acdream/docs/research/2026-04-22-sky-lighting-decompile.md

Sky Lighting Formula Analysis: Retail vs acdream

Date: 2026-04-22
Status: DECOMPILE-INCOMPLETE — retail D3D render code not located
Finding: Formula inferred from observed behavior + r12 architecture docs + test evidence

Executive Summary

Retail AC sky meshes are tinted by the ambient color from the current SkyTimeOfDay keyframe.

Additive meshes (sun/moon/stars) render unlit with texture colors preserved.
Alpha-blended meshes (clouds) multiply texture by ambient color to pick up time-of-day hue.

At midnight: AmbientColor = (0.05, 0.05, 0.12) → clouds appear purple
At dusk: AmbientColor = (0.35, 0.25, 0.25) → clouds appear pink/orange
At noon: AmbientColor = (0.5, 0.5, 0.55) → clouds stay light gray

Our bug: SkyRenderer.cs:200 always sets uTint = Vector4.One (white), so clouds stay neutral gray.

1. Sky Mesh Draw Entry Point

Retail decompile search: NOT FOUND

Searched chunk_00400000.c through chunk_007F0000.c for:

• String literals: "sky", "Sky", "SkyObject"
• D3D constants: D3DRS_LIGHTING, D3DRS_AMBIENT, SetMaterial, SetLight
• Environment/weather functions

No matches. Retail sky code likely in undecompiled section or heavily compiler-optimized.

Retail reference: WorldBuilder SkyboxRenderManager

File: references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/SkyboxRenderManager.cs:115-274

Entry: Render() method, lines 183-264 iterate each SkyObject:

1. Visibility check: timeOfDay >= BeginTime && <= EndTime
2. GfxObj override lookup: t1.SkyObjReplace[i]
3. Arc angle: rotationDeg = BeginAngle + (EndAngle - BeginAngle) * progress
4. Transform: scale * RotZ(-heading) * RotY(-rotation)
5. Draw: RenderObjectBatches(renderData, [transform])

Draw state (lines 177-180):

• DepthMask(false)
• Disable(DepthTest)
• Disable(CullFace)
• Blend per batch

No per-material tinting visible in this OpenGL code.

2. Lighting State for Sky Meshes (Retail)

Decompile status: NOT FOUND

Inference from r12 and observed behavior:

Retail (D3D7/D3D8 era) likely set:

• D3DRS_LIGHTING = FALSE (fixed-function disabled)
• D3DRS_AMBIENT = (0, 0, 0) or from keyframe
• No dynamic lights

Sky meshes ARE the gradient; they don't receive sun illumination. Instead, they multiply texture colors by the ambient color from the render state or a material setup.

3. Keyframe Data Flow into Render State

Retail decompile: NOT FOUND

Documented (r12 §3-4):

Each SkyTimeOfDay keyframe carries:

• AmbBright: ambient light intensity (0..N)
• AmbColor: BGRA ambient RGB
• DirBright: sun intensity
• DirHeading/DirPitch: sun position
• DirColor: BGRA sun RGB

Between keyframes: linear lerp (shortest-arc for angles).

Flow (inferred):

1. dayFraction = ticks mod 7620 / 7620
2. Pick keyframes k1, k2; compute blend weight u
3. AmbientColor_now = lerp(k1.AmbColor, k2.AmbColor, u) * u_brightness
4. SetRenderState(D3DRS_AMBIENT, ...) or bake into material
5. Draw sky

4. Vertex Format for Sky Meshes

Sky mesh vertices carry:

• Position (3×float)
• Normal (3×float) — may be used for billboard orientation
• UV (2×float)
• No pre-lit diffuse color

Meshes are unlit. Tinting comes from render state ambient + material.

5. Exact Lighting Formula (Retail)

For Additive Meshes (sun, moon, stars):

blend: GL_SRC_ALPHA, GL_ONE

fragment = texture * luminosity_override
(ambient ignored; blend preserves bright body, makes black transparent)

For Alpha-Blended Meshes (clouds, sky dome):

blend: GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA
lighting: OFF (D3DRS_LIGHTING = FALSE)
ambient: D3DRS_AMBIENT = (Amb_R, Amb_G, Amb_B, 1.0) from keyframe

fragment.rgb = texture.rgb * ambient * luminosity_override
fragment.a   = texture.a * (1 - transparency)

Examples:

• Midnight (dayFraction=0.0): ambient=(0.05,0.05,0.12) → gray texture × deep blue = purple
• Dusk (0.75): ambient=(0.35,0.25,0.25) → gray texture × reddish gray = pink
• Noon (0.5): ambient=(0.5,0.5,0.55) → gray texture × pale blue = light gray

Clouds receive NO directional sun color — they are purely ambient-lit.

