Three.js中实现碰撞检测

1. 引言

碰撞检测是三维场景中常见的需求，Three.js是常用的前端三维JavaScript库，本文就如何在Three.js中进行碰撞检测进行记述

主要使用到的方法有：

射线法Raycaster
包围盒bounding box
物理引擎Cannon.js

2. Raycaster

Raycaster用于进行raycasting（光线投射），光线投射用于进行鼠标拾取（在三维空间中计算出鼠标移过了什么物体）

在某些情况下也能用于初略的碰撞检测

示例如下：

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="UTF-8">

  <meta http-equiv="X-UA-Compatible" content="IE=edge">

  <meta name="viewport" content="width=device-width, initial-scale=1.0">

  <title>Document</title>

  <style>

    html,

    body,

    canvas {

      height: 100%;

      width: 100%;

      margin: 0;

    }

  </style>

</head>

<body>

  <canvas id="canvas"></canvas>

  <script type="importmap">

		{

			"imports": {

				"three": "https://unpkg.com/three/build/three.module.js",

				"three/addons/": "https://unpkg.com/three/examples/jsm/"

			}

		}

	</script>

  <script type="module">

    import * as THREE from 'three';

    import { OrbitControls } from 'three/addons/controls/OrbitControls.js';

    import Stats from 'three/addons/libs/stats.module.js'

    const scene = new THREE.Scene();

    const raycaster = new THREE.Raycaster();

    const geometry = new THREE.BoxGeometry(1, 1, 1);

    const material = new THREE.MeshBasicMaterial({ color: 0x0000ff });

    const cube = new THREE.Mesh(geometry, material);

    scene.add(cube);

    // 创建性能监视器

    let stats = new Stats();

    // 将监视器添加到页面中

    document.body.appendChild(stats.domElement)

    const canvas = document.querySelector('#canvas');

    const camera = new THREE.PerspectiveCamera(75, canvas.clientWidth / canvas.clientHeight, 0.1, 100000);

    camera.position.set(0, 0, 10);

    // 添加环境光

    const ambient = new THREE.AmbientLight("#FFFFFF");

    ambient.intensity = 5;

    scene.add(ambient);

    // 添加平行光

    const directionalLight = new THREE.DirectionalLight("#FFFFFF");

    directionalLight.position.set(0, 0, 0);

    directionalLight.intensity = 16;

    scene.add(directionalLight);

    // 添加Box

    const box = new THREE.BoxGeometry(1, 1, 1);

    const boxMaterial = new THREE.MeshBasicMaterial({ color: 0x00ff00 });

    const boxMesh = new THREE.Mesh(box, boxMaterial);

    boxMesh.position.set(6, 0, 0);

    scene.add(boxMesh);

    const renderer = new THREE.WebGLRenderer({

      canvas: document.querySelector('#canvas'),

      antialias: true

    });

    renderer.setSize(window.innerWidth, window.innerHeight, false)

    const controls = new OrbitControls(camera, renderer.domElement);

    function animate() {

      // 更新帧数

      stats.update()

      boxMesh.position.x -= 0.01;

      cube.material.color.set(0x0000ff);

      raycaster.set(boxMesh.position, new THREE.Vector3(-1, 0, 0).normalize());

      const intersection = raycaster.intersectObject(cube);

      if (intersection.length > 0) {

        if (intersection[0].distance < 0.5) {

          intersection[0].object.material.color.set(0xff0000);

        }

      }

      raycaster.set(boxMesh.position, new THREE.Vector3(1, 0, 0).normalize());

      const intersection2 = raycaster.intersectObject(cube);

      if (intersection2.length > 0) {

        if (intersection2[0].distance < 0.5) {

          intersection2[0].object.material.color.set(0xff0000);

        }

      }

      requestAnimationFrame(animate);

      renderer.render(scene, camera);

    }

    animate();

  </script>

</body>

</html>

可以看到，两个立方体在刚接触时和要分开时检测出了碰撞，但是在两个立方体接近重合时却没检测出碰撞

这是因为Raycaster使用的是一根射线来检测，射线需要起点和方向，上述例子中将起点设为绿色立方体的中心，当绿色立方体中心在蓝色立方体内时，就检测不出碰撞了

另外，射线是需要方向的，上述示例中设置为检测左右两个方向，然而方向是难以穷举的，太多的Raycaster也严重损耗性能

所以说，Raycaster在某些情况下也能用于初略的碰撞检测，然而问题是显著的

3. bounding box

bounding box，在Three.js中为Box3类，表示三维空间中的一个轴对齐包围盒（axis-aligned bounding box，AABB）

利用bounding box，可以用来检测物体是否相交（即，碰撞）

示例如下（和Raycaster部分的代码相比只修改了animate函数）：

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="UTF-8">

  <meta http-equiv="X-UA-Compatible" content="IE=edge">

  <meta name="viewport" content="width=device-width, initial-scale=1.0">

  <title>Document</title>

  <style>

    html,

    body,

    canvas {

      height: 100%;

      width: 100%;

      margin: 0;

