src/AcDream.App/Rendering/TerrainRenderer.cs`:
```csharp
uint stride = (uint)sizeof(Vertex);          // now 48
// aPos
_gl.EnableVertexAttribArray(0);
_gl.VertexAttribPointer (0, 3, Float, false, stride, (void*)0);
// aNormal
_gl.EnableVertexAttribArray(1);
_gl.VertexAttribPointer (1, 3, Float, false, stride, (void*)(3 * sizeof(float)));
// aTex
_gl.EnableVertexAttribArray(2);
_gl.VertexAttribPointer (2, 2, Float, false, stride, (void*)(6 * sizeof(float)));
// aData0..3 (uint attributes need VertexAttribIPointer)
_gl.EnableVertexAttribArray(3);
_gl.VertexAttribIPointer(3, 1, UnsignedInt, stride, (void*)(8 * sizeof(float)));
_gl.EnableVertexAttribArray(4);
_gl.VertexAttribIPointer(4, 1, UnsignedInt, stride, (void*)(8 * sizeof(float) + 4));
_gl.EnableVertexAttribArray(5);
_gl.VertexAttribIPointer(5, 1, UnsignedInt, stride, (void*)(8 * sizeof(float) + 8));
_gl.EnableVertexAttribArray(6);
_gl.VertexAttribIPointer(6, 1, UnsignedInt, stride, (void*)(8 * sizeof(float) + 12));
```

Binds both texture arrays before drawing:
```csharp
_gl.ActiveTexture(Texture0);
_gl.BindTexture(Texture2DArray, _atlas.GlTerrainTexture);
_gl.ActiveTexture(Texture1);
_gl.BindTexture(Texture2DArray, _atlas.GlAlphaTexture);
_shader.SetInt("uTerrain", 0);
_shader.SetInt("uAlpha",   1);
```

`StaticMeshRenderer` bumps to the same 7-attribute layout even though mesh.vert/frag don't use attribs 3-6 (they'll just sit ignored).

---

## 8. Shader rewrite

### terrain.vert
```glsl
#version 430 core
layout(location = 0) in vec3  aPos;
layout(location = 1) in vec3  aNormal;
layout(location = 2) in vec2  aTex;
layout(location = 3) in uint  aData0;
layout(location = 4) in uint  aData1;
layout(location = 5) in uint  aData2;
layout(location = 6) in uint  aData3;

uniform mat4 uModel, uView, uProjection;

out vec2  vBaseUV;
out vec3  vWorldNormal;
flat out uvec4 vPacked0;  // base(tex,alpha) ovl0(tex,alpha)  -- each 8 bits
flat out uvec4 vPacked1;  // ovl1(tex,alpha) ovl2(tex,alpha)
flat out uvec4 vPacked2;  // road0(tex,alpha) road1(tex,alpha)
flat out uvec4 vPacked3;  // rotations + split direction

void main() {
    vBaseUV      = aTex;
    vWorldNormal = normalize(mat3(uModel) * aNormal);

    vPacked0 = uvec4( aData0        & 0xFFu, (aData0 >>  8) & 0xFFu,
                     (aData0 >> 16) & 0xFFu, (aData0 >> 24) & 0xFFu);
    vPacked1 = uvec4( aData1        & 0xFFu, (aData1 >>  8) & 0xFFu,
                     (aData1 >> 16) & 0xFFu, (aData1 >> 24) & 0xFFu);
    vPacked2 = uvec4( aData2        & 0xFFu, (aData2 >>  8) & 0xFFu,
                     (aData2 >> 16) & 0xFFu, (aData2 >> 24) & 0xFFu);
    vPacked3 = uvec4( aData3        & 0xFFu, (aData3 >>  8) & 0xFFu,
                     (aData3 >> 16) & 0xFFu, (aData3 >> 24) & 0xFFu);

    gl_Position = uProjection * uView * uModel * vec4(aPos, 1.0);
}
```

