.HMP -- Terrain Heightmap Grid

Author: HeavenHM • Game: Project I.G.I.: I'm Going In (2000) • Engine: Innerloop Engine v1.0
Project I.G.I. 1 Formats:
All IGI 1 QVM QSC RES MEF TEX SPR PIC MTP DAT Graph FNT IFF BEF OLM HMP LMP CTR CMD BIT WAV AFP/AVI

1. Architectural Role & Engine Purpose

The HMP (Heightmap File) format stores the massive outdoor terrain elevations, surface normal vectors, and material associations that define the expansive outdoor landscapes in Project I.G.I. 1.

Innerloop's flight simulator pedigree shines in the HMP format. Vast multi-kilometer outdoor valleys are represented as a uniform regular grid of cells using compact 12-byte records (hmp_item_s), dynamically tessellated via continuous level-of-detail (CLOD) algorithms at 60 FPS.

2. Binary File Layout & Memory Map Diagram

An HMP file contains a terrain grid header followed by a contiguous 2D elevation grid array:

HMP HEIGHTMAP — 12-BYTE TERRAIN GRID BINARY LAYOUT 0x00: Terrain Grid Header (24 Bytes) 24 BYTES Magic "HMP " | Grid Width (513), Height (513) | Horizontal Cell Step (8.0m) | Min/Max Height magic[4], grid_w, grid_h, cell_step (8.0f), min_height, max_height Contiguous Elevation Cell Stream: 12 Bytes per Cell (hmp_item_s) 12B STRIDE Elevation (float 32-bit height in meters) | Normal X (int16) | Normal Y (int16) | Material ID (uint16) | Flags Nz = sqrt(1 - Nx^2 - Ny^2) • 513 * 513 * 12 Bytes = 3.15 MB contiguous memory map Precomputed Quadtree LOD Bounding Hierarchy QUADTREE LOD Min/Max height bounds per quad block for rapid frustum culling and collision raycasts Enables flight-simulator scale multi-kilometer mountain valleys

3. Continuous Level-of-Detail (CLOD) Terrain Pipeline Diagram

The terrain engine evaluates camera distance per quad block and tessellates terrain meshes on the fly:

HMP TERRAIN — CONTINUOUS LEVEL-OF-DETAIL (CLOD) PIPELINE 1. CAMERA VIEW • Camera Pos & Frustum • Quadtree Bounding Test • Cull Invisible Valleys 2. LOD SELECTION • Distance-Based Step • Near: Step 1 (Full Res) • Far: Step 4 (Coarse) 3. DYNAMIC MESH • Crack-Free Stitching • Emit Index Strips • Read 12B hmp_item_s 4. D3D7 TERRAIN DRAW • Multitexture Shading • DrawIndexedPrimitive • Smooth 60 FPS Horizon

4. Architectural Mechanics & Engine Implementation Details

The 12-byte HMP structure is a masterclass in year-2000 graphics engine optimization:

  • Fixed-Point Normals: Storing surface normal vectors as two 16-bit integers (reconstructing the third component via sqrt(1 - Nx^2 - Ny^2)) saves 8 bytes per vertex compared to 3 floating-point coordinates.
  • Direct Physics Height Query: When guards or vehicles walk across the terrain, calculating height at arbitrary $(X, Z)$ coordinates requires simple bilinear interpolation between four adjacent cell elevations in $O(1)$ time.

5. C++ Struct Definitions

#pragma pack(push, 1)
struct HmpHeader {
    char     magic[4];       // "HMP "
    uint32_t grid_width;     // Number of vertices along X (e.g., 513)
    uint32_t grid_height;    // Number of vertices along Z (e.g., 513)
    float    cell_step;      // Horizontal spacing in meters (e.g., 8.0f)
    float    min_height;     // Minimum valley elevation
    float    max_height;     // Maximum mountain elevation
};

// IGI 1 12-Byte Heightmap Cell Descriptor
struct hmp_item_s {
    float    elevation;      // 32-bit floating-point height in meters
    int16_t  normal_x;       // Fixed-point surface normal X
    int16_t  normal_y;       // Fixed-point surface normal Y
    uint16_t material_id;    // Texture splatting index
    uint16_t flags;          // Surface flags
};
static_assert(sizeof(hmp_item_s) == 12, "hmp_item_s must be 12 bytes");
#pragma pack(pop)
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