    }

  </style>

</head>

<body>

  <canvas id="canvas"></canvas>

  <script type="importmap">

		{

			"imports": {

				"three": "https://unpkg.com/three/build/three.module.js",

				"three/addons/": "https://unpkg.com/three/examples/jsm/"

			}

		}

	</script>

  <script type="module">

    import * as THREE from 'three';

    import { OrbitControls } from 'three/addons/controls/OrbitControls.js';

    import Stats from 'three/addons/libs/stats.module.js'

    const scene = new THREE.Scene();

    const raycaster = new THREE.Raycaster();

    const geometry = new THREE.BoxGeometry(1, 1, 1);

    const material = new THREE.MeshBasicMaterial({ color: 0x0000ff });

    const cube = new THREE.Mesh(geometry, material);

    scene.add(cube);

    // 创建性能监视器

    let stats = new Stats();

    // 将监视器添加到页面中

    document.body.appendChild(stats.domElement)

    const canvas = document.querySelector('#canvas');

    const camera = new THREE.PerspectiveCamera(75, canvas.clientWidth / canvas.clientHeight, 0.1, 100000);

    camera.position.set(0, 0, 10);

    // 添加环境光

    const ambient = new THREE.AmbientLight("#FFFFFF");

    ambient.intensity = 5;

    scene.add(ambient);

    // 添加平行光

    const directionalLight = new THREE.DirectionalLight("#FFFFFF");

    directionalLight.position.set(0, 0, 0);

    directionalLight.intensity = 16;

    scene.add(directionalLight);

    // 添加Box

    const box = new THREE.BoxGeometry(1, 1, 1);

    const boxMaterial = new THREE.MeshBasicMaterial({ color: 0x00ff00 });

    const boxMesh = new THREE.Mesh(box, boxMaterial);

    boxMesh.position.set(6, 0, 0);

    scene.add(boxMesh);

    const renderer = new THREE.WebGLRenderer({

      canvas: document.querySelector('#canvas'),

      antialias: true

    });

    renderer.setSize(window.innerWidth, window.innerHeight, false)

    const controls = new OrbitControls(camera, renderer.domElement);

    function animate() {

      // 更新帧数

      stats.update()

      boxMesh.position.x -=  0.02;

      const cubeBox = new THREE.Box3().setFromObject(cube);

      const boxMeshBox = new THREE.Box3().setFromObject(boxMesh);

      cubeBox.intersectsBox(boxMeshBox) ? cube.material.color.set(0xff0000) : cube.material.color.set(0x0000ff);

      requestAnimationFrame(animate);

      renderer.render(scene, camera);

    }

    animate();

  </script>

</body>

</html>

可以看到，在Three.js中使用bounding box来检测碰撞效果还可以，当然，AABB这种bounding box是将物体用一个立方体或长方体包围起来，如果物体的形状很不规则，那么使用bounding box来检测碰撞可能是不够精细的，比如下面这个例子：

示例中绿色立方体还没撞到蓝色锥体，但是bounding box已经检测出碰撞

所以，利用bounding box来检测物体是否相交是大体可行的

4. Cannon.js

Cannon.js是一个3d物理引擎，它能实现常见的碰撞检测，各种体形，接触，摩擦和约束功能

这里笔者想借助物理引擎来实现碰撞检测，当然，其他的物理引擎（如，Ammo.js，Oimo.js等）也是可以的

使用Cannon.js进行两个Cube的碰撞检测示例如下：

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="UTF-8">

  <meta http-equiv="X-UA-Compatible" content="IE=edge">

  <meta name="viewport" content="width=device-width, initial-scale=1.0">

  <title>Document</title>

  <style>

    html,

    body,

    canvas {

      height: 100%;

      width: 100%;

      margin: 0;

    }

  </style>

</head>

<body>

  <canvas id="canvas"></canvas>

  <script type="importmap">

		{

			"imports": {

				"three": "https://unpkg.com/three/build/three.module.js",

				"three/addons/": "https://unpkg.com/three/examples/jsm/"

