Performance and instancing

• Set instanceMatrix.needsUpdate after every batch of setMatrixAt calls.
• Merge static geometry with BufferGeometryUtils.mergeGeometries after baking transforms.
• Call lod.update(camera) inside the render loop for distance-based switching.

const COUNT = 8000;

const trunk = new THREE.CylinderGeometry(0.6, 0.9, 12, 6);
const leaf = new THREE.SphereGeometry(3.4, 8, 6);

const trunkMesh = new THREE.InstancedMesh(trunk, trunkMaterial, COUNT);
const leafMesh = new THREE.InstancedMesh(leaf, leafMaterial, COUNT);

const dummy = new THREE.Object3D();
const color = new THREE.Color();

for (let i = 0; i < COUNT; i++) {
  const x = (Math.random() - 0.5) * 1600;
  const z = (Math.random() - 0.5) * 1600;
  const s = 0.7 + Math.random() * 0.6;

  dummy.position.set(x, 6 * s, z);
  dummy.rotation.y = Math.random() * Math.PI * 2;
  dummy.scale.set(s, s, s);
  dummy.updateMatrix();
  trunkMesh.setMatrixAt(i, dummy.matrix);

  dummy.position.y = 12 * s + 2.4 * s;
  dummy.scale.setScalar(s);
  dummy.updateMatrix();
  leafMesh.setMatrixAt(i, dummy.matrix);

  color.setHSL(0.28 + Math.random() * 0.08, 0.45, 0.32 + Math.random() * 0.12);
  leafMesh.setColorAt(i, color);
}

trunkMesh.instanceMatrix.needsUpdate = true;
leafMesh.instanceMatrix.needsUpdate = true;
leafMesh.instanceColor.needsUpdate = true;

scene.add(trunkMesh, leafMesh);

import { mergeGeometries } from 'three/addons/utils/BufferGeometryUtils.js';

const parts = [];

buildings.forEach(function (b) {
  const geo = new THREE.BoxGeometry(b.w, b.h, b.d);
  geo.translate(b.x, b.h / 2, b.z);
  geo.deleteAttribute('uv');          // 属性不一致会导致合并失败
  parts.push(geo);
});

const merged = mergeGeometries(parts, false);
const mesh = new THREE.Mesh(merged, new THREE.MeshStandardMaterial({
  color: 0x9aa7b4,
  roughness: 0.9
}));

scene.add(mesh);

• Instance counts are fixed at construction, and extra setMatrixAt calls write out of bounds.
• Merging geometries with mismatched attributes returns null with only a console warning.