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<!DOCTYPE html>
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<html lang="zh">
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<head>
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<meta charset="utf-8">
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<title>广告牌(Billboards)</title>
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<meta name="viewport" content="width=device-width, user-scalable=no, minimum-scale=1.0, maximum-scale=1.0">
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<meta name="twitter:card" content="summary_large_image">
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<meta name="twitter:site" content="@threejs">
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<meta name="twitter:title" content="Three.js – Billboards">
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<meta property="og:image" content="https://threejs.org/files/share.png">
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<link rel="shortcut icon" href="../../files/favicon_white.ico" media="(prefers-color-scheme: dark)">
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<link rel="shortcut icon" href="../../files/favicon.ico" media="(prefers-color-scheme: light)">
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<link rel="stylesheet" href="../resources/lesson.css">
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<link rel="stylesheet" href="../resources/lang.css">
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<script type="importmap">
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{
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"imports": {
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"three": "../../build/three.module.js"
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}
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}
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</script>
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<link rel="stylesheet" href="/manual/zh/lang.css">
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</head>
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<body>
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<div class="container">
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<div class="lesson-title">
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<h1>广告牌(Billboards)</h1>
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</div>
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<div class="lesson">
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<div class="lesson-main">
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<p>在 <a href="canvas-textures.html">上一篇文章</a> 我们使用了一个 <a href="/docs/#api/en/textures/CanvasTexture"><code
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class="notranslate" translate="no">CanvasTexture</code></a>
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在人物上创作标签(Labels)和徽标(Badges)。有时我们想制作一些总是面对相机的东西。Three.js提供了 <a href="/docs/#api/en/objects/Sprite"><code
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class="notranslate" translate="no">Sprite</code></a> 和
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<a href="/docs/#api/en/materials/SpriteMaterial"><code class="notranslate"
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translate="no">SpriteMaterial</code></a> 来实现这个功能。
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</p>
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<p>我们修改这个徽标的例子 <a href="canvas-textures.html">使用Canvas作为纹理</a>，
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应用 <a href="/docs/#api/en/objects/Sprite"><code class="notranslate" translate="no">Sprite</code></a> and
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<a href="/docs/#api/en/materials/SpriteMaterial"><code class="notranslate"
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translate="no">SpriteMaterial</code></a>
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</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">function makePerson(x, labelWidth, size, name, color) {
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const canvas = makeLabelCanvas(labelWidth, size, name);
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const texture = new THREE.CanvasTexture(canvas);
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// 因为我们的Canvas的尺寸可能不是2的N次方
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// 在两个维度上适当地设置filter属性
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texture.minFilter = THREE.LinearFilter;
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texture.wrapS = THREE.ClampToEdgeWrapping;
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texture.wrapT = THREE.ClampToEdgeWrapping;
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- const labelMaterial = new THREE.MeshBasicMaterial({
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+ const labelMaterial = new THREE.SpriteMaterial({
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map: texture,
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- side: THREE.DoubleSide,
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transparent: true,
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});
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const root = new THREE.Object3D();
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root.position.x = x;
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const body = new THREE.Mesh(bodyGeometry, bodyMaterial);
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root.add(body);
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body.position.y = bodyHeight / 2;
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const head = new THREE.Mesh(headGeometry, bodyMaterial);
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root.add(head);
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head.position.y = bodyHeight + headRadius * 1.1;
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- const label = new THREE.Mesh(labelGeometry, labelMaterial);
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+ const label = new THREE.Sprite(labelMaterial);
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root.add(label);
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label.position.y = bodyHeight * 4 / 5;
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label.position.z = bodyRadiusTop * 1.01;</pre>
