# Phase N — WorldBuilder Rendering Migration: Design

**Date:** 2026-05-08
**Status:** Design complete, awaiting plan generation for N.1.

## Goal

Stop re-porting AC-specific rendering and dat-handling algorithms from
retail decomp. Instead, depend on a fork of WorldBuilder
(`github.com/Chorizite/WorldBuilder`, MIT) for terrain, scenery, static
objects, EnvCells, portals, sky, particles, texture decoding, mesh
extraction, and visibility / culling. Acdream keeps its own network,
physics, animation, motion, UI, plugin, audio, and chat layers — those
are not in WorldBuilder.

## Why

acdream has accumulated a recurring pattern of subtle porting bugs in
its own rendering algorithms (the latest: a tree near the road at
landblock `0xA9B1` that retail and WorldBuilder do not show but our
re-port did, despite the algorithm code looking byte-identical to
WorldBuilder's). The triangle-Z bug, the hover-over-terrain bug, and
the edge-vertex spawn bug are all in the same family: small porting
errors that survive surface-level review.

WorldBuilder is verified by visual inspection to render the AC world
correctly. It uses the same Silk.NET + .NET stack we already target.
It is MIT-licensed. It has fewer subtle bugs because its developers
have run it against the entire client_cell + client_portal dat content
and fixed everything users have reported.

The cost of "we re-port retail algorithms ourselves" is now higher than
the cost of "we depend on someone else's tested port and inherit their
fixes." Migrating the rendering+dat layer to WorldBuilder is the
right call.

## Inventory reference

The full taxonomy of "what WorldBuilder has, what we keep porting
ourselves" lives at
[`docs/architecture/worldbuilder-inventory.md`](../../architecture/worldbuilder-inventory.md).
Before re-implementing any rendering or dat-handling algorithm, **check
the inventory first**. CLAUDE.md is updated to enforce this.

## Architecture

### Integration model

**Fork upstream WorldBuilder, depend on the fork via git submodule.**

- Fork: `https://github.com/eriknihlen/WorldBuilder` (already created;
  upstream: `Chorizite/WorldBuilder`).
- Long-lived branch in fork: `acdream`. Upstream `master` merges into
  `acdream` periodically; our acdream-specific changes (delete editor
  files, expose hooks for our scene state) live on `acdream`.
- The current read-only snapshot at `references/WorldBuilder/` is
  **replaced** by a git submodule pointing at the fork's `acdream`
  branch. Existing CLAUDE.md path references and research docs that
  cite `references/WorldBuilder/...` keep working.
- Our solution adds two `<ProjectReference>`s:
  - `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Chorizite.OpenGLSDLBackend.csproj`
  - `references/WorldBuilder/WorldBuilder.Shared/WorldBuilder.Shared.csproj`
- Transitive NuGet dependencies (`Chorizite.Core`,
  `Chorizite.DatReaderWriter.Extensions`, `BCnEncoder.Net`,
  `SixLabors.ImageSharp`, `Silk.NET.SDL`, `MP3Sharp`) flow through.
- Editor-only files in WorldBuilder (Modules/Landscape/{Tools,
  Commands, Services, Migrations, Hubs}, LandscapeDocument, etc.) stay
  in the fork's source tree but are simply not referenced by acdream.
  They impose no runtime cost. We can prune later if upstream stays
  well-organized.

### Phasing — strangler fig, subsystem by subsystem

Each sub-phase is independently shippable behind a feature flag
(`ACDREAM_USE_WB_<NAME>=1`). After visual verification the flag becomes
default-on, then is removed and the old code is deleted. This gives us
a one-line revert if a phase regresses.

| # | Sub-phase | Effort | Risk |
|---|---|---|---|
| **N.0** | Submodule + project references + build green | 1-2 hrs | Low |
| **N.1** | Scenery algorithm calls | 1-2 days | Low |
| **N.2** | Terrain math helpers | 1-2 days | Low |
| **N.3** | Texture decoding | 2-3 days | Medium |
| **N.4** | Object meshing (Setup/GfxObj) | 1 week | Medium |
| **N.5** | Terrain rendering (full pipeline) | 2 weeks | High |
| **N.6** | Static objects rendering | 2 weeks | High |
| **N.7** | EnvCells / dungeons | 2 weeks | High |
| **N.8** | Sky + particles | 1 week | Medium |
| **N.9** | Visibility / culling | 3-5 days | Medium |
| **N.10** | GL infrastructure consolidation (optional) | 1 week | Medium |

Total estimated calendar: 2-3 months. Engineering effort: 6-8 weeks.

### What WorldBuilder does NOT cover (keep porting from retail decomp)

- Network protocol (UDP, ISAAC, ACE messages) — keep ours
- Physics: collision, BSP queries, sphere sweeps, walkable validation
  — keep ours (partial), continue porting from retail decomp
- Animation: motion sequencer, cycle/non-cycle parts — keep ours
- Movement: WASD → MoveToState wire, remote-entity motion via
  UpdateMotion + dead-reckoning — keep ours
- Game UI: chat, vitals, inventory, spell book — keep ours (ImGui
  today, custom-toolkit later)
- Plugin API: IGameState, IEvents, IActions, IPacketPipeline,
  IOverlay — keep ours (acdream-unique)
- Game events: combat, allegiance, spell casting — keep ours
- Audio (OpenAL pipeline) — keep ours
- TurbineChat + slash commands — keep ours
- Login + character selection flow — keep ours

Per CLAUDE.md update, these still follow the
"grep named → decompile → verify → port" workflow against retail decomp
at `docs/research/named-retail/`.

