forked from Ports/goldeneye-pc-port
James Dansereau
811e9a62cf
Recoil byte order, crouch-tank interactions, auto-fire cadence in ticks, hipfire turret tracking, light-fixture Gfx index parsing, hit-type byte reads. Rule-2 sign-offs recorded for D424 and D427. Documentation re-triaged onto a single tracker (docs/ROADMAP.md). Refs: D424 D425 D426 D427 D428 D430 D431 D432 D434 #114 #115 #116 #117 #118 #119
128 lines
6.9 KiB
Markdown
128 lines
6.9 KiB
Markdown
# D176(a) Sky — consolidated research notes (read-only)
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Companion to the (since retired) `GRAPHICS-BACKLOG.md` D176(a) and findings.md M-37/M-47.
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Purpose: make the next implementation session mechanical. No code changed by
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this doc.
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## State of play
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- **Root cause (M-37):** `skyRenderTri`/`skyRenderFull` emit geometry only as
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`G_RDPHALF_*` immediates; `gfx_pc.cpp:2901-2903` deliberately no-ops them
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(same in the PD port) → solid black sky on every cloud-sky level. Clean
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headless repro: `-level_22` (Statue), frame ~360.
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- **M-47 draft (Path B)** — `scratchpad/sky.patch`: `#ifdef PORT` hooks in
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both functions re-emit the sky as ordinary screen-space GBI via
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`skyPortRenderPoly` (ortho pixel projection + `gSPVertex`/triangles).
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Geometry reaches GL and textures (top-half luma 0.1 → ~174 on `-level_22`).
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Two open defects (below). Draft, not shippable.
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- **Path A** (decode RDPHALF in fast3d) is the backlog's original suggestion;
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the wire format is now fully decoded (next section), so it is a viable
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fallback if Path B's units can't be pinned.
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## RDPHALF wire format (decoded from `src/game/sky.c` + `include/PR/gbi.h`)
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- Opcodes (`gbi.h:102-164,216-223`): `G_RDPHALF_1 = 0xe1`,
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`G_RDPHALF_2 = 0xf1` (final word of a stream), `G_RDPHALF_CONT`
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(= `G_IMMFIRST-13`). Tri ops: `G_TRI_FILL 0xc8`, `G_TRI_FILL_ZBUFF 0xc9`,
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`G_TRI_SHADE_TXTR 0xce`, `G_TRI_SHADE_TXTR_ZBUFF 0xcf`.
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- **All payload values are S15.1 fixed point** — converter is
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`sub_GAME_7F094298` (`sky.c:218`): `clamp(x, ±32767.9) × 65536`, u32.
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- **Packing:** each `(G_RDPHALF_1, G_RDPHALF_CONT)` pair carries four
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halfwords = **two S15.1 values**: `valueA = (w1.hi16, w2.lo16)`,
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`valueB = (w1.lo16, w2.hi16)`. Multi-value groups are streamed in two
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passes — all high halves first, then all low halves (see the explicit
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`(x & 0xffff0000) | (y & 0xffff0000) >> 16` packing at `sky.c:1854-1870`).
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- **`skyRenderTri` header** (`sky.c:1725-1728`): first word =
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`(op << 24) | (winding/sign flag bit23) | lo16(first value)`; the stream
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then carries screen-space positions (`unk28×0.25` = pixel X, `unk2c` = Y×4
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family) plus per-vertex attributes. The RSP ucode reassembles and DMAs a
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real edge-walked RDP tri command (per the M-46 comment in sky.patch).
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- **Vertex attribute vectors are 8 components** (`sky.c:1759-1776`):
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`(r, g, b, a, S·w′, T·w′, 32767·w′, z)` with perspective weight
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`w′ = unk34 × min(unk0c)/2` — i.e. color + perspective-corrected S/T +
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depth. `SkyRelated38` fields: `unk28 = screenX×4`, `unk2c = screenY×4`,
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`unk30 = z (0..0x7fff)`, `unk34 = 1/w`, `unk20/unk24 = S/T`.
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- GE emits RDPHALF **nowhere except sky.c** — the command surface is closed
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and small.
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## Defect 1 — texcoord shear (clouds smeared into streaks)
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`skyPortRenderPoly` uses `tc = unk20 × 32.0f` — a magic number.