### terrain.frag (sketch — exact compositing math is 1:1 from WorldBuilder's `Landscape.frag`)
```glsl
#version 430 core
in vec2  vBaseUV;
in vec3  vWorldNormal;
flat in uvec4 vPacked0, vPacked1, vPacked2, vPacked3;

out vec4 fragColor;

uniform sampler2DArray uTerrain;   // 36 layers
uniform sampler2DArray uAlpha;     // 16 layers

// Lighting (unchanged from Phase 3a tune)
const vec3 SUN_DIR = normalize(vec3(0.5, 0.4, 0.6));
const float AMBIENT = 0.25;
const float DIFFUSE = 0.75;

vec4 sampleT(uint layer, vec2 uv) {
    // Uniform tiling for now; can add per-layer uTexTiling[] later if needed.
    return texture(uTerrain, vec3(uv, float(layer)));
}

vec4 sampleA(uint layer, vec2 uv) {
    return texture(uAlpha, vec3(uv, float(layer)));
}

// Port of WorldBuilder maskBlend3: three-layer alpha-weighted composite.
vec4 combineOverlays(vec2 uv,
                     uint t0, uint a0,
                     uint t1, uint a1,
                     uint t2, uint a2) {
    // ... see references/WorldBuilder/Chorizite.OpenGLSDLBackend/Shaders/Landscape.frag
    //     for the exact math; port verbatim with attribution in a comment
}

vec4 combineRoads(vec2 uv, uint t0, uint a0, uint t1, uint a1) {
    // Port of WorldBuilder combineRoad: inverted-alpha multiply
}

void main() {
    uint baseTex = vPacked0.x;
    vec4 baseColor = sampleT(baseTex, vBaseUV);

    vec4 overlays = vec4(0.0);
    if (vPacked0.z != 255u)  // ovl0 present
        overlays = combineOverlays(vBaseUV,
            vPacked0.z, vPacked0.w, vPacked1.x, vPacked1.y, vPacked1.z, vPacked1.w);

    vec4 roads = vec4(0.0);
    if (vPacked2.x != 255u)  // road0 present
        roads = combineRoads(vBaseUV, vPacked2.x, vPacked2.y, vPacked2.z, vPacked2.w);

    // Composite: base × (1-ovlA) × (1-rdA) + ovl × ovlA × (1-rdA) + road × rdA
    vec3 b = baseColor.rgb * ((1.0 - overlays.a) * (1.0 - roads.a));
    vec3 o = overlays.rgb  * ( overlays.a       * (1.0 - roads.a));
    vec3 r = roads.rgb     *   roads.a;
    vec3 rgb = b + o + r;

    // Phase 3a lighting
    float ndotl    = max(dot(normalize(vWorldNormal), SUN_DIR), 0.0);
    float lighting = AMBIENT + DIFFUSE * ndotl;
    fragColor = vec4(rgb * lighting, 1.0);
}
```

---

## 9. Tests

### 9.1 Pure logic (`TerrainBlendingTests`)
- `GetPalCode_AllGrass_MatchesGolden` — hand-computed bit value
- `GetPalCode_DeterministicFromInputs`
- `GetCellSplitDirection_Deterministic`
- `GetCellSplitDirection_ProducesKnownValueForHoltburgCell` — golden from WorldBuilder instrumented
- `FillCellData_AllGrass_EmitsBaseOnly` — `d0.baseTex = grass, d0.ovl0Tex = 255`
- `FillCellData_GrassBorderingDirt_EmitsOverlay`
- `PseudoRandomIndex_InRange`

### 9.2 Mesh generation (`LandblockMeshTests` rewritten)
- Old "81 vertices per landblock" asserts replaced with "384 vertices per landblock"
- `AllSixVerticesOfACellShareData0123` — within each 6-vertex stride, Data0..3 are identical
- `SplitDirectionFlipsTriangleOrder` — with a stub cell that hashes NwToSe, the emitted triangle corners differ from SwToNe
- Flat landblock still produces coherent mesh (no NaN normals, positions in expected range)

### 9.3 Atlas build (`TerrainAtlasTests`)
- Extend existing tests: alpha atlas has ≥ 14 layers, each non-empty
- Corner/Side/Road layer index lists have expected sizes

### 9.4 Visual smoke (no automated harness)
- Fly around Holtburg; terrain type boundaries should blend smoothly
- If roads exist in the loaded landblocks, they should overlay terrain correctly
- Lighting still works (Phase 3a/3b preserved in the rewritten fragment shader)

---

## 10. Execution order (best sequence to catch bugs early)

1. **Palette math, pure CPU** — write `TerrainBlending` + unit tests first. No GL, no visuals. Just prove the palCode/FillCellData math is byte-identical to WorldBuilder via golden values. Commit as `feat(core): terrain palette blending math (Phase 3c.1)`.

