docs(backlog): local interstellar-dust volume (Edenhofer 2024)

docs/BACKLOG.md

@@ -44,6 +44,7 @@ Items with a **→ details** link have a full write-up in [`backlog/`](backlog/)
 ## Rendering
 
 - [ ] **Ultra-real Earth** `needs-design` — atmosphere scattering, day/night lights, specular oceans on the dedicated `earthRenderer` seam; scope in a brainstorm first.
+- [ ] **Local interstellar-dust volume (Edenhofer 2024)** `needs-design` — Sun-centered per-parsec extinction cube as an SCFD field (MCPM-clone, ~3.2 GB one-time via `dustmaps`); blocked on a sub-kpc render slab (COSMO near-clip = 10 kpc) + emissive-vs-absorptive compositing choice. → [details](backlog/2026-07-18-local-dust-volume.md)
 - [ ] **Lower-res offscreen star-aggregate pass** `ready` — try `STAR_AGGREGATE_DIVISOR` 2 → 4 (`renderTargets.ts`); ~4× further fill cut if the upsampled glow field survives visually.
 - [ ] **Bright star clump at ~5.9 kpc** `deferred` — flux verified conserved; residual over-exposure is display policy (mid-anchor slider + summed knee shipped; retune or tone-map shoulder next). → [details](backlog/2026-07-17-star-clump-brightness-5-9kpc.md)
 - [ ] **Field-star resolve predicate duplicated** `ready` — `starResolves` (focusedFieldStarSphereLayer) restates partitionStarsByResolution's per-star diameter+resolve rule; extract a shared `starResolvesToSphere` util both call.

