效果图
在项目中升级threejs版本，原版本中用到了Geometry.merge()方法。升级后的threejs删除了Geometry相关类，转而使用BufferGeometry实现生成几何体。BufferGeometry中的merge()方法使用后，并未将多个几何体进行合并，故自实现了合并多个几何体的功能。
本文中的合并几何体函数只对position、normal、uv、groups进行了合并，合并的几何体为shape拉伸的多个几何体，且为表面和侧立面分别使用不同的material。
以下为实现合并多个几何体方法代码（方法参照javascript - How to merge two BufferGeometries in one BufferGeometry in Three.JS? - Stack Overflow）：
const mergeBufferGeometry = function (objects) {const sumPosArr = new Array();const sumNormArr = new Array();const sumUvArr = new Array();const modelGeometry = new THREE.BufferGeometry();let sumPosCursor = 0;let sumNormCursor = 0;let sumUvCursor = 0;let startGroupCount = 0;let lastGroupCount = 0;for (let a = 0; a < objects.length; a++ ){const posAttArr = objects[a].geometry.getAttribute('position').array;for (let b = 0; b < posAttArr.length; b++){sumPosArr[b + sumPosCursor] = posAttArr[b];}sumPosCursor += posAttArr.length;const numAttArr = objects[a].geometry.getAttribute('normal').array;for (let b = 0; b < numAttArr.length; b++){sumNormArr[b + sumNormCursor] = numAttArr[b];}sumNormCursor += numAttArr.length;const uvAttArr = objects[a].geometry.getAttribute('uv').array;for (let b = 0; b < uvAttArr.length; b++){sumUvArr[b + sumUvCursor] = uvAttArr[b];}sumUvCursor += uvAttArr.length;const groupArr = objects[a].geometry.groups;for (let b = 0; b < groupArr.length; b++){startGroupCount = lastGroupCountmodelGeometry.addGroup(startGroupCount, groupArr[b].count, groupArr[b].materialIndex)lastGroupCount = startGroupCount + groupArr[b].count}}modelGeometry.setAttribute('position', new THREE.Float32BufferAttribute(sumPosArr, 3 ));sumNormArr.length && modelGeometry.setAttribute('normal', new THREE.Float32BufferAttribute(sumNormArr, 3 ));sumUvArr.length && modelGeometry.setAttribute('uv', new THREE.Float32BufferAttribute(sumUvArr, 2 ));return modelGeometry}
1）objects 传参：需要合并的几何体数组，mesh数组
2）position 合并：将 objects 数组中几何体的 position 合并至一个数组中，最后将全部 position 添加至新创建的 modelGeometry 中并返回
3）normal 合并：将 objects 数组中几何体的 normal 合并至一个数组中，最后将全部 normal 添加至新创建的 modelGeometry 中并返回
4）uv 合并：将 objects 数组中几何体的 uv 合并至一个数组中，最后将全部 uv 添加至新创建的 modelGeometry 中并返回
5）groups 合并：单个 groups：{ start: Integer, count: Integer, materialIndex: Integer }，合并 groups 需要重新计算 start 的值（注意此处的 count 个数并不是 position 个数），下一个 start 的值总是等于上一个 start + count 。本文中 groups 来源为创建ExtrudeGeometry几何体时，传入了表面和侧立面的 material。
生成地图3D图形代码部分
drawChinaMap() {const chinaGroup = new THREE.Group()const bumpMap = new THREE.TextureLoader().load(require('./assets/img/normal1.jpg'), texture => {texture.wrapS = texture.wrapT = THREE.RepeatWrapping;texture.offset.set( 0, 0.5 )texture.repeat.set( 0.01, 0.01 )})chinaGeometry.features.forEach(chinaItem => {let oneCityLineGroup = new THREE.Group()let oneCityLineArr = []let oneCityShapelet oneCityShapeArr = []let material1 = new THREE.ShaderMaterial({uniforms: Shader1.uniforms,vertexShader: Shader1.vertexShader,fragmentShader: Shader1.fragmentShader,side: THREE.DoubleSide,transparent: true,// blending: THREE.AdditiveBlending,})let material2 = new THREE.MeshStandardMaterial({// color: '#7293b5',bumpMap: bumpMap,// map: bumpMap,opacity: 0.8,bumpScale: 10,roughness: 1,metalness: 0.0,})chinaItem.geometry.coordinates.forEach(chinaChildItem => {if (chinaItem.geometry.type === "MultiPolygon") {chinaChildItem.forEach((item) => {const oneCityShape = this.drawShape(item, [material2, material1])oneCityShapeArr.push(oneCityShape)})} else {const oneCityShape = this.drawShape(chinaChildItem, [material2, material1])oneCityShapeArr.push(oneCityShape)}})const newGeometry = this.mergeBufferGeometry(oneCityShapeArr)oneCityShape = new THREE.Mesh(newGeometry, [material2, material1])oneCityShape.name = chinaItem.properties.namethis.outlinePass.selectedObjects.push(oneCityShape)chinaGroup.add(oneCityShape)})chinaGroup.rotateX(-Math.PI/2)chinaGroup.scale.set(0.0015, 0.0015, 0.0015)this.scene.add(chinaGroup)}
拉伸shape任意几何体方法如下：
drawShape(position, material) {const pointArr = []const shape = new THREE.Shape()const extrudeSettings = {steps: 2,depth: 100000,curveSegments: 128,bevelEnabled: true,bevelThickness: 1,bevelSize: 1,bevelOffset: 10,bevelSegments: 1};position.forEach(point => {const coord = geographicToVector2(point)pointArr.push([coord[0]-center[0], coord[1] - center[1]]);})pointArr.forEach((p, i) => {i === 0 ? shape.moveTo(p[0], p[1]) : shape.lineTo(p[0], p[1])})const geometry = new THREE.ExtrudeGeometry(shape, extrudeSettings)return new THREE.Mesh(geometry, material)}
