From 56ca482cd2d675deb39d92b6be7d6396372a1551 Mon Sep 17 00:00:00 2001
From: Victor Giers <mail@victorgiers.com>
Date: Sat, 18 Apr 2026 19:43:58 +0200
Subject: [PATCH] Add terrain mesh generation logic

---
 src/geometry/terrain-mesh.ts | 166 +++++++++++++++++++++++++++++++++++
 1 file changed, 166 insertions(+)
 create mode 100644 src/geometry/terrain-mesh.ts

diff --git a/src/geometry/terrain-mesh.ts b/src/geometry/terrain-mesh.ts
new file mode 100644
index 00000000..f8413da1
--- /dev/null
+++ b/src/geometry/terrain-mesh.ts
@@ -0,0 +1,166 @@
+import { BufferAttribute, BufferGeometry } from "three";
+
+import type { Vec3 } from "../core/vector";
+import {
+  getTerrainHeightAtSample,
+  type Terrain
+} from "../document/terrains";
+
+export type TerrainCellDiagonal = "forward" | "backward";
+
+export interface TerrainCellTriangulation {
+  cellX: number;
+  cellZ: number;
+  diagonal: TerrainCellDiagonal;
+}
+
+export interface DerivedTerrainMeshData {
+  geometry: BufferGeometry;
+  positions: Float32Array;
+  normals: Float32Array;
+  uvs: Float32Array;
+  indices: Uint32Array;
+  cellTriangulation: TerrainCellTriangulation[];
+  localBounds: {
+    min: Vec3;
+    max: Vec3;
+  };
+}
+
+function createEmptyLocalBounds(): { min: Vec3; max: Vec3 } {
+  return {
+    min: {
+      x: Number.POSITIVE_INFINITY,
+      y: Number.POSITIVE_INFINITY,
+      z: Number.POSITIVE_INFINITY
+    },
+    max: {
+      x: Number.NEGATIVE_INFINITY,
+      y: Number.NEGATIVE_INFINITY,
+      z: Number.NEGATIVE_INFINITY
+    }
+  };
+}
+
+function chooseCellDiagonal(
+  topLeft: number,
+  topRight: number,
+  bottomLeft: number,
+  bottomRight: number
+): TerrainCellDiagonal {
+  return Math.abs(topLeft - bottomRight) <= Math.abs(topRight - bottomLeft)
+    ? "forward"
+    : "backward";
+}
+
+function pushCellIndices(
+  indices: number[],
+  cellTriangulation: TerrainCellTriangulation[],
+  cellX: number,
+  cellZ: number,
+  diagonal: TerrainCellDiagonal,
+  topLeft: number,
+  topRight: number,
+  bottomLeft: number,
+  bottomRight: number
+) {
+  cellTriangulation.push({
+    cellX,
+    cellZ,
+    diagonal
+  });
+
+  if (diagonal === "forward") {
+    indices.push(topLeft, bottomLeft, bottomRight);
+    indices.push(topLeft, bottomRight, topRight);
+    return;
+  }
+
+  indices.push(topLeft, bottomLeft, topRight);
+  indices.push(topRight, bottomLeft, bottomRight);
+}
+
+export function buildTerrainDerivedMeshData(
+  terrain: Terrain
+): DerivedTerrainMeshData {
+  const vertexCount = terrain.sampleCountX * terrain.sampleCountZ;
+  const positions = new Float32Array(vertexCount * 3);
+  const uvs = new Float32Array(vertexCount * 2);
+  const localBounds = createEmptyLocalBounds();
+  let vertexOffset = 0;
+  let uvOffset = 0;
+
+  for (let sampleZ = 0; sampleZ < terrain.sampleCountZ; sampleZ += 1) {
+    for (let sampleX = 0; sampleX < terrain.sampleCountX; sampleX += 1) {
+      const localX = sampleX * terrain.cellSize;
+      const localY = getTerrainHeightAtSample(terrain, sampleX, sampleZ);
+      const localZ = sampleZ * terrain.cellSize;
+      positions[vertexOffset] = localX;
+      positions[vertexOffset + 1] = localY;
+      positions[vertexOffset + 2] = localZ;
+      vertexOffset += 3;
+
+      localBounds.min.x = Math.min(localBounds.min.x, localX);
+      localBounds.min.y = Math.min(localBounds.min.y, localY);
+      localBounds.min.z = Math.min(localBounds.min.z, localZ);
+      localBounds.max.x = Math.max(localBounds.max.x, localX);
+      localBounds.max.y = Math.max(localBounds.max.y, localY);
+      localBounds.max.z = Math.max(localBounds.max.z, localZ);
+
+      uvs[uvOffset] = terrain.position.x + localX;
+      uvs[uvOffset + 1] = terrain.position.z + localZ;
+      uvOffset += 2;
+    }
+  }
+
+  const indexValues: number[] = [];
+  const cellTriangulation: TerrainCellTriangulation[] = [];
+
+  for (let cellZ = 0; cellZ < terrain.sampleCountZ - 1; cellZ += 1) {
+    for (let cellX = 0; cellX < terrain.sampleCountX - 1; cellX += 1) {
+      const topLeft = cellZ * terrain.sampleCountX + cellX;
+      const topRight = topLeft + 1;
+      const bottomLeft = (cellZ + 1) * terrain.sampleCountX + cellX;
+      const bottomRight = bottomLeft + 1;
+      const diagonal = chooseCellDiagonal(
+        getTerrainHeightAtSample(terrain, cellX, cellZ),
+        getTerrainHeightAtSample(terrain, cellX + 1, cellZ),
+        getTerrainHeightAtSample(terrain, cellX, cellZ + 1),
+        getTerrainHeightAtSample(terrain, cellX + 1, cellZ + 1)
+      );
+
+      pushCellIndices(
+        indexValues,
+        cellTriangulation,
+        cellX,
+        cellZ,
+        diagonal,
+        topLeft,
+        topRight,
+        bottomLeft,
+        bottomRight
+      );
+    }
+  }
+
+  const indices = new Uint32Array(indexValues);
+  const geometry = new BufferGeometry();
+  geometry.setAttribute("position", new BufferAttribute(positions, 3));
+  geometry.setAttribute("uv", new BufferAttribute(uvs, 2));
+  geometry.setIndex(new BufferAttribute(indices, 1));
+  geometry.computeVertexNormals();
+
+  const normalAttribute = geometry.getAttribute("normal");
+  const normals = new Float32Array(normalAttribute.array.length);
+  normals.set(normalAttribute.array as ArrayLike<number>);
+
+  return {
+    geometry,
+    positions,
+    normals,
+    uvs,
+    indices,
+    cellTriangulation,
+    localBounds
+  };
+}