diablo2-web/docs/lighting_reverse_engineering_113c.md

Diablo II v1.13c Reverse Engineering Specification: dLightMap.c Lighting Pipeline

Ground Truth Binary: samples/d2/D2Client.dll (v1.13c, PE ImageBase 0x6fab0000, file timestamp 2010-03-09).
Source File Origin: ..\Source\D2Client\DRAW\dLightMap.c (string located at VA 0x6fb84104).
Target Assembly Range: 0x6fb58000 – 0x6fb5a000 (dLightMap.c), 0x6faef000 – 0x6faefee0 (light debug & map tile interface), 0x6fb10000 – 0x6fb10500 (floor tile rasterizer).

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1. Light Grid Architecture & Topography

1.1 Dimensions & Coordinate Space

• Grid Dimensions: Exactly 48 × 48 cells (2,304 cells total, hex 0x900).
  • Column index x \in [0, 47] (cmp ecx, 0x30).
  • Row index y \in [0, 47] (cmp eax, 0x30).
  • Linear address mapping: index = 48 * y + x (lea eax, [eax + eax*2]; shl eax, 4; add eax, ecx).
• Spatial Resolution & Scale:
  • The lighting grid steps by 8 sub-tiles per cell (sar ecx, 3 / add esi, 8).
  • World sub-tile coordinates (x_{\text{sub}}, y_{\text{sub}}) are converted to grid coordinates by:

    x_{\text{grid}} = (x_{\text{sub}} \gg 3) - x_{\text{origin}}

    y_{\text{grid}} = (y_{\text{sub}} \gg 3) - y_{\text{origin}}

    Where (x_{\text{origin}}, y_{\text{origin}}) are the grid origin sub-tile offsets (ds:0x6fbb4df0, ds:0x6fbb4df4).

1.2 Cell Memory Layout (8 Bytes per Cell)

The primary lighting array is located at base address 0x6fbb5208 with an 8-byte stride per cell (ecx * 8 + 0x6fbb5208):

struct D2LightCellStrc {
    uint8_t nFlags;          // +0x00: Obstacle / cell flag
    uint8_t pad[3];          // +0x01..+0x03
    uint8_t nIntensity;      // +0x04: Clamped intensity [0..255]
    uint8_t nRed;            // +0x05: Red color channel [0..255]
    uint8_t nGreen;          // +0x06: Green color channel [0..255]
    uint8_t nBlue;           // +0x07: Blue color channel [0..255]
};

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2. Mathematical Formulas (Decompiled from Assembly)

2.1 Fast Euclidean Distance Approximation (Alpha Max Plus Beta Min)

In 0x6fb597b4 – 0x6fb597d9, Blizzard avoids floating-point square root sqrt(dx^2 + dy^2) using an integer approximation:

6fb597b4: cmp ecx, eax
6fb597b6: jl 0x6fb597cb
6fb597b8: imul ecx, ecx, 0x3d7 ; 983
6fb597be: imul eax, eax, 0x197 ; 407
6fb597c4: add ecx, eax
6fb597c6: shr ecx, 0xa         ; >> 10 (1024)

Mathematical representation:

\text{dist}(dx, dy) = \frac{983 \cdot \max(|dx|, |dy|) + 407 \cdot \min(|dx|, |dy|)}{1024}

Where:

• \frac{983}{1024} \approx 0.95996
• \frac{407}{1024} \approx 0.39746
• Maximum theoretical error across all angles is \le 3.96\%.

2.2 Light Falloff & Obstacle Attenuation Formula

In 0x6fb597de – 0x6fb597fa:

6fb597de: mov edx, [esi+0x18]  ; Light Radius R
6fb597e1: sub edx, ecx         ; (R - dist)
6fb597e3: imul edx, [esp+0x3c] ; * intensitySlope (16.16 fixed point)
6fb597f1: sar edx, 16          ; >> 16
6fb597e8: sar edi, 1           ; edi = obstacle factor
6fb597ea: mov eax, 8
6fb597ef: sub eax, edi         ; (8 - obstacle)
6fb597f4: imul eax, edx        ; (8 - obstacle) * attenuated
6fb597f7: sar eax, 3           ; >> 3 (/ 8)

Mathematical representation:

\text{IntensitySlope} = \frac{\text{BaseIntensity} \ll 16}{R}

\text{Attenuated} = \max\left(0, \frac{(R - \text{dist}) \cdot \text{IntensitySlope}}{65536}\right)

\text{FinalCellIntensity} = \frac{(8 - \text{ObstacleFactor}) \cdot \text{Attenuated}}{8}

• When \text{ObstacleFactor} = 0: Light transmits at 100\% (\frac{8-0}{8} = 1.0).
• When \text{ObstacleFactor} \ge 16: Light is completely blocked (cmp edi, 0x10; jge skip).

2.3 Multi-Source Color Blending (Weighted Energy Conservation)

In 0x6fb58a91 – 0x6fb58b45: When multiple lights illuminate the same grid cell, their colors are combined via intensity-weighted averaging:

6fb58a91: lea ecx, [esi+edx*1] ; newIntensity = existing + incoming
6fb58a94: cmp ecx, 0xff
...
6fb58abd: mov edi, [ebx*4 + 0x6fbb4e00] ; 65536 / newIntensity table lookup
6fb58ac4: imul ecx, esi        ; existingRed * existingIntensity
6fb58acd: imul ebp, edx        ; incomingRed * incomingIntensity
6fb58ad0: add ecx, ebp
6fb58ad2: imul ecx, edi        ; sum * (65536 / newIntensity)
6fb58ad5: shr ecx, 0x10        ; >> 16

Mathematical representation:

I_{\text{total}} = \min(255, I_{\text{existing}} + I_{\text{incoming}})

R_{\text{blended}} = \min\left(255, \frac{R_{\text{existing}} \cdot I_{\text{existing}} + R_{\text{incoming}} \cdot I_{\text{incoming}}}{I_{\text{total}}}\right)

G_{\text{blended}} = \min\left(255, \frac{G_{\text{existing}} \cdot I_{\text{existing}} + G_{\text{incoming}} \cdot I_{\text{incoming}}}{I_{\text{total}}}\right)

B_{\text{blended}} = \min\left(255, \frac{B_{\text{existing}} \cdot I_{\text{existing}} + B_{\text{incoming}} \cdot I_{\text{incoming}}}{I_{\text{total}}}\right)

2.4 Sub-Tile Bilinear Interpolation (8.8 Fixed Point)

In 0x6fb58373 – 0x6fb58385:

mov ebx, 0x100       ; 256
sub ebx, eax         ; 256 - factor
imul eax, edx        ; factor * L2
imul ebx, edi        ; (256 - factor) * L1
add ebx, eax
sar ebx, 0x8         ; >> 8

Interpolates smoothly between neighboring cells using fixed-point factor f \in [0, 256]:

\text{Sample} = \frac{(256 - f) \cdot L_1 + f \cdot L_2}{256}

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3. Light Sources & Defaults

• Player Base Light Radius: Default is radius 10–13 sub-tiles adjusted by item STAT_LIGHTRADIUS (CUnit.c / dLightMap.c).
• Monster Default Radius: 8 sub-tiles (mov ebx, 0x8 at 0x6fb59894).
• Missile Lights: Read directly from Missiles.txt Light, Red, Green, Blue columns.
• Flicker: Perturbation index updated every logic frame (25Hz) modifying radius by \pm 1 sub-tile.
• 2D Software Mode Fallback: When bColoredLighting is false (ds:0x6fb8e830 == 0), R=G=B=I and the intensity directly drives the 32-level PL2 colormap row.