Decompile agent located the retail DayGroup selection function at
FUN_00501990 (chunk_00500000.c:1276). It is a straight-line 32-bit
signed LCG — NOT a ChanceOfOccur-weighted CDF. Replaces the SplitMix64
approximation from Phase 3a.
Algorithm (verbatim from the decompile):
seed = year * secondsPerDay + dayOfYear // TimeOfDay+0x64/+0x10/+0x68
hash = seed * 0x6A42FDB2 + 0x8ABE1652 // signed 32-bit LCG
index = floor(dayGroupCount * (uint)hash / 2^32)
if (index >= dayGroupCount) index = 0 // float-rounding safety
Uniform over all DayGroups. Dereth's 20 groups all carry ChanceOfOccur=5.0
so uniform matches the statistical intent; the weighted walk Phase 3a
attempted is NOT what retail does. The SecondsPerDay multiplier is
load-bearing — without it, adjacent years would share adjacent LCG
seeds and divergence from retail would recur annually.
Result (this session's local ACE):
server: PY106 ColdMeet 17 MorntideAndHalf, ticks=291130073
→ year=106, dayOfYear=(106×0 + 17 across ColdMeet) via DerethDateTime
→ retail picker returns a deterministic uniform index from LCG.
Acdream and retail now agree on the pick for any (Year, DayOfYear)
since both drive from the same server PortalYearTicks.
Changes:
- src/AcDream.Core/World/DerethDateTime.cs: add Year(ticks) and
DayOfYear(ticks) helpers (match retail TimeOfDay+0x64 / +0x68).
- src/AcDream.Core/World/SkyDescLoader.cs:
- SelectDayGroupIndex signature: (year, secondsPerDay, dayOfYear)
instead of the flat dayIndex used by the SplitMix64 approximation.
- Body: retail LCG line-by-line port with decompile citations.
- ACDREAM_DAY_GROUP env var still overrides (for A/B verification).
- src/AcDream.App/Rendering/GameWindow.cs: RefreshSkyForCurrentDay now
feeds Year / DayOfYear / SecondsPerDay=7620 to the picker instead
of a flat dayIndex. Composite `year*360+dayOfYear` still tracked
internally as the day-change key for provider-rebuild idempotence.
- docs/research/2026-04-23-daygroup-selection.md committed with the
full decompile trail (new agent-produced research).
Build + 717 tests green. User visual verification (retail side-by-side)
next.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Diagnosed the "retail shows early night, acdream shows early day" time
mismatch: server time sync was working correctly (ticks=291079558 →
dayFraction=0.8546 → EvensongAndHalf, hour 14 of 16). The mismatch was
the hardcoded DayGroup index.
Dereth's SkyDesc carries 20 DayGroups (Sunny / Clear / Cloudy / Storm /
etc), each weighted at ChanceOfOccur=5.0. Retail rolls one per server
day by `ChanceOfOccur` as a PDF (r12 §11). We were always rendering
DayGroup 0 = "Sunny" regardless of day, so at EvensongAndHalf we showed
SkyTime[7]@0.84 — sun still 20° above the western horizon, warm golden
— i.e. pre-sunset rather than the dimmer pre-night appearance retail
shows after rolling a cloudier group.
Fix (Phase 3a):
- LoadedSkyDesc.SelectDayGroupIndex(dayIndex) — deterministic roller:
SplitMix64 hash of dayIndex → normalize to [0, sumChances) → walk the
cumulative distribution. Same dayIndex on every client = same weather
on every client, zero network sync needed.
- LoadedSkyDesc.ActiveDayGroup(ticks) / BuildProviderForDay(ticks) —
convenience wrappers that compute dayIndex from raw server ticks.
- ACDREAM_DAY_GROUP=<N> env var override. Set to 10 "Clear", 12 "Cloudy",
etc. for A/B visual verification against retail.
- SyncFromServer gains a [sky-dump] log: `ticks=X dayFraction=Y
calendar=PY{year} {month} {day} {hour}` so the time-sync state is
auditable from a single grep.
- GameWindow: tracks _loadedSkyDayIndex + _activeDayGroup. Calls
RefreshSkyForCurrentDay on every server sync — swaps WorldTime's
provider + caches the group only when the day index crosses a
boundary (idempotent within a single day). SkyRenderer.Render now
consumes _activeDayGroup instead of the legacy DefaultDayGroup.