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<p>现在标签始终是面向相机了。</p>
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<p></p>
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<div translate="no" class="threejs_example_container notranslate">
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<div><iframe class="threejs_example notranslate" translate="no" style=" "
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src="/manual/examples/resources/editor.html?url=/manual/examples/billboard-labels-w-sprites.html"></iframe>
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</div>
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<a class="threejs_center" href="/manual/examples/billboard-labels-w-sprites.html" target="_blank">点击在新窗口打开</a>
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</div>
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<p></p>
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<p>一个问题是，从某些角度来看的话，标签与人物重合了。 </p>
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<div class="threejs_center"><img src="../resources/images/billboard-label-z-issue.png" style="width: 455px;">
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</div>
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<p>我们可以通过移动标签的位置来解决此问题。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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+// 如果单位是米，这里就用0.01
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+// 也就是以厘米作为标签的单位
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+const labelBaseScale = 0.01;
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const label = new THREE.Sprite(labelMaterial);
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root.add(label);
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-label.position.y = bodyHeight * 4 / 5;
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-label.position.z = bodyRadiusTop * 1.01;
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+label.position.y = head.position.y + headRadius + size * labelBaseScale;
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-// 如果单位是米，这里就用0.01
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-// 也就是以厘米作为标签的单位
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-const labelBaseScale = 0.01;
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label.scale.x = canvas.width * labelBaseScale;
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label.scale.y = canvas.height * labelBaseScale;</pre>
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<p></p>
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<div translate="no" class="threejs_example_container notranslate">
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<div><iframe class="threejs_example notranslate" translate="no" style=" "
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src="/manual/examples/resources/editor.html?url=/manual/examples/billboard-labels-w-sprites-adjust-height.html"></iframe>
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</div>
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<a class="threejs_center" href="/manual/examples/billboard-labels-w-sprites-adjust-height.html"
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target="_blank">点击在新窗口打开</a>
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</div>
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<p></p>
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<p>我们可以用Billboard做的另一件事是绘制立面(Facades)。</p>
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<p>我们不绘制 3D 对象，而是使用图片绘制 2D 平面化的 3D 对象，这通常比绘制 3D 对象要快。</p>
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<p>例如，我们用树木网络制作一个场景，我们让每一棵树的底部是圆柱体，顶部是圆锥体。</p>
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<p>第一步，我们创建圆锥体和圆柱体的Geometry和Material，所有的树都会复用这些。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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const trunkRadius = .2;
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const trunkHeight = 1;
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const trunkRadialSegments = 12;
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const trunkGeometry = new THREE.CylinderGeometry(
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trunkRadius, trunkRadius, trunkHeight, trunkRadialSegments);
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const topRadius = trunkRadius * 4;
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const topHeight = trunkHeight * 2;
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const topSegments = 12;
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const topGeometry = new THREE.ConeGeometry(
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topRadius, topHeight, topSegments);
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const trunkMaterial = new THREE.MeshPhongMaterial({color: 'brown'});
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const topMaterial = new THREE.MeshPhongMaterial({color: 'green'});</pre>
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<p>然后我们创建一个函数，对每一棵树的树干和树顶创建一个 <a href="/docs/#api/en/objects/Mesh"><code class="notranslate"
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translate="no">Mesh</code></a>
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，并把它们都加入到一个 <a href="/docs/#api/en/core/Object3D"><code class="notranslate"
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translate="no">Object3D</code></a>对象下。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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function makeTree(x, z) {
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const root = new THREE.Object3D();
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const trunk = new THREE.Mesh(trunkGeometry, trunkMaterial);
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trunk.position.y = trunkHeight / 2;
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root.add(trunk);
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const top = new THREE.Mesh(topGeometry, topMaterial);
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top.position.y = trunkHeight + topHeight / 2;
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root.add(top);
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root.position.set(x, 0, z);
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scene.add(root);
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return root;
