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以下是一个web端cad测量面积的示例代码,基于Three.js和jQuery库实现:
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    HTML部分: 复制代码<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Web端CAD测量面积</title>
    <style>
      canvas {
        width: 100%;
        height: 100%;
        display: block;
      }
    </style>
  </head>
  <body>
    <div id="container"></div>
    <div id="output">
      <p>请点击屏幕选择点开始绘制,再次点击结束绘制并显示面积。</p >
    </div>
    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
    <script src="https://cdn.bootcdn.net/ajax/libs/three.js/r125/three.min.js"></script>
    <script src="app.js"></script>
  </body>
</html>
JavaScript部分(app.js):
// 初始化Three.js场景、相机、渲染器
var scene = new THREE.Scene();
var camera = new THREE.PerspectiveCamera( 
  75, window.innerWidth/window.innerHeight, 0.1, 1000 );
var renderer = new THREE.WebGLRenderer({ antialias: true });
renderer.setSize( window.innerWidth, window.innerHeight );
document.getElementById("container").appendChild(renderer.domElement);
// 初始化画线的材质
var material = new THREE.LineBasicMaterial( { color: 0x0000ff } );
// 初始化测量变量
var linePoints = [];
var area = 0;
// 监听鼠标点击事件,绘制线段并计算面积
document.addEventListener( 'click', onClick, false );
function onClick( event ) {
  // 获取鼠标点击坐标
  var mouse = new THREE.Vector2();
  mouse.x = ( event.clientX / window.innerWidth ) * 2 - 1;
  mouse.y = - ( event.clientY / window.innerHeight ) * 2 + 1;
  // 创建射线
  var raycaster = new THREE.Raycaster();
  raycaster.setFromCamera( mouse, camera );
  // 计算射线和平面的交点,并绘制线段
  var plane = new THREE.Plane(new THREE.Vector3(0, 0, 1), 0);
  var intersection = new THREE.Vector3();
  raycaster.ray.intersectPlane(plane, intersection);
  linePoints.push(intersection);
  if (linePoints.length > 1) {
    var geometry = new THREE.BufferGeometry().setFromPoints(linePoints);
    var line = new THREE.Line(geometry, material);
    scene.add(line);
    // 计算面积
    var v1 = linePoints[linePoints.length-2];
    var v2 = linePoints[linePoints.length-1];
    area += (v1.x * v2.y - v2.x * v1.y) / 2;
    $("#output").html("<p>面积: " + Math.abs(area).toFixed(2) + "</p >");
  }
}
// 渲染场景
function animate() {
  requestAnimationFrame( animate );
  renderer.render( scene, camera );
}
animate();
这个示例代码可以在浏览器中运行,并实现了在Three.js场景中绘制线段、计算面积等功能,在线CAD有很多其他功能,可以到梦想CAD控件官网查看。以上代码可以根据实际需求对代码进行修改和拓展。 
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