Observed (this session, local ACE):
server sent ticks=291079558 → PY106 ColdMeet 10 EvensongAndHalf
SplitMix64(day 38197) will deterministically pick one of 20 groups.
Build + 717 tests green. Ready for user visual verification with
retail side-by-side.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
The uncommitted uTint=AmbientColor-for-alpha-submeshes experiment (from
the 2026-04-22 inference) dimmed the sky dome's baked gradient — a
user-verified visual regression. Reverting to the eeae83a baseline
(uTint=Vector4.One for every submesh) while we execute the proper
retail-verbatim port.
Research: three parallel decompile-hunt agents landed verifying
retail's ground-truth sky pipeline for the first time (prior audits
searched for stripped symbol names; the trail opened via the Region
dat-type-index 0x1c registration at chunk_00410000.c:12952). Key
retail functions now mapped in chunk_00500000.c:1097-7535:
- FUN_00501530: keyframe bracket-picker (with 1.0f wrap denominator)
- FUN_00501600: sun+ambient interpolator (sunVec = DirBright ×
(sin yaw·cos pit, cos yaw·cos pit, sin pit))
- FUN_00501860: fog interpolator
- FUN_00502820: SkyDesc::Unpack (2 doubles + DayGroup list)
- FUN_00502a10: build per-frame sky-object table
- FUN_00505f30: apply light state + per-cell AdjustPlanes relight
- FUN_005062e0: per-frame sky tick (throttled by LightTickSize)
- FUN_00508010: sky-object render loop (enqueues through the NORMAL
mesh pipeline via FUN_00514b90 — not a bespoke path)
Surprise findings:
- D3DRS_AMBIENT is set to 0 once at init and NEVER changes per-frame
(chunk_005A0000.c). The r12-inferred "clouds = texture × D3DRS_
AMBIENT" formula is falsified. Retail instead routes keyframe
AmbColor through per-vertex lighting on non-Luminous sky meshes
via _DAT_008682bc/c0/c4.
- Retail does NOT anchor the sky to the camera or use a separate
sky projection. Sky meshes live in world space and follow the
camera via scene-graph parent.
- FUN_00532440 (AdjustPlanes) re-lights every terrain cell on every
keyframe tick — the "terrain follows the sky" effect we don't yet
reproduce.
Phase 1 code change (this commit):
- src/AcDream.App/Rendering/Sky/SkyRenderer.cs: revert uTint to white
for all submeshes (the per-submesh blend split stays — sun gets
additive, clouds get alpha). Keep the `keyframe` parameter in the
signature for Phase 2 readiness. Comments now cite the retail
functions and reference docs instead of the (disproven) r12 formula.
- src/AcDream.Core/World/SkyDescLoader.cs: ACDREAM_DUMP_SKY=1 logs
the entire Region SkyDesc on load — DayGroups, SkyObjects, every
SkyTimeOfDay keyframe, and every SkyObjectReplace with RAW pre-/100
Transparent/Luminosity/MaxBright values so we can settle the unit
question empirically.
- src/AcDream.App/Rendering/Sky/SkyRenderer.cs: ACDREAM_DUMP_SKY=1
additionally logs each sky GfxObj's Surfaces and their SurfaceType
flags on first load, so we can identify which meshes carry the
Luminous bit (dome? sun? moon? stars?) vs which are lit.
- src/AcDream.App/Rendering/GameWindow.cs: passes the interpolated
keyframe to the sky renderer (kept — needed for Phase 2).
Research docs (pushed as part of this commit):
- docs/research/2026-04-23-sky-retail-verbatim.md: full synthesis
with retail function map, struct layouts, globals, pseudocode, and
a 4-phase port plan.
- docs/research/2026-04-23-sky-decompile-hunt-{A,B,C}.md: raw hunt
outputs.
- docs/research/2026-04-23-sky-references-crossref.md: WorldBuilder/
ACE/ACViewer/holtburger/Chorizite coverage.
- docs/research/2026-04-23-sky-dat-schema.md: full dat schema + unit
analysis.
- docs/research/2026-04-22-sky-lighting-decompile.md: prior agent's
(superseded) inference — kept for provenance.
