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- <title>拾取</title>
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- <h1>拾取</h1>
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- <div class="lesson">
- <div class="lesson-main">
- <p><em>拾取</em> 指代推断用户点击或触碰了哪个对象的过程。有很多方式实现拾取,但是,每一种都有相应的成本,使用时需有所取舍。下面是最常用的两种方式:</p>
- <p><strong>射线追踪法</strong>(raycasting)很可能是最常用的方法,其基本原理是:从鼠标处发射一条射线,穿透场景的视椎体,通过计算,找出视锥体中哪些对象与射线相交。</p>
- <p>首先,获取鼠标的屏幕坐标.其次,对其应用摄像机的投影和方向的矩阵变换,得到其在世界空间的坐标。然后,计算出一条射线,从视锥体的近端平面射向远端平面。再然后,对于场景中每一个对象的每一个三角,检查其是否与射线相交。假设你的场景中有1000个对象,每个对象有1000个三角,那么就需要检查一百万个三角。</p>
- <p>对此,可以做一些优化,先检查对象的包围球或包围盒是否与射线相交,包围球或包围盒是指包含整个对象的球体或者立方体,如果射线未相交,就不需要检查组成该对象的三角们了。</p>
- <p>THREE.js 提供了 <code class="notranslate" translate="no">RayCaster</code> 类来做这些事情。</p>
- <p>接下来,让我们先创建一个包含100个对象的场景,然后试着去拾取这些对象。可以从样例<a href="responsive.html">threejs-响应式</a>开始。</p>
- <p>改动一些代码
- 使摄像机成为一个对象的子元素,旋转这个对象时,摄像机会像绑定在自拍杆上一样,在场景中游弋。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">*const fov = 60;
- const aspect = 2; // 画布默认纵横比为2
- const near = 0.1;
- *const far = 200;
- const camera = new THREE.PerspectiveCamera(fov, aspect, near, far);
- *camera.position.z = 30;
- const scene = new THREE.Scene();
- +scene.background = new THREE.Color('white');
- +// 把摄像机放到自拍杆上 (把它添加为一个对象的子元素)
- +// 如此,我们就能通过旋转自拍杆,来移动摄像机
- +const cameraPole = new THREE.Object3D();
- +scene.add(cameraPole);
- +cameraPole.add(camera);
- </pre>
- <p>在 <code class="notranslate" translate="no">render</code> 函数中旋转摄像机端点。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">cameraPole.rotation.y = time * .1;
- </pre>
- <p>把光源也绑定到摄像机上,这样光源就会随着摄像机移动。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">-scene.add(light);
- +camera.add(light);
- </pre>
- <p>生成100个立方体,每个立方体的颜色,位置,朝向,缩放都随机。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">const boxWidth = 1;
- const boxHeight = 1;
- const boxDepth = 1;
- const geometry = new THREE.BoxGeometry(boxWidth, boxHeight, boxDepth);
- function rand(min, max) {
- if (max === undefined) {
- max = min;
- min = 0;
- }
- return min + (max - min) * Math.random();
- }
- function randomColor() {
- return `hsl(${rand(360) | 0}, ${rand(50, 100) | 0}%, 50%)`;
- }
- const numObjects = 100;
- for (let i = 0; i < numObjects; ++i) {
- const material = new THREE.MeshPhongMaterial({
- color: randomColor(),
- });
- const cube = new THREE.Mesh(geometry, material);
- scene.add(cube);
- cube.position.set(rand(-20, 20), rand(-20, 20), rand(-20, 20));
- cube.rotation.set(rand(Math.PI), rand(Math.PI), 0);
- cube.scale.set(rand(3, 6), rand(3, 6), rand(3, 6));
- }
- </pre>
- <p>最后,让我们来完成拾取功能。
- 写一个简单的类来管理拾取操作</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">class PickHelper {
- constructor() {
- this.raycaster = new THREE.Raycaster();
- this.pickedObject = null;
- this.pickedObjectSavedColor = 0;
- }