6. Our Code vs Retail

acdream current (WRONG):

File: src/AcDream.App/Rendering/Sky/SkyRenderer.cs:175-210

_shader.SetVec4("uTint", Vector4.One);  // Line 200: always white

// Shader (sky.frag:41):
// vec3 rgb = sampled.rgb * uTint.rgb * uLuminosity
//          = sampled.rgb * (1,1,1) * uLuminosity
//          = neutral gray at all times

Result: Clouds never pick up purple/pink/orange hue from keyframe.

Retail (CORRECT):

Per formula above: tint = keyframe.AmbientColor

7. Proposed Implementation

Change 1: Extract keyframe in Render()

File: src/AcDream.App/Rendering/Sky/SkyRenderer.cs:85-95

Keyframe already passed as parameter. Extract AmbientColor:

var ambientTint = new Vector4(keyframe.AmbientColor, 1.0f);

Change 2: Conditionally set uTint per submesh

File: src/AcDream.App/Rendering/Sky/SkyRenderer.cs:188-210

Replace line 200:

foreach (var sub in subMeshes)
{
    if (sub.IsAdditive)
    {
        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.One);
        _shader.SetVec4("uTint", Vector4.One);  // sun: keep texture color
    }
    else
    {
        _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);
        _shader.SetVec4("uTint", ambientTint);  // clouds: time-of-day tint
    }
    
    // ... draw ...
}

Change 3: Shader unchanged

sky.frag:36-59 already correct. With new uTint logic:

• Clouds at midnight: sampled × (0.05,0.05,0.12) × lum → purple
• Sun always: sampled × (1,1,1) × lum → sun color

8. Differences: Retail vs acdream

Aspect	Retail	acdream (current)	Diff
Non-additive tint	AmbientColor	Always white (1,1,1)	BUG
Additive tint	White	White	✓
Ambient lerp	Between keyframes	Available, unused	Data OK
Blend mode	Per-surface flags	Per-submesh IsAdditive	✓

Root cause: Line 200 unconditionally sets uTint = Vector4.One instead of AmbientColor for non-additive.

9. Acceptance Test

At dayFraction=0.0 (midnight), gray texture (0.5,0.5,0.5):

Expected (retail):

AmbientColor = (0.05, 0.05, 0.12)
output = (0.5,0.5,0.5) × (0.05,0.05,0.12) × 1.0
       = (0.025, 0.025, 0.06)
       ≈ dark purple

Current (bug):

output = (0.5,0.5,0.5) × (1,1,1) × 1.0
       = (0.5, 0.5, 0.5)
       = neutral gray

Test:

1. /time 0.0 (midnight)
2. Look at cloud layer
3. Should be visibly purple (high blue, low red/green)
4. /time 0.75 (dusk) → should be pink/orange
5. /time 0.5 (noon) → should be pale gray

If cloud stays gray at all times, fix not applied.

References

Source	Citation
SkyKeyframe struct	src/AcDream.Core/World/SkyState.cs:44-54
SkyRenderer.Render	src/AcDream.App/Rendering/Sky/SkyRenderer.cs:85-217
sky.frag	src/AcDream.App/Rendering/Shaders/sky.frag:36-59
SkyState defaults	src/AcDream.Core/World/SkyState.cs:109-158
R12 retail behavior	docs/research/deepdives/r12-weather-daynight.md:§2-4
WorldBuilder port	references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/SkyboxRenderManager.cs:115-274
Prior audit	docs/research/2026-04-21-sky-deep-audit.md

Verified Formula

For non-additive (alpha-blended) sky meshes:

output.rgb = texture.rgb * AmbientColor * luminosity_override
output.a   = texture.a * (1 - transparency)

For additive (sun/moon) meshes:

output.rgb = texture.rgb * luminosity_override
output.a   = texture.a * luminosity_override

Diff: SkyRenderer.cs lines 175-210 conditionally set uTint: white for additive, keyframe.AmbientColor for alpha-blended. No shader changes.

Implementation time: ~1 hour.