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}</pre>
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<p>然后我们会创建一个循环，生成树网络。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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for (let z = -50; z <= 50; z += 10) {
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for (let x = -50; x <= 50; x += 10) {
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makeTree(x, z);
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}
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}</pre>
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<p>让我们再增加一个地平面。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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// 添加地面
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{
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const size = 400;
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const geometry = new THREE.PlaneGeometry(size, size);
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const material = new THREE.MeshPhongMaterial({color: 'gray'});
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const mesh = new THREE.Mesh(geometry, material);
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mesh.rotation.x = Math.PI * -0.5;
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scene.add(mesh);
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}</pre>
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<p>然后把背景调整为浅蓝(lightblue)</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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const scene = new THREE.Scene();
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-scene.background = new THREE.Color('white');
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+scene.background = new THREE.Color('lightblue');</pre>
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<p>我们得到了一个树木网络</p>
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<p></p>
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<div translate="no" class="threejs_example_container notranslate">
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<div><iframe class="threejs_example notranslate" translate="no" style=" "
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src="/manual/examples/resources/editor.html?url=/manual/examples/billboard-trees-no-billboards.html"></iframe>
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</div>
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<a class="threejs_center" href="/manual/examples/billboard-trees-no-billboards.html"
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target="_blank">点击在新窗口打开</a>
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</div>
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<p></p>
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<p>这里有11x11或者121棵树，每棵树由12个多边形组成的锥体 +
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48个多边形组成的树干组成，所以每棵树包含60个多边形。121*60的结果是7260，这并不是很多，当然更精细的3D树可能有1000-3000个多边形构成。如果3000个多边形构成的树，那么121棵树将包含363000个多边形。
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</p>
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<p>使用Facades，可以降低这个数字。</p>
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<p>我们可以在一些绘图应用中手动创建一个Facade，现在让我们编写一些代码来手动生成一个。</p>
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<p>现在写一些代码把对象绘制到纹理中，使用一个 <code class="notranslate" translate="no">RenderTarget</code>，我们提到过使用
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<code class="notranslate" translate="no">RenderTarget</code>
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来渲染，具体在这篇 <a href="rendertargets.html">渲染目标</a> 文章里。
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</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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function frameArea(sizeToFitOnScreen, boxSize, boxCenter, camera) {
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const halfSizeToFitOnScreen = sizeToFitOnScreen * 0.5;
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const halfFovY = THREE.MathUtils.degToRad(camera.fov * .5);
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const distance = halfSizeToFitOnScreen / Math.tan(halfFovY);
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camera.position.copy(boxCenter);
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camera.position.z += distance;
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// 为视锥体选择合适的near和far值
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// 可以把盒模型包裹进来
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camera.near = boxSize / 100;
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camera.far = boxSize * 100;
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camera.updateProjectionMatrix();
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}
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function makeSpriteTexture(textureSize, obj) {
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const rt = new THREE.WebGLRenderTarget(textureSize, textureSize);
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const aspect = 1; // 因为Render Target是正方形
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const camera = new THREE.PerspectiveCamera(fov, aspect, near, far);
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scene.add(obj);
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// 计算对象的盒模型
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const box = new THREE.Box3().setFromObject(obj);
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const boxSize = box.getSize(new THREE.Vector3());
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const boxCenter = box.getCenter(new THREE.Vector3());
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// 设置相机去构建盒模型
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const fudge = 1.1;
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const size = Math.max(...boxSize.toArray()) * fudge;
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frameArea(size, size, boxCenter, camera);
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renderer.autoClear = false;
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renderer.setRenderTarget(rt);
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renderer.render(scene, camera);
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renderer.setRenderTarget(null);
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renderer.autoClear = true;