			}

		}

	</script>

  <script src="https://cdnjs.cloudflare.com/ajax/libs/cannon.js/0.6.2/cannon.js"></script>

  <script type="module">

    import * as THREE from 'three';

    import { OrbitControls } from 'three/addons/controls/OrbitControls.js';

    import Stats from 'three/addons/libs/stats.module.js'

    const scene = new THREE.Scene();

    const world = new CANNON.World()

    // 创建性能监视器

    let stats = new Stats();

    // 将监视器添加到页面中

    document.body.appendChild(stats.domElement)

    const canvas = document.querySelector('#canvas');

    const camera = new THREE.PerspectiveCamera(75, canvas.clientWidth / canvas.clientHeight, 0.1, 100000);

    camera.position.set(0, 0, 10);

    // 添加环境光

    const ambient = new THREE.AmbientLight("#FFFFFF");

    ambient.intensity = 5;

    scene.add(ambient);

    // 添加平行光

    const directionalLight = new THREE.DirectionalLight("#FFFFFF");

    directionalLight.position.set(0, 0, 0);

    directionalLight.intensity = 16;

    scene.add(directionalLight);

    // 创建第一个Cube的Three.js模型

    const cubeGeometry1 = new THREE.BoxGeometry(1, 1, 1);

    const cubeMaterial1 = new THREE.MeshBasicMaterial({ color: 0x0000ff });

    const cube1 = new THREE.Mesh(cubeGeometry1, cubeMaterial1);

    scene.add(cube1);

    // 创建第一个Cube的Cannon.js刚体

    const cubeShape1 = new CANNON.Box(new CANNON.Vec3(0.5, 0.5, 0.5));

    const cubeBody1 = new CANNON.Body({ mass: 1, shape: cubeShape1 });

    cubeBody1.position.set(1, 0, 0);

    world.addBody(cubeBody1);

    // 创建第二个Cube的Three.js模型

    const cubeGeometry2 = new THREE.BoxGeometry(1, 1, 1);

    const cubeMaterial2 = new THREE.MeshBasicMaterial({ color: 0x00ff00 });

    const cube2 = new THREE.Mesh(cubeGeometry2, cubeMaterial2);

    scene.add(cube2);

    // 创建第二个Cube的Cannon.js刚体

    const cubeShape2 = new CANNON.Box(new CANNON.Vec3(0.5, 0.5, 0.5));

    const cubeBody2 = new CANNON.Body({ mass: 1, shape: cubeShape2 });

    cubeBody2.position.set(-1, 0, 0);

    world.addBody(cubeBody2);

    // 监听碰撞事件

    cubeBody2.addEventListener("collide", function (e) {

      cube2.material.color.set(0xff0000);

    });

    const renderer = new THREE.WebGLRenderer({

      canvas: document.querySelector('#canvas'),

      antialias: true

    });

    renderer.setSize(window.innerWidth, window.innerHeight, false)

    const controls = new OrbitControls(camera, renderer.domElement);

    function animate() {

      // 更新帧数

      stats.update()

      world.step(1 / 60);

      cubeBody1.position.x -= 0.02;

      // 更新Three.js模型的位置

      cube1.position.copy(cubeBody1.position);

      cube1.quaternion.copy(cubeBody1.quaternion);

      cube2.position.copy(cubeBody2.position);

      cube2.quaternion.copy(cubeBody2.quaternion);

      requestAnimationFrame(animate);

      renderer.render(scene, camera);

    }

    animate();

  </script>

</body>

</html>

至于精确性呢，使用Cannon.js也是不错的，示例如下：

看上去，使用Cannon.js的效果是相当不错的，在追求效果的情况下使用物理引擎是不错的选择，当然，增加的编码成本、计算开销也是不少

5. 参考资料

[1] Raycaster – three.js docs (three3d.cn)

[2] Box3 – three.js docs (threejs.org)

[3] schteppe/cannon.js: A lightweight 3D physics engine written in JavaScript. (github.com)

[4] Three.js - 物体碰撞检测（二十六） - 掘金 (juejin.cn)

[5] Three.js 进阶之旅：物理效果-碰撞和声音 - 掘金 (juejin.cn)

[6] pmndrs/cannon-es: A lightweight 3D physics engine written in JavaScript. (github.com)

[7] Cannon.js -- 3d物理引擎_cannon-es_acqui~Zhang的博客-CSDN博客