### Network reference posture

`references/Chorizite.ACProtocol/` (separate Chorizite repo) remains
the Primary Oracle for protocol field order and packed-dword
conventions per CLAUDE.md's reference table. No fork needed there. We
will lean on it harder during future network-conformance phases (Phase
M is already on the roadmap for that).

## Components

### N.0 — Setup (must land before N.1)

**Files / actions:**
- Remove `references/WorldBuilder/` from working tree (it's currently a
  checked-in snapshot). Add it back as a submodule pointing at
  `git@github.com:eriknihlen/WorldBuilder.git` tracking the `acdream`
  branch (created off `master`).
- Add `<ProjectReference>` entries in
  `src/AcDream.Core/AcDream.Core.csproj` and
  `src/AcDream.App/AcDream.App.csproj` for the two WB projects.
- Update `.gitmodules` to reflect the new submodule.
- Verify `dotnet build` and `dotnet test` are green.
- Commit.

**Done criteria:**
- `git submodule status` shows `references/WorldBuilder` at the fork's
  `acdream` HEAD.
- Solution builds clean with no new warnings.
- Existing 870+ tests still pass.

### N.1 — Scenery algorithm calls

See companion design doc:
[`2026-05-08-phase-n1-scenery-via-wb-helpers-design.md`](2026-05-08-phase-n1-scenery-via-wb-helpers-design.md).

Brief: replace the algorithm guts inside `SceneryGenerator.Generate()`
with calls to WB's `SceneryHelpers` (Displace, RotateObj, ScaleObj,
ObjAlign, CheckSlope) and `TerrainUtils` (OnRoad, GetNormal). Keep our
data flow, our `ScenerySpawn` shape, our renderer integration. Add a
small adapter `LandBlock → TerrainEntry[]`.

### N.2-N.10 — separately brainstormed when we get there

Each sub-phase will get its own brainstorm + spec when we reach it.
Estimating ahead is unreliable for the bigger phases (N.5, N.6, N.7);
we'll know more after N.1 ships and we have hands-on experience with
the WB integration.

## Risks

1. **Chorizite.Core dependency footprint.** Each render manager we
   take pulls in `Chorizite.Core.Lib` and `Chorizite.Core.Render`.
   Mitigation: take the NuGet dep, don't try to strip it. Risk is
   mostly cosmetic (an extra package).

2. **WB's data-flow is editor-shaped.** `LandscapeDocument`,
   `LandscapeChunk`, etc. are editor concepts. Mitigation: write small
   adapters that produce the editor-shaped data from our dat reads.
   Phase N.1 is intentionally chosen to avoid this — we use only the
   stateless helpers, not the full `SceneryRenderManager`. Larger
   phases (N.5+) will need real adapter layers.

3. **Upstream divergence.** WB's `master` will keep moving. Mitigation:
   merge upstream `master` into our `acdream` branch periodically (at
   minimum, before each new phase starts). Our acdream-specific
   changes are isolated to deletions and additions on the `acdream`
   branch, which merges cleanly with upstream most of the time.

4. **Behaviors WB doesn't have.** WB is a dat editor; some
   in-game-only behaviors (creature appearance via CreaturePalette /
   GfxObjRemapping / HiddenParts) aren't in WB and we'll still need to
   handle them ourselves at the integration boundary. Mitigation:
   ACME's `StaticObjectManager.cs` covers these and is documented in
   CLAUDE.md as the secondary oracle for character appearance.

5. **Visual regression during migration.** Mitigation: feature flag
   per phase. Visual verification at known-good locations (Holtburg,
   Foundry statue, dungeon entrances) before flag becomes default-on.

## Testing

- **N.0:** existing 870+ tests stay green; `dotnet build` clean.
- **N.1:** new conformance test that runs both our `SceneryGenerator`
  and a parallel call into WB's helpers against the same fixture data,
  asserts identical spawn list. Visual verification at landblock
  `0xA9B1` — the offending tree should be gone, Issue #49's missing
  scenery should still be visible.
- **N.2-N.10:** each phase will define its own conformance and visual
  verification criteria when brainstormed.

## Documentation impact

- [x] `docs/architecture/worldbuilder-inventory.md` — created.
- [x] `CLAUDE.md` — updated with new posture (top-level rule + reference
  table + per-domain oracle hierarchy).
- [ ] `docs/plans/2026-04-11-roadmap.md` — add Phase N entry alongside
  L, M, etc. (this happens in the same commit as the spec).
- [ ] `docs/architecture/acdream-architecture.md` — needs an
  acknowledging note that the rendering layer is now WB-backed; can
  follow in a later commit, not blocking.

## Out of scope for this design

- Phase N.2-N.10 detailed scope (each gets own brainstorm).
- Network conformance work (separate Phase M).
- Animation, physics, motion ports (continue against retail decomp,
  not WB).
- UI, plugin, chat work (separate phases, not affected).