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fix(sky): translucency-as-opacity + sky fog floor + additive fog-skip

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Three retail-faithful sky/weather composite fixes (one cohesive commit
because they touch the same per-Surface flag plumbing path).

1. Surface.Translucency is OPACITY, not (1 - opacity).
   Retail D3DPolyRender::SetSurface at 0x59c7a6 (decomp 425255-425260)
   computes `curr_alpha = _ftol2(translucency × 255)` and writes that
   directly as vertex.color.alpha. ACViewer (TextureCache.cs:142) and
   WorldBuilder (ObjectMeshManager.cs:1115) both use `1 - translucency`
   and are wrong by the same misread. Cloud surface 0x08000023 has
   Translucency=0.25; under the old (1-x) formula opacity was 0.75,
   making clouds 3× too bright vs retail. Flipped to use translucency
   directly. Gated on the Translucent flag (0x10) so non-Translucent
   surfaces (which carry Translucency=0 in the dat) keep opacity 1.0
   instead of going invisible.

2. Sky fog re-enabled with a "fog floor" mitigation.
   Disabled 2026-04-24 because Dereth sky meshes are authored at radii
   1050-1820m while storm-keyframe FogEnd is ~400m, which would saturate
   the entire dome to flat fogColor and destroy stars/moon/dome texture.
   Retail visibly DOES fog its sky, mechanism still un-pinned. Workaround:
   clamp `vFogFactor` to a minimum of SKY_FOG_FLOOR=0.2 so the dome shows
   AT LEAST 20% raw texture even at extreme distances. Tuned via dual-
   client visual comparison; preserves stars/moon while letting the
   horizon haze visibly in low-FogEnd keyframes.

3. Additive sky surfaces skip fog entirely.
   Retail D3DPolyRender::SetSurface at 0x59c882 calls
   SetFFFogAlphaDisabled(1) when the Additive flag (0x10000) is set —
   sun, moon, stars, additive cloud sheets render unfogged. Without this
   gate the sun dimmed to fog color at horizon dusk/dawn instead of
   staying bright. Plumbed via new `uApplyFog` shader uniform driven by
   the existing SubMeshGpu.IsAdditive boolean (already set from
   TranslucencyKind.Additive at upload time).

User visually verified all three vs retail screenshots in Holtburg.
Tests: 1223 pass.
This commit is contained in:
Erik 2026-04-27 19:49:51 +02:00
parent 63b50c5291
commit 97fc1b51d8
4 changed files with 121 additions and 55 deletions
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32
src/AcDream.Core/Meshing/GfxObjMesh.cs
View file

@ -200,7 +200,21 @@ public static class GfxObjMesh
// docs/research/2026-04-23-sky-retail-verbatim.md §6).
var translucency = TranslucencyKind.Opaque;
var luminosity = 0f;
var surfTranslucency = 0f;
// SurfTranslucency = the OPACITY multiplier the shader applies
// to fragment alpha. 1.0 = fully opaque (default, non-Translucent
// surfaces). For Translucent-flag surfaces, retail's
// D3DPolyRender::SetSurface at 0x59c7a6 (decomp lines 425255-
// 425260) computes curr_alpha = _ftol2(translucency × 255) and
// feeds that as vertex.color.alpha — so the dat's Translucency
// float is the OPACITY directly (NOT inverted). For rain
// (translucency=0.5) opacity is 0.5; for cloud surface
// 0x08000023 (translucency=0.25) opacity is 0.25 — that's why
// retail's clouds are dim and acdream's were 3× too bright
// before this fix (we used 1-translucency, inverting the
// semantic). ACViewer's TextureCache.cs:142 and WorldBuilder's
// ObjectMeshManager.cs:1115 also use 1-translucency and are
// both wrong by the same misread.
var surfTranslucency = 1.0f;
if (dats is not null)
{
var surface = dats.Get<Surface>(surfaceId);
@ -208,15 +222,13 @@ public static class GfxObjMesh
{
translucency = TranslucencyKindExtensions.FromSurfaceType(surface.Type);
luminosity = surface.Luminosity;
// Retail D3DPolyRender::SetSurface at 0x59c767: when the
// Translucent (0x10) flag is set, the surface's
// Translucency float drives per-vertex alpha. Both
// ACViewer and WorldBuilder apply opacity = (1 - x).
// For the rain Surface 0x080000C5 this is 0.5. Carrying
// the float verbatim and converting to opacity in the
// shader keeps non-Translucent surfaces (Translucency=0)
// identical to the previous behavior.
surfTranslucency = surface.Translucency;
// Apply the dat's Translucency value as opacity ONLY
// when the Translucent flag (0x10) is set on the
// Surface. Without this gate, surfaces with
// Translucency=0 (non-Translucent default) would
// render fully transparent.
if (((uint)surface.Type & (uint)DatReaderWriter.Enums.SurfaceType.Translucent) != 0)
surfTranslucency = surface.Translucency;
}
}