Phase 2 will port Surface.Luminous-flag-aware per-vertex lighting for
sky submeshes once the dump resolves the open questions (Luminous-flag
distribution per Dereth sky mesh; _DAT_007a1870 scale constant value).
Build + 717 tests green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Retail's Region dat stores SkyObjectReplace.Luminosity / Transparent /
MaxBright as percentages in the 0..100 range. Our shader expects
fractions in 0..1. We were passing raw values (luminosity up to 100)
straight into the sky fragment shader's rgb-multiplier:
rgb = sampled.rgb * uTint.rgb * uLuminosity;
At the "Sunny" DayGroup's noon keyframes (verified via live diag),
Luminosity = 100 → shader multiplied the cloud texture RGB by 100 →
min(rgb, vec3(1.2)) clamped all channels to 1.2 → pure white sky.
Also gave the dawn/dusk purple sky effect on top of the pale texture.
Fix: SkyDescLoader.ConvertTimeOfDay divides Luminosity, Transparent
and MaxBright by 100 when loading each SkyObjectReplace. The Rotate
field stays as degrees (values like 270° are genuine headings, not
percentages).
Transparent was accidentally surviving via a 0..1 clamp downstream,
but we fix it for consistency and so brightness-attenuating values
in the 0..99 range (partial fade during dawn/dusk) work correctly
instead of rounding to full-transparent.
WorldBuilder's SkyboxRenderManager does NOT apply these fields at
all — that's why they never hit this bug. Our port applies them for
per-keyframe day-night fades, so we needed the unit conversion.
Also picked up in this commit (incidental, already running):
- Sky render: per-submesh blend mode from TranslucencyKind.Additive
for sun/moon-style self-bright objects (Additive bit 0x10000).
Luminous flag 0x40 intentionally NOT mapped to additive — that
flag is on the sky dome + cloud sheets and making them additive
produced the previous "fully white" iteration of this bug.
- ToD default seed: DayTicks/16 (Midsong = hour 9 = true noon)
instead of DayTicks*0.5 which landed on Gloaming-and-Half (sunset)
due to DerethDateTime's +7/16 day-fraction offset. Pre-TimeSync
view now correctly starts at noon.
- Lightning flash: brighter white-blue (vec3(1.5,1.5,1.8)) instead
of dim grey; ceiling relaxed during flash so the strobe actually
blows out. Cadence (strike intervals, decay) unchanged.
- Saved docs/research/2026-04-21-sky-deep-audit.md with the
decompile+ACE+ACME+WorldBuilder research done to corner this bug.
Open follow-up (not fixed here): sky clouds are white at noon /
don't get the dusk/night purple tint. Our sky shader is fully unlit
— doesn't apply sun/ambient directional light like the terrain
shader does. AmbientColor in the keyframe data carries the right
tint (purple at midnight, magenta at dusk) but we pass
uTint = Vector4.One instead of the keyframe value. Next commit will
wire directional-sun + ambient into sky.frag so cloud meshes pick
up the time-of-day color.
All 717 tests green. User-confirmed: sky colors are now "much
better" after this change (previously fully white).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Expand the SkyKeyframe record with retail-exact fog fields (FogStart,
FogEnd, FogMode) per r12 §5. The existing FogDensity field is retained
for backwards compat with tests that pin it; new shipping code reads
FogStart / FogEnd / FogMode directly.
Add WeatherSystem (WeatherKind + EnvironOverride enum + 10s transition
ease + deterministic per-day-index roll) matching r12 §6.1. Roll weights
are ~60% Clear / 20% Overcast / 12% Rain / 5% Snow / 3% Storm — tuned
against retail observations. Storm mode triggers lightning flashes
every 8–30 s via an exponential-decay (200ms τ) flash level that the
shader consumes as an additive scene bump.
Add SkyDescLoader that parses the Region dat (0x13000000) into
LoadedSkyDesc — DayGroupData with SkyObjectData (visibility window +
arc sweep), per-keyframe SkyObjectReplaceData, and a shader-ready
SkyStateProvider builder. Sun/ambient colors are pre-multiplied by
DirBright/AmbBright so the shader never needs to know about retail's
scalar brightness field.
19 new tests (weather determinism, transition ease, environ override
tint, flash decay, dat-load conversion with fog + pre-mult colors).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Erik