- pick(normalizedPosition, scene, camera, time) {
- // 恢复上一个被拾取对象的颜色
- if (this.pickedObject) {
- this.pickedObject.material.emissive.setHex(this.pickedObjectSavedColor);
- this.pickedObject = undefined;
- }
- // 发出射线
- this.raycaster.setFromCamera(normalizedPosition, camera);
- // 获取与射线相交的对象
- const intersectedObjects = this.raycaster.intersectObjects(scene.children);
- if (intersectedObjects.length) {
- // 找到第一个对象,它是离鼠标最近的对象
- this.pickedObject = intersectedObjects[0].object;
- // 保存它的颜色
- this.pickedObjectSavedColor = this.pickedObject.material.emissive.getHex();
- // 设置它的发光为 黄色/红色闪烁
- this.pickedObject.material.emissive.setHex((time * 8) % 2 > 1 ? 0xFFFF00 : 0xFF0000);
- }
- }
- }
- </pre>
- <p>可以看到,我们创建了一个 <code class="notranslate" translate="no">RayCaster</code> 实例,调用该实例的 <code class="notranslate" translate="no">pick</code> 方法可以在场景中发出一条射线。如果,射线有撞击到场景中的物体,修改撞击到的第一个物体的颜色。</p>
- <p>当然,也可以在用户点击鼠标时,调用这个方法,这恐怕是最常见的应用场景。但是,在本范例中,不管在鼠标下方是什么,在每一帧中都会进行拾取操作,为此,需要跟踪鼠标的位置。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">const pickPosition = {x: 0, y: 0};
- clearPickPosition();
- ...
- function getCanvasRelativePosition(event) {
- const rect = canvas.getBoundingClientRect();
- return {
- x: (event.clientX - rect.left) * canvas.width / rect.width,
- y: (event.clientY - rect.top ) * canvas.height / rect.height,
- };
- }
- function setPickPosition(event) {
- const pos = getCanvasRelativePosition(event);
- pickPosition.x = (pos.x / canvas.width ) * 2 - 1;
- pickPosition.y = (pos.y / canvas.height) * -2 + 1; // note we flip Y
- }
- function clearPickPosition() {
- // 对于触屏,不像鼠标总是能有一个位置坐标,
- // 如果用户不在触摸屏幕,我们希望停止拾取操作。
- // 因此,我们选取一个特别的值,表明什么都没选中
- pickPosition.x = -100000;
- pickPosition.y = -100000;
- }
- window.addEventListener('mousemove', setPickPosition);
- window.addEventListener('mouseout', clearPickPosition);
- window.addEventListener('mouseleave', clearPickPosition);
- </pre>
- <p>需要注意的是,我们记录了归一化的鼠标位置。无论画布的尺寸,我们需要一个从左到右,落入区间(-1,1)的值,类似的,也需要一个从下到上,落入区间(-1,1)的值。</p>
- <p>完成以后,我们再添加对移动端的支持:</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">window.addEventListener('touchstart', (event) => {
- // 阻止窗口滚动行为
- event.preventDefault();
- setPickPosition(event.touches[0]);
- }, {passive: false});
- window.addEventListener('touchmove', (event) => {
- setPickPosition(event.touches[0]);
- });
- window.addEventListener('touchend', clearPickPosition);
- </pre>
- <p>最终,在 <code class="notranslate" translate="no">render</code> 方法中,我们调用了 <code class="notranslate" translate="no">PickHelper</code> 的 <code class="notranslate" translate="no">pick</code> 方法</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">+const pickHelper = new PickHelper();
- function render(time) {
- time *= 0.001; //将毫秒单位转换为秒单位;
- ...
- + pickHelper.pick(pickPosition, scene, camera, time);
- renderer.render(scene, camera);
- ...