2. **Alpha atlas loading** — extend `TerrainAtlas` to load the alpha maps from the Region dat. Add a debug output mode (optional: render alpha atlas full-screen as a grid) to verify the loaded masks look right. Commit as `feat(app): alpha atlas texture array (Phase 3c.2)`.

3. **Vertex format + mesh generation** — rewrite `Vertex` struct, `LandblockMesh.Build`, and the VAO bindings in both `TerrainRenderer` and `StaticMeshRenderer`. The old terrain.frag will still render (reads only baseTex from Data0, ignores overlays) so the world should still look recognizable — just with per-cell texture choices instead of per-vertex. Commit as `refactor(core+app): per-cell terrain vertex layout (Phase 3c.3)`.

4. **Shader rewrite** — the new terrain.vert/.frag with full blending. This is the visual-win commit. Iterate with you watching. Commit as `feat(app): per-cell terrain blending shader (Phase 3c.4)`.

5. **Memory + state doc** — update `MEMORY.md`, write `project_phase_3c_state.md`.

Each step is ~a few hundred lines, testable in isolation, and preserves a runnable program after commit. If step 4 looks wrong, we haven't broken steps 1-3.

---

## 11. Risks

| Risk | Mitigation |
|---|---|
| Bit layout in `FillCellData` drifts from the shader's `uvec4` unpack | Unit-test the CPU packer round-trip: `Pack → Unpack (same shifts as GLSL) → SurfaceInfo` and assert equality |
| `GetCellSplitDirection` magic constants copied wrong → wrong triangles | Golden-value test for ~5 known (lb,cell) → split from a WorldBuilder instrumented run |
| Triangle winding flipped → culled terrain (screen goes dark) | Disable face culling for the bring-up; enable once visually correct |
| Alpha atlas channel swizzle wrong (stored red but shader reads alpha) | Dedicated debug shader output mode showing only the alpha sample; short iterative loop |
| `TexMerge.TerrainDesc[i].MTileSize` unavailable in our DatReaderWriter version | Grep the reference crate first; fall back to uniform 1.0 tiling if missing |
| `StaticMeshRenderer` and `TerrainRenderer` VAO layouts drift apart after shared Vertex extension | Keep attribute enables identical in both; add a test that both call the same attribute-setup helper if drift becomes a concern |
| Visual iteration on blend math takes many runs | Leave the grayscale-lighting debug toggle easy to re-enable; have a "show alpha only" flag ready |

---

## 12. Deliverables checklist

- [ ] `src/AcDream.Core/Terrain/TerrainBlending.cs` + `SurfaceInfo` + `CellSplitDirection`
- [ ] `src/AcDream.Core.Tests/Terrain/TerrainBlendingTests.cs` with golden values
- [ ] `Vertex` struct extended (+16 bytes)
- [ ] `LandblockMesh.Build` rewritten to 64-cell layout
- [ ] `LandblockMeshTests` migrated to 384-vertex expectations
- [ ] `TerrainAtlas.Build` loads corner/side/road alpha maps, exposes `GlAlphaTexture` + index lists
- [ ] `TerrainRenderer` + `StaticMeshRenderer` VAO bindings extended (stride 48, 7 attributes)
- [ ] `terrain.vert` + `terrain.frag` rewritten with blending
- [ ] Visual smoke: Holtburg terrain boundaries blend smoothly
- [ ] Memory update: `project_phase_3c_state.md` + `MEMORY.md` index entry

---

## 13. Not in 3c, tracked for later

- **Phase 3d (if/when we need it):** chunking — global VBO slot pool, `TerrainChunk`, `TerrainGeometryGenerator` stateless chunk builder, multi-draw indirect. Pays off once we stream >20 landblocks.
- **Phase 4:** networking (design doc already at `docs/plans/2026-04-10-phase-4-networking-design.md`). Foundry statue comes online here.
- **Phase 5:** character appearance + animation.
- **Phase 6:** collision / physics (port ACViewer `Physics/`).