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scene.remove(obj);
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return {
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position: boxCenter.multiplyScalar(fudge),
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scale: size,
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texture: rt.texture,
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};
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}</pre>
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<p>关于上面代码的一些注意事项：</p>
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<p>我们使用之前代码定义好的视图的 (<code class="notranslate" translate="no">fov</code>) 属性，
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</p>
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<p>我们计算一个包含树的盒模型，这和 <a href="load-obj.html">加载.obj的文件</a> 中提到的方式一致，有一点微小的改变。</p>
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<p>我们再次调用 <code class="notranslate" translate="no">frameArea</code>，稍微改写了一下<a
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href="load-obj.html">加载.obj的文件</a>。
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在这种情况下，我们计算相机需要离物体多远，从而让它的视野以包含对象。然后我们将相机的-z值设置为从对象盒模型的中心到此的距离。</p>
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<p>我们将想要适应的大小乘以1.1倍(<code class="notranslate" translate="no">fudge</code>)
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来确保树完全在渲染目标中。这是因为我们用来计算对象是否适合相机的视口的尺寸，没有考虑到对象的边缘可能会超出我们的可视区域之外。我们可以计算出如何让盒子100%合适，但这会浪费很多空间，所以我们就是蒙混
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<em>(fudge)</em> 一下。
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</p>
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<p>然后我们渲染到RenderTarget中，然后从场景中移除此对象。 </p>
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<p>重点是需要场景中的灯光，但我们需要确保场景中没有其他东西。</p>
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<p>我们也不能给场景设置背景色。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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const scene = new THREE.Scene();
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-scene.background = new THREE.Color('lightblue');</pre>
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<p>最后我们返回了纹理，位置和缩放比例，我们需要创建Facade，让它看起来在同一个地方。</p>
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<p>然后我们制作一棵树，调用此代码：</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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// 创建Billboard纹理
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const tree = makeTree(0, 0);
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const facadeSize = 64;
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const treeSpriteInfo = makeSpriteTexture(facadeSize, tree);</pre>
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<p>然后我们可以制作一个Facade网络，而不是树网络。</p>
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<pre class="prettyprint showlinemods notranslate lang-js" translate="no">
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+function makeSprite(spriteInfo, x, z) {
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+ const {texture, offset, scale} = spriteInfo;
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+ const mat = new THREE.SpriteMaterial({
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+ map: texture,
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+ transparent: true,
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+ });
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+ const sprite = new THREE.Sprite(mat);
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+ scene.add(sprite);
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+ sprite.position.set(
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+ offset.x + x,
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+ offset.y,
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+ offset.z + z);
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+ sprite.scale.set(scale, scale, scale);
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+}
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for (let z = -50; z <= 50; z += 10) {
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for (let x = -50; x <= 50; x += 10) {
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- makeTree(x, z);
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+ makeSprite(treeSpriteInfo, x, z);
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}
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}</pre>
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<p>在上面的代码中，我们应用了定位Facade所需的偏移量和缩放比例，因此他会出现在和原树同一个地方。</p>
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<p>现在我们已经完成了Facade纹理的制作，我们可以再次设置背景。</p>
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<pre class="prettyprint showlinemods notranslate lang-js"
|
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translate="no">scene.background = new THREE.Color('lightblue');</pre>
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<p>现在我们得到了一个全是树Facades的场景。</p>
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<p></p>
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<div translate="no" class="threejs_example_container notranslate">
|
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|
<div><iframe class="threejs_example notranslate" translate="no" style=" "
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|
src="/manual/examples/resources/editor.html?url=/manual/examples/billboard-trees-static-billboards.html"></iframe>
|
|
|
</div>
|
|
|
<a class="threejs_center" href="/manual/examples/billboard-trees-static-billboards.html"
|
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|
target="_blank">点击在新窗口打开</a>
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|
</div>
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<p></p>
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<p>
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与上面的树模型相比，它们看起来非常相似。我们使用了低分辨率纹理，只有64x64像素，所以Facade是块状的，你当然可以提高分辨率。通常Facade只会用在非常远处的物体，因为当它们非常小的时候，低分辨率纹理就足够了。它节省了绘制远处只有几个像素的精致树模型时间。
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</p>
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<p>另一个问题是我们只能从一侧查看树。这往往是通过渲染更多的Facade来解决，比如绘制对象周围的8个方向，然后根据实际相机的方向来设置要展示的Facade。</p>
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<p>是否使用Facade由你决定，如果你决定去使用它们，希望这篇文章给了你一些想法和解决方案。</p>
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