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### M-82 update — the scale is right, the magnitude is the bug
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- **Cloud texture is `s_skywaterimages[0]` = `IMAGE_CLOUDS_GRAYSCALE`,
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`0x40 × 0x40` (64×64), `G_IM_FMT_IA` / `G_IM_SIZ_8b`, `G_TX_WRAP`/`G_TX_WRAP`**
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(`assets/oddtextures.c:646`). Not a fraction-indexed texture — it *tiles*.
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- **GE's own screen-space textured-quad idiom is `tc = width << 5`**
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(`src/game/glass2.c:678,689` — `sImageTableEntry.width << 5` /
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`.height << 5`). `<< 5` = ×32 = the S10.5 texel fixed-point fast3d expects,
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and it divides `tc` by `tex_width` before GL_REPEAT. So the patch's `× 32.0f`
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**is the correct scale** — provided `unk20`/`unk24` are already in *texel*
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units (they are: the cloud-tiling math in `sub_GAME_7F097388` /
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`CloudRepeat(skyheight)=10000` produces texel coords in the thousands, and
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M-47 measured `unk20` reaching ±20 000 near the horizon).
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- **The bug: `(s16)(20000 * 32)` overflows.** `Vtx.tc` is `s16` (±32767); a
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horizon vertex needs `tc` ≈ 640 000. It wraps/saturates → the streak.
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- **Fix (no fast3d change, no grid needed for the near/mid sky):** phase-fold
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every vertex of *one primitive* by the **same** multiple of the tile period
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(64 texels), so inter-vertex deltas are preserved but magnitudes stay in
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s16 range:
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```c
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s32 foldS = (s32)floorf(vmin_unk20 / 64.0f) * 64; // vmin over this prim's verts
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s32 foldT = (s32)floorf(vmin_unk24 / 64.0f) * 64;
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tc[0] = (s16)((v[i]->unk20 - foldS) * 32.0f);
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tc[1] = (s16)((v[i]->unk24 - foldT) * 32.0f);
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```
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This is exact wherever the S/T *span across a single triangle* is
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< ~1000 texels (s16 range ÷ 32). It holds for the corner-quad tessellation
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the current patch emits everywhere except triangles that straddle the
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horizon line, where the span diverges — those still need Defect-2's
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finer/adaptive tessellation. Land the fold first (fixes most of the frame),
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keep the grid as the horizon-band follow-up.
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## Defect 2 — vertical coverage (band fills only top ~25%)
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Leading hypothesis, from reading the N64 path: **the quad is not a plain
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corner quad.** The N64 stream's four vertices are *perspective-interpolated*
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points computed in the big interpolation block (`sky.c:1797-1813`, the
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`sp210/sp290/sp2b0/sp230` vectors), not the raw TL/TR/BL/BR corners that
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Path B's hook passes through. If those interpolated y-values differ from the
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corner ys (they will, wherever w varies across the quad), the re-emitted
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quad lands short.
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Next steps (one focused session):
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1. Per-call probe on `skyPortRenderPoly`: log nverts + each vertex's
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(x, y) + the tri op-header word; A/B against N64 reference footage of
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`-level_22`/`-level_36`.
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2. If confirmed, either (a) replicate the interpolation block under PORT and
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emit the 4 computed vertices, or (b) drop to Path A with the decoder spec
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above (the ucode's own reassembly is then irrelevant — we decode the same
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values directly).
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## Recommendation
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Finish **Path B** first — geometry already works; both defects are unit/
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vertex-set questions answerable in one render-iterate session. Keep Path A
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as fallback; it is now fully specified by this doc if B stalls.
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## Verification ask (M-82 — for the next session with a display)
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The Defect-1 phase-fold is committed to PR #18 but **unverified visually**
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(this session had no display). One check settles it:
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1. `./build-pc/ge007.x86_64.exe -level_22` (Statue, night). At the intro the
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camera faces the horizon within a few seconds.
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2. Look at the upper half of the frame:
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- **Black** → Path B still not emitting (regression from the rebase —
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check `skyPortRenderPoly` is reached; `GE_D176=1`).
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- **Clouds, but smeared into horizontal streaks** → the fold didn't take;
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re-check `unk20`/`unk24` really are texels (probe their range).
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- **Recognisable tiled cloud texture, correct near/mid, ragged or missing
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right at the horizon line** → fold works; that ragged band is Defect 2
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→ adaptive tessellation is the next task.
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- **Looks right** → close Defect 1, mark PR #18 ready pending a
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`-level_36`/`-29`/Dam-exterior spot check.
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3. A `GE_PCDUMP="330-390:30"` capture of that run attached to PR #18 is enough
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for a remote reviewer to make the same call.
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