- </pre>
- <p>这是最终结果
- </p><div translate="no" class="threejs_example_container notranslate">
- <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/picking-raycaster.html"></iframe></div>
- <a class="threejs_center" href="/manual/examples/picking-raycaster.html" target="_blank">点击此处在新标签页中打开</a>
- </div>
- <p></p>
- <p>这种方式看起来效果不错,而且能处理很多用户场景,但是也存在几个问题:</p>
- <ol>
- <li>这是基于CPU运算的
- Javascript遍历每一个对象,检查其包围盒或包围球是否与射线相交,如果相交,它必须遍历组成该对象的每一个三角,检查它们是否与射线相交。<br>好处是,JavaScript能够很容易计算出射线在哪里与三角相交,并为我们提供相关数据。举个例子,如果你想要在相交的位置放置一个标记。<br>缺点是,CPU要做大量的工作,当你的对象由大量的三角组成时,这个过程会有些慢。</li>
- <li>它无法处理一些奇怪的着色器或者位移
- 如果,你有一个变形或者拟态几何形状的着色器,Javascript无法理解这个变形,它会给出错误的答案。举例:据我所知,你不能对有皮肤的对象使用这种方式。</li>
- <li>无法处理透明的孔洞
- 举个例子,为立方体应用这个纹理<div class="threejs_center"><img class="checkerboard" src="../examples/resources/images/frame.png"></div>
- </li>
- </ol>
- <p>改动代码如下:</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">+const loader = new THREE.TextureLoader();
- +const texture = loader.load('resources/images/frame.png');
- const numObjects = 100;
- for (let i = 0; i < numObjects; ++i) {
- const material = new THREE.MeshPhongMaterial({
- color: randomColor(),
- +map: texture,
- +transparent: true,
- +side: THREE.DoubleSide,
- +alphaTest: 0.1,
- });
- const cube = new THREE.Mesh(geometry, material);
- scene.add(cube);
- ...
- </pre>
- <p>运行后,你就能看到问题所在。
- </p><div translate="no" class="threejs_example_container notranslate">
- <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/picking-raycaster-transparency.html"></iframe></div>
- <a class="threejs_center" href="/manual/examples/picking-raycaster-transparency.html" target="_blank">点击此处在新标签页中打开</a>
- </div>
- <p></p>
- <p>试着透过盒子拾取一些物体,但是你无法做到</p>
- <div class="threejs_center"><img src="../resources/images/picking-transparent-issue.jpg" style="width: 635px;"></div>
- <p>这是因为 JavaScript 无法通过简单的查看纹理和材质,就推测出你的对象是否存在一部分是透明的或者不透明。</p>
- <p>对于这些问题的解决方案,就是使用基于GPU的拾取方法。可惜,该方法概念上简单,但是相比于射线追踪法,用起来就复杂了。</p>
- <p>为了完成GPU拾取,对每一个对象使用唯一的颜色进行离屏渲染。然后,检查鼠标位置关联的像素的颜色。这个颜色就能告诉我们哪个对象被选中。</p>
- <p>这能解决上面的问题2,3。至于问题1的速度问题,这取决于业务场景。每个对象会被绘制两次,一次用于观看,一次用于拾取。也许存在开脑洞的解决方案,可以只绘制一次就完成查看和拾取,此处我们不会尝试。</p>
- <p>但是有一件事值得去做,因为拾取时我们只需读取1px,所以我们可以设置摄像机,只绘制1px,通过 <a href="/docs/#api/zh/cameras/PerspectiveCamera.setViewOffset"><code class="notranslate" translate="no">PerspectiveCamera.setViewOffset</code></a> 方法,可以告诉THREE.js 计算出一个摄像机 只呈现一个大矩形的一个很小的部分。这应该能节省一些运行时间。</p>
- <p>此时,要在THREE.js中实现这种拾取方式,需要创建两个场景。一个使用正常的网格对象填充。另外一个使用“拾取材质”的网格对象填充。</p>
- <p>因此,首先创建第二个场景,并将其清理为黑色背景。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">const scene = new THREE.Scene();
- scene.background = new THREE.Color('white');
- const pickingScene = new THREE.Scene();
- pickingScene.background = new THREE.Color(0);
- </pre>
- <p>然后,对于在主场景中的每一个立方体,在 <code class="notranslate" translate="no">pickingScene</code> 中,在同样的位置,创建一个与原立方体相似的,相关联的“可拾取立方体”,用对象的id生成颜色值,去设置对象的材质。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">const idToObject = {};
