# Phase N.3 handoff — texture decoding via WorldBuilder

**Use this whole document as the prompt** when handing off to a fresh
agent. Everything they need to pick up cold is below.

---

## Background you'll need

You're working in `acdream`, a from-scratch C# .NET 10 reimplementation
of Asheron's Call's retail client. The project's house rule (in
`CLAUDE.md`) is **the code is modern, the behavior is retail**.

acdream just shipped **Phase N.1** (commits `26cf2b8` through `ad8b931`),
the first sub-phase of a strategic migration to fork WorldBuilder
(`github.com/Chorizite/WorldBuilder`, MIT) and depend on its tested
rendering + dat-handling code instead of porting algorithms from retail
decomp ourselves.

**Read first:**
- `docs/architecture/worldbuilder-inventory.md` — the full taxonomy of
  what WB has and what we keep porting ourselves
- `docs/superpowers/specs/2026-05-08-phase-n-worldbuilder-migration-design.md`
  — the parent design doc for Phase N
- `CLAUDE.md` — especially the "Reference repos" section (now points at
  WB as the rendering BASE) and the workflow rules

**Phase N.1 commit history (just shipped):** read
`git log --oneline c8782c9..ad8b931` to see how N.0 + N.1 were
structured. The pattern repeats for N.3.

## What N.3 is

Replace acdream's texture decoding pipeline with WorldBuilder's
`Chorizite.OpenGLSDLBackend.Lib.TextureHelpers`. WB handles INDEX16,
P8, BGRA, DXT, and alpha-channel decoding. Our existing implementations
of these are scattered across `src/AcDream.App/Rendering/TextureCache.cs`
and possibly `src/AcDream.Core/Meshing/` — find them with
`grep -rln "INDEX16\|P8 decode\|DXT\|BGRA" src/`.

## Acceptance criteria

- Build green (`dotnet build`)
- All existing tests green (the 8 pre-existing `DispatcherToMovementIntegrationTests`
  failures don't count — they exist on main)
- New conformance tests added per format that's substituted (one xUnit
  Theory per: INDEX16, P8, BGRA, DXT). Each compares a fixed input byte
  array decoded by our path vs WB's path; assertions on output pixel array.
- Visual verification at Holtburg (or wherever) shows no texture
  regressions: terrain texturing, mesh texturing, particle textures all
  look the same.
- ISSUES.md updated with any known cosmetic deltas (the N.1 pattern —
  if WB and retail disagree on something subtle, file it, don't try
  to fix it inline).

## Tasks (suggested decomposition)

Follow the N.1 plan structure (`docs/superpowers/plans/2026-05-08-phase-n1-scenery-via-wb-helpers.md`)
as the template. Concretely:

1. **Audit our texture decode paths.** Grep, list every file/method that
   decodes a texture. Map each to the WB equivalent in
   `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/TextureHelpers.cs`
   (read it end to end first).
2. **Per-format conformance test.** TDD style: write the test, run it
   to fail, then plumb the substitution. Conformance test fixture inputs
   should include real-dat byte sequences (read a known-good texture from
   a dat, encode the bytes as a hex blob in the test).
3. **Substitution.** Replace each decode site with the WB call. Keep our
   GL upload pathways — those are NOT WB's responsibility.
4. **Visual verification.** Launch the client at Holtburg, walk around,
   look at a tree (mesh texture), the ground (atlas texture), particles
   (the recent C.1 rain/clouds/aurora work), and a building (composite
   texture). Compare against retail or against a screenshot before the
   change.
5. **Delete legacy decoders** once visual verification passes.
6. **Update roadmap + ISSUES** as the final commit.

## Watchouts (lessons from N.1)

- **ACME has a downstream fork with extra filters** (`references/WorldBuilder-ACME-Edition/`).
  WB's `TextureHelpers` may have ACME-specific patches not yet in upstream.
  Compare both before assuming WB's version is canonical. We forked
  upstream WB; ACME is reference-only.
- **Conformance tests are non-negotiable.** Phase N.1's rotation bug was
  caught by the conformance test. Don't skip them. If a test fails, it's
  a real divergence — investigate before "fixing" the test.
- **Whackamole stops the migration.** If 3+ visual regressions appear on
  default-on, stop, file as ISSUES, ship. The migration goal is "use WB's
  tested code"; pixel-perfect equivalence with our broken hand-ports is
  not the goal.
- **`Setup.SortingSphere` ≠ `Setup.CylSphere`.** The N.1 attempt at
  `obj_within_block` over-suppressed because we used the wrong radius
  source (sorting sphere too large). For texture decoding this likely
  doesn't matter, but the general lesson is: read WB's full source
  carefully before adapting; don't assume parallel methods do parallel
  things.
- **Per-vertex road check — STOP signal.** If you find yourself reading
  ACME for "what's missing" and considering a per-vertex filter, STOP.
  N.1 tried this (commit `e279c46`), regressed visually, reverted in
  `677a726`. ACME's filter set works as a coherent unit; pick-and-choose
  fails. If the N.3 work uncovers a similar ACME-only filter, file it
  in ISSUES and move on, don't port it inline.

## Where to start

1. `git pull` on main to get the latest (Phase N.1 just merged).
2. Create a new worktree for the work:
   `git worktree add .claude/worktrees/<your-name> -b claude/<your-name>`.
3. Read the three "read first" docs above.
4. Run `dotnet build && dotnet test` to confirm clean baseline.
5. Read `references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/TextureHelpers.cs`
   end to end. Take notes on the public API surface.
6. Run the audit task (#1 in Tasks above). Output should be a markdown
   table of "our function / file:line / WB equivalent / format covered."
7. Use `superpowers:writing-plans` to convert the audit into a concrete
   per-format plan. Then use `superpowers:subagent-driven-development`
   to execute it with fresh subagents per format.

## Useful greps

- `grep -rln "INDEX16\|IndexedSurface\|P8\|DXT\|BGRA\|TextureFormat" src/` — find decode paths
- `grep -rln "TextureCache" src/` — find our cache layer
- `grep -n "public static.*Decode\|public static.*Convert" references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/TextureHelpers.cs` — WB's public API

## Open question to resolve early

Does `Chorizite.OpenGLSDLBackend.Lib.TextureHelpers` cover ALL the
formats we use, or does it have gaps? Audit our texture types against
WB's API in step 1. If WB is missing a format we need, the migration for
that format gets deferred (file in ISSUES; keep our decoder for it; note
in the roadmap).
