.LMP -- Terrain Grayscale Lightmap

1. Architectural Role & Engine Purpose

The LMP (Lightmap File) format stores precomputed static lighting intensity and building shadow masks for outdoor terrain in Project I.G.I. 1.

LMP files encode pre-baked sunlight illumination and mountain cast shadows as an 8-bit grayscale grid. This allowed the year-2000 engine to render realistic sunlight transitions and shadowed forest valleys without requiring any runtime dynamic shadow casting from the GPU.

2. Binary File Layout & Memory Map Diagram

An LMP file contains lightmap dimensions and sun vector metadata, followed by an 8-bit intensity raster:

3. Multi-Texture Terrain Shading Pipeline Diagram

During terrain rendering, the engine binds the LMP buffer to Direct3D Texture Stage 1 for multiplicative modulation:

4. Architectural Mechanics & Engine Implementation Details

The 8-bit LMP lightmap format solved the harsh lighting limitations of early Direct3D hardware:

• D3DTOP_MODULATE2X: By using the 2X multiply blending stage, the lightmap value of 128 represented neutral lighting, values below 128 represented shadowed terrain, and values above 128 represented bright specular highlights.
• Zero Memory Waste: Because color was already supplied by the diffuse terrain textures, storing lightmaps in 8-bit grayscale reduced VRAM consumption by 75% compared to 32-bit RGBA lightmaps.

5. C++ Struct Definitions

#pragma pack(push, 1)
struct LmpHeader {
    char     magic[4];       // "LMP "
    uint16_t width;          // Lightmap horizontal resolution
    uint16_t height;         // Lightmap vertical resolution
    float    world_scale;    // Meters per lightmap texel
    float    sun_dir[3];     // Sun direction vector
    float    ambient_base;   // Minimum ambient illumination (0.0 - 1.0)
};
static_assert(sizeof(LmpHeader) == 28, "LmpHeader must be 28 bytes");
#pragma pack(pop)