- +const numObjects = 100;
- for (let i = 0; i < numObjects; ++i) {
- + const id = i + 1;
- const material = new THREE.MeshPhongMaterial({
- color: randomColor(),
- map: texture,
- transparent: true,
- side: THREE.DoubleSide,
- alphaTest: 0.1,
- });
- const cube = new THREE.Mesh(geometry, material);
- scene.add(cube);
- + idToObject[id] = cube;
- cube.position.set(rand(-20, 20), rand(-20, 20), rand(-20, 20));
- cube.rotation.set(rand(Math.PI), rand(Math.PI), 0);
- cube.scale.set(rand(3, 6), rand(3, 6), rand(3, 6));
- + const pickingMaterial = new THREE.MeshPhongMaterial({
- + emissive: new THREE.Color().setHex(id, THREE.NoColorSpace),
- + color: new THREE.Color(0, 0, 0),
- + specular: new THREE.Color(0, 0, 0),
- + map: texture,
- + transparent: true,
- + side: THREE.DoubleSide,
- + alphaTest: 0.5,
- + blending: THREE.NoBlending,
- + });
- + const pickingCube = new THREE.Mesh(geometry, pickingMaterial);
- + pickingScene.add(pickingCube);
- + pickingCube.position.copy(cube.position);
- + pickingCube.rotation.copy(cube.rotation);
- + pickingCube.scale.copy(cube.scale);
- }
- </pre>
- <p>注意到,此时,我们利用 <a href="/docs/#api/zh/materials/MeshPhongMaterial"><code class="notranslate" translate="no">MeshPhongMaterial</code></a> 创建材质,使用id生成颜色,设置到它的<code class="notranslate" translate="no">emissive</code>属性,<code class="notranslate" translate="no">color</code> 和 <code class="notranslate" translate="no">specular</code>属性设置为0,设置 <code class="notranslate" translate="no">alphaTest</code> 属性,只渲染纹理的alpha值大于该属性值的部分,还需要将<code class="notranslate" translate="no">blending</code> 设置为 <code class="notranslate" translate="no">NoBlending</code>,这样alpha通道不会作用到id生成色</p>
- <p>注意到,利用 <a href="/docs/#api/zh/materials/MeshPhongMaterial"><code class="notranslate" translate="no">MeshPhongMaterial</code></a> 可能并不是最优的解决方案,因为,在绘制拾取场景时,仍然需要计算所有的光线,尽管我们不需要这些计算。一个更优的方案是使用自定义的着色器,只为纹理alpha值大于 <code class="notranslate" translate="no">alphaTest</code> 属性值的部分,输出id生成色</p>
- <p>由于我们是从像素点拾取,而不是射线追踪,只需将代码修改为使用像素拾取方式,获取拾取位置。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">function setPickPosition(event) {
- const pos = getCanvasRelativePosition(event);
- - pickPosition.x = (pos.x / canvas.clientWidth ) * 2 - 1;
- - pickPosition.y = (pos.y / canvas.clientHeight) * -2 + 1; // 注意,翻转了y轴
- + pickPosition.x = pos.x;
- + pickPosition.y = pos.y;
- }
- </pre>
- <p>首先,我们将 <code class="notranslate" translate="no">PickHelper</code> 修改为 <code class="notranslate" translate="no">GPUPickHelper</code>。这里使用了 <a href="/docs/#api/zh/renderers/WebGLRenderTarget"><code class="notranslate" translate="no">WebGLRenderTarget</code></a>,如同我们在 <a href="rendertargets.html">多个渲染目标</a>中介绍的一样,此处,我们的渲染目标只有1像素的尺寸,1×1。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">-class PickHelper {
- +class GPUPickHelper {
- constructor() {
- - this.raycaster = new THREE.Raycaster();
- + // 创建一个1px的渲染目标
- + this.pickingTexture = new THREE.WebGLRenderTarget(1, 1);
