mtgodot-poc/formats/terrain_mesh.cpp

#include "terrain_mesh.h"

#include "m2_coord.h"

#include <algorithm>
#include <cmath>

namespace fmt {

namespace {
constexpr int CS = m2coord::CELLSCALE;          // 200 cm
constexpr double CS_M = double(CS) * m2coord::CM_TO_M; // 2.0 m
} // namespace

double terrain_height_at(const HeightMap& hm, double lx_cm, double ly_cm, double hs) {
    // 照 CTerrain::GetHeight（本地坐标版，调用方已把全局->本地 + y 取绝对值）
    if (lx_cm < 0 || ly_cm < 0 || lx_cm > double(m2coord::TERRAIN_SIZE) * CS ||
        ly_cm > double(m2coord::TERRAIN_SIZE) * CS)
        return 0.0;

    long xi = (long)lx_cm, yi = (long)ly_cm;
    long xdist = xi % CS, ydist = yi % CS;
    long x = xi / CS, y = yi / CS;
    const double oo = 1.0 / double(CS);

    double h1 = hm.at((int)x, (int)y) * hs;         // TL
    double h2 = hm.at((int)x + 1, (int)y + 1) * hs; // BR
    if (xdist <= ydist) {                            // 左三角：TL, BR, BL
        double h3 = hm.at((int)x, (int)y + 1) * hs;
        double xslope = (h2 - h3) * oo;
        double yslope = (h3 - h1) * oo;
        return h1 + (xdist * xslope + ydist * yslope);
    }
    // 右三角：TL, BR, TR
    double h3 = hm.at((int)x + 1, (int)y) * hs;
    double xslope = (h3 - h1) * oo;
    double yslope = (h2 - h3) * oo;
    return h1 + (xdist * xslope + ydist * yslope);
}

void build_terrain_mesh(const HeightMap& hm, int tile_x, int tile_y, double hs,
                        TerrainMesh& out, bool ccw_from_above) {
    out = TerrainMesh{};
    const int N = TerrainMesh::VERTS_XY; // 129
    out.positions.resize(size_t(N) * N * 3);
    out.normals.resize(size_t(N) * N * 3);
    out.uvs.resize(size_t(N) * N * 2);
    out.indices.reserve(size_t(TerrainMesh::QUADS_XY) * TerrainMesh::QUADS_XY * 6);

    // Godot 空间原点：见 area_data.h 说明 —— 区块 y 取负，position_to_godot 再翻正。
    const double X0 = double(tile_x) * m2coord::CHUNK_CM * m2coord::CM_TO_M;
    const double Z0 = double(tile_y) * m2coord::CHUNK_CM * m2coord::CM_TO_M;

    auto height_m = [&](int i, int j) -> double {
        return hm.at(i, j) * hs * m2coord::CM_TO_M;
    };

    double miny = 1e30, maxy = -1e30;
    for (int j = 0; j < N; ++j) {
        for (int i = 0; i < N; ++i) {
            const double gy = height_m(i, j);
            const size_t o = (size_t(j) * N + i) * 3;
            out.positions[o + 0] = float(X0 + i * CS_M);
            out.positions[o + 1] = float(gy);
            out.positions[o + 2] = float(Z0 + j * CS_M);
            const size_t uo = (size_t(j) * N + i) * 2;
            out.uvs[uo + 0] = float(i) / float(TerrainMesh::QUADS_XY);
            out.uvs[uo + 1] = float(j) / float(TerrainMesh::QUADS_XY);
            miny = std::min(miny, gy);
            maxy = std::max(maxy, gy);

            // 中心差分（用边界样本，跨区块无缝）；y=f(x,z) 的法线 = (-df/dx, 1, -df/dz)
            const double dh_di = (height_m(i + 1, j) - height_m(i - 1, j)) / (2.0 * CS_M);
            const double dh_dj = (height_m(i, j + 1) - height_m(i, j - 1)) / (2.0 * CS_M);
            double nx = -dh_di, ny = 1.0, nz = -dh_dj;
            const double len = std::sqrt(nx * nx + ny * ny + nz * nz);
            out.normals[o + 0] = float(nx / len);
            out.normals[o + 1] = float(ny / len);
            out.normals[o + 2] = float(nz / len);
        }
    }
    out.min_y = float(miny);
    out.max_y = float(maxy);

    auto vid = [N](int i, int j) { return int32_t(j * N + i); };
    for (int j = 0; j < TerrainMesh::QUADS_XY; ++j) {
        for (int i = 0; i < TerrainMesh::QUADS_XY; ++i) {
            int32_t TL = vid(i, j), TR = vid(i + 1, j);
            int32_t BL = vid(i, j + 1), BR = vid(i + 1, j + 1);
            // 对角线 TL-BR（= GetHeight 的 h1/h2）
            if (ccw_from_above) {
                out.indices.insert(out.indices.end(), {TL, BR, BL});
                out.indices.insert(out.indices.end(), {TL, TR, BR});
            } else {
                out.indices.insert(out.indices.end(), {TL, BL, BR});
                out.indices.insert(out.indices.end(), {TL, BR, TR});
            }
        }
    }
}

} // namespace fmt