docs/backlog/2026-07-18-local-dust-volume.md

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+# Local interstellar-dust volume (Edenhofer 2024)
+
+**Status:** needs-design
+**Area:** Rendering / volumes + data source
+
+The 10¹⁹ "stars around the Sun" rung of the Powers-of-Ten ladder
+(`docs/powers-of-ten/data.js`) is marked _buildable_ precisely because the
+Gaia star field already renders and this dust volume is the missing half. It
+is the natural companion to the near-field star bin: stars + the dust they
+sit inside, at the Local-Bubble scale.
+
+## The data is available and ideal
+
+Edenhofer et al. (2023), "A parsec-scale Galactic 3D dust map out to 1.25 kpc
+from the Sun." Zenodo DOI [10.5281/zenodo.8187943](https://doi.org/10.5281/zenodo.8187943).
+
+- **Quantity:** differential dust _extinction density_ — magnitude per parsec
+  (the Zhang–Green–Rix 2023 extinction unit). A genuine continuous density
+  field, not integrated columns — exactly what a raymarch wants.
+- **Native grid:** HEALPix × distance — 516 log-spaced shells 69→1250 pc,
+  Nside 256, 12 posterior samples, **centered on the Sun**. A validation run
+  extends to 2 kpc. The Local Bubble (~300 pc) sits well inside.
+- **Also provided pre-resampled** to cartesian `xyz` (mean+std, 15.7 GB,
+  2 pc voxels, |X|,|Y| ≤ 2100 pc, |Z| ≤ 700 pc) and galactic `lbd`. Full
+  deposit is 122.7 GB — but we need almost none of it.
+- **Light acquisition path:** the [`dustmaps`](https://dustmaps.readthedocs.io/)
+  Python package. `Edenhofer2023Query` samples at arbitrary `(l, b, distance)`
+  with `mode='mean'`; `fetch()` pulls only **~3.2 GB** (mean+std HEALPix) once
+  on a build machine. Sample it onto whatever cartesian grid we choose — no
+  15.7 GB download.
+
+**This is the MCPM pattern, exactly.** MCPM already documents a Python +
+domain-package extraction (`tools/volumes/extractMcpmCube.py`, "requires
+Python + pyslime, once per VAC release; contributors curl the pre-extracted
+`.npy` tiers from R2 and run `npm run build-mcpm`"). Replace pyslime with
+`dustmaps` and the flow is identical.
+
+A modest cube — 256³ at ~10 pc voxels (±1.25 kpc) — is **~32 MB as f16 SCFD**
+(384³ ≈ 108 MB), trivial next to `glade-large.bin` (130 MB). Tier it
+small/medium/large like MCPM.
+
+**Pre-spec verification (one fact):** confirm the Zenodo record's reuse
+licence before ingest — it is citable (Edenhofer et al. 2023) but the licence
+was not confirmed during this survey.
+
+## Rendering: most of the stack exists; one real engine gap
+
+skymap already has a complete scalar-volume pipeline. Adding a scalar field is
+a well-trodden ~12-site mechanical job (mirror MCPM/CF-4), no new format:
+
+- **Format** — `src/data/volume/scalarFieldFormat.ts` (SCFD v3, 96-byte
+  header, f16 voxels). Extent is **per-field** in the header
+  (`origin` + `voxelSize` + `dims`); nothing in the format or
+  `buildCubeModelMatrix` assumes a cosmological voxel size, so a sub-parsec
+  cube encodes and places fine.
+- **Builder** — new `tools/volumes/buildDustVolume.ts` mirroring
+  `buildMcpmVolume.ts` (npy → f16 → `encodeScalarField`, tiered), fed by a
+  new `extractDustCube.py` mirroring `extractMcpmCube.py`.
+- **Registration surface** — Source enum (`src/data/source.ts`), registry
+  entry (`src/data/sources/<id>.ts` + `SOURCE_REGISTRY` row), fetcher + slot
+  (mirror `mcpmFetcher`/`mcpmSlot`), `AssetKey`, demand row
+  (`assetWiring.ts`), raw-data registry, R2 allow-list (`tools/deploy/syncR2.ts`).
+  Settings/UI need no edits — volume rows are keyed generically by
+  `VolumeFieldId`.
+
+**The load-bearing gap — the render slab.** The raymarch runs on the **COSMO**
+slab (`scalarVolumeLayer.slab = COSMO`), whose near plane is hardcoded at
+`COSMO_NEAR_MPC = 0.01` (**10 kpc**) in `src/services/engine/frame/slabs.ts`.
+A ±1.25 kpc dust cube (≈0.0025 Mpc across) sits _entirely inside_ 10 kpc: to
+look at it the camera is a few kpc from the Sun, so the whole cube is nearer
+than the COSMO near-clip and gets thrown away. The volume _format/placement_
+is scale-agnostic; the _render slab_ is not. Options: re-home the volume
+offscreen target onto the **NEAR0** slab (today it carries Sun/Earth/stars/
+orbits but not the volume target), or add a sub-kpc-near slab. Either is a
+real engine change, and it overlaps the slab-restructuring theme in
+[`2026-07-13-star-field-own-slab.md`](2026-07-13-star-field-own-slab.md) —
+design them together.
+
+**The aesthetic-vs-physical axis — compositing.** The existing pass
+(`shaders/scalarVolume/fragment.wesl`) is **additive/emissive** — front-to-back
+accumulation, palette LUT, bright-end exposure blow-out — tuned to make the
+_cosmic web glow_. Dust is physically **absorptive**: it should dim and redden
+the stars _behind_ it (dark nebulae), not glow.
+
+- Cheapest: reuse the emissive pass with a warm/brown palette → glowing dust
+  clouds. Looks fine, isn't physical.
+- Right: a Beer–Lambert absorption / over-compositing mode, ordered against
+  the star passes so the dust occludes them. Additional rendering-infra work,
+  and it interacts with the slab/pass ordering above.
+
+## Smaller, table-driven pieces
+
+- **Fade band** — a `SCALE_FADE_BANDS` entry (#438) so the field fades in only
+  around the Local-Bubble zoom (meaningless at cosmological _and_ planetary
+  zoom); gate via `volumeLiveness`.
+- **Colour ramp** — pick an existing palette or add one brown/absorptive
+  branch in `src/data/volume/scalarFieldPalettes.ts` (+ `ScalarFieldPaletteId`).
+- **densityScale/intensity** — tune per-cube on the registry entry; local-space
+  math is [0,1]³ so per-step integration is extent-independent, only the data's
+  dynamic range matters.
+
+## Design questions
+
+1. Slab: re-home the volume target to NEAR0, or a new sub-kpc slab? Resolve
+   jointly with the star-field-own-slab item.
+2. Compositing: emissive glow (ship-fast) vs Beer–Lambert absorption
+   (physical) — and if absorption, the dust pass's order relative to the
+   star/Earth passes and the HDR composite.
+3. Cube resolution + tiers: 256³ vs 384³; voxel size vs the map's native
+   ~2–10 pc effective resolution (don't oversample past the data).
+4. Extent: full 1.25 kpc, or a tighter Local-Bubble crop for the small tier?
+
+## References
+
+- `docs/powers-of-ten/data.js` — the 10¹⁹ rung this completes
+- `src/data/volume/scalarFieldFormat.ts` — SCFD format
+- `tools/volumes/buildMcpmVolume.ts` + `extractMcpmCube.py` — the clone target
+- `src/services/engine/frame/slabs.ts` — `COSMO_NEAR_MPC` = 0.01 (the gap)
+- `src/services/gpu/shaders/scalarVolume/fragment.wesl` — additive/emissive raymarch
+- [`2026-07-13-star-field-own-slab.md`](2026-07-13-star-field-own-slab.md) — co-design the slab work
+- Edenhofer et al. 2023, Zenodo [10.5281/zenodo.8187943](https://doi.org/10.5281/zenodo.8187943); [`dustmaps`](https://dustmaps.readthedocs.io/)