- + this.pixelBuffer = new Uint8Array(4);
- this.pickedObject = null;
- this.pickedObjectSavedColor = 0;
- }
- pick(cssPosition, scene, camera, time) {
- + const {pickingTexture, pixelBuffer} = this;
- // 如果已经存在拾取的对象,将其颜色恢复
- if (this.pickedObject) {
- this.pickedObject.material.emissive.setHex(this.pickedObjectSavedColor);
- this.pickedObject = undefined;
- }
- + // 设置视野偏移来表现鼠标下的1px
- + const pixelRatio = renderer.getPixelRatio();
- + camera.setViewOffset(
- + renderer.getContext().drawingBufferWidth, // 全宽
- + renderer.getContext().drawingBufferHeight, // 全高
- + cssPosition.x * pixelRatio | 0, // rect x
- + cssPosition.y * pixelRatio | 0, // rect y
- + 1, // rect width
- + 1, // rect height
- + );
- + // 渲染场景
- + renderer.setRenderTarget(pickingTexture)
- + renderer.render(scene, camera);
- + renderer.setRenderTarget(null);
- +
- + // 清理视野偏移,回归正常
- + camera.clearViewOffset();
- + // 读取像素
- + renderer.readRenderTargetPixels(
- + pickingTexture,
- + 0, // x
- + 0, // y
- + 1, // width
- + 1, // height
- + pixelBuffer);
- +
- + const id =
- + (pixelBuffer[0] << 16) |
- + (pixelBuffer[1] << 8) |
- + (pixelBuffer[2] );
- - // 射线穿越视锥体
- - this.raycaster.setFromCamera(normalizedPosition, camera);
- - // 获取与射线相交的对象
- - const intersectedObjects = this.raycaster.intersectObjects(scene.children);
- - if (intersectedObjects.length) {
- - // 获取第一个对象,他是离鼠标最近的一个
- - this.pickedObject = intersectedObjects[0].object;
- + const intersectedObject = idToObject[id];
- + if (intersectedObject) {
- + //获取第一个对象,他是离鼠标最近的一个
- + this.pickedObject = intersectedObject;
- // 保存颜色
- this.pickedObjectSavedColor = this.pickedObject.material.emissive.getHex();
- // 设置对象在黄/红两色间闪烁
- this.pickedObject.material.emissive.setHex((time * 8) % 2 > 1 ? 0xFFFF00 : 0xFF0000);
- }
- }
- }
- </pre>
- <p>接下来,我们就可以使用它了</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">-const pickHelper = new PickHelper();
- +const pickHelper = new GPUPickHelper();
- </pre>
- <p>这里是将 <code class="notranslate" translate="no">pickScene</code> 传给helper,而不是<code class="notranslate" translate="no">scene</code>。</p>
- <pre class="prettyprint showlinemods notranslate lang-js" translate="no">- pickHelper.pick(pickPosition, scene, camera, time);
- + pickHelper.pick(pickPosition, pickScene, camera, time);
- </pre>
- <p>现在,你应该可以透过透明的部分进行拾取操作了</p>
- <p></p><div translate="no" class="threejs_example_container notranslate">
- <div><iframe class="threejs_example notranslate" translate="no" style=" " src="/manual/examples/resources/editor.html?url=/manual/examples/picking-gpu.html"></iframe></div>
- <a class="threejs_center" href="/manual/examples/picking-gpu.html" target="_blank">点击此处在新标签页中打开</a>
- </div>
- <p></p>
- <p>至此,对于如何实现拾取,希望此文能给你一些灵感。在后续的文章中,也许,我们可以看看如何使用鼠标操作对象。</p>
- </div>
- </div>
- </div>
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- <script src="../resources/lesson.js"></script>
- </body></html>
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