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    <title>渲染目标</title>
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        <h1>渲染目标</h1>
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      <div class="lesson">
        <div class="lesson-main">
          <p>在three.js中，渲染目标大体上指的是可以被渲染的纹理。当它被渲染之后，你可以像使用其他纹理一样使用它。</p>
<p>让我们举个简单的例子。我们将从<a href="responsive.html">the article on responsiveness</a>开始。</p>
<p>渲染到渲染目标基本上跟通常的渲染一样。首先我们创建一个 <a href="/docs/#api/zh/renderers/WebGLRenderTarget"><code class="notranslate" translate="no">WebGLRenderTarget</code></a>。</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const rtWidth = 512;
const rtHeight = 512;
const renderTarget = new THREE.WebGLRenderTarget(rtWidth, rtHeight);
</pre>
<p>然后我们需要一个 <a href="/docs/#api/zh/cameras/Camera"><code class="notranslate" translate="no">Camera</code></a>  和一个 <a href="/docs/#api/zh/scenes/Scene"><code class="notranslate" translate="no">Scene</code></a></p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const rtFov = 75;
const rtAspect = rtWidth / rtHeight;
const rtNear = 0.1;
const rtFar = 5;
const rtCamera = new THREE.PerspectiveCamera(rtFov, rtAspect, rtNear, rtFar);
rtCamera.position.z = 2;

const rtScene = new THREE.Scene();
rtScene.background = new THREE.Color('red');
</pre>
<p>注意我们设置长宽比(aspect)是相对渲染目标而言的，不是画布(canvas)。
正确的长宽比取决于我们要渲染的对象。在本例，我们要将渲染目标的纹理用在方块的一个面，基于方块的面我们设置长宽比为1.0。</p>
<p>我们将需要的东西添加到场景中。在本例我们使用灯光和三个方块<a href="responsive.html">from the previous article</a>。</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">{
  const color = 0xFFFFFF;
  const intensity = 1;
  const light = new THREE.DirectionalLight(color, intensity);
  light.position.set(-1, 2, 4);
*  rtScene.add(light);
}

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

function makeInstance(geometry, color, x) {
  const material = new THREE.MeshPhongMaterial({color});

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

  cube.position.x = x;

  return cube;
}

*const rtCubes = [
  makeInstance(geometry, 0x44aa88,  0),
  makeInstance(geometry, 0x8844aa, -2),
  makeInstance(geometry, 0xaa8844,  2),
];
</pre>
<p>在上个例子中的 <a href="/docs/#api/zh/scenes/Scene"><code class="notranslate" translate="no">Scene</code></a> 和 <a href="/docs/#api/zh/cameras/Camera"><code class="notranslate" translate="no">Camera</code></a> 保持不变，我们将在画布中继续使用它们，只需要添加渲染的物体。</p>
<p>让我们添加使用了渲染目标纹理的方块。</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">const material = new THREE.MeshPhongMaterial({
  map: renderTarget.texture,
});
const cube = new THREE.Mesh(geometry, material);
scene.add(cube);
</pre>
<p>现在在渲染的时候，我们首先将渲染目标的场景(rtScene)，渲染到渲染目标(注：这里有点绕，需要结合代码理解)。</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">function render(time) {
  time *= 0.001;

  ...

  // rotate all the cubes in the render target scene
  rtCubes.forEach((cube, ndx) =&gt; {
    const speed = 1 + ndx * .1;
    const rot = time * speed;
    cube.rotation.x = rot;
    cube.rotation.y = rot;
  });

  // draw render target scene to render target
  renderer.setRenderTarget(renderTarget);
  renderer.render(rtScene, rtCamera);
  renderer.setRenderTarget(null);
</pre>
<p>然后我们在画布中，渲染使用了渲染目标纹理的方块的场景。</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">  // rotate the cube in the scene
  cube.rotation.x = time;
  cube.rotation.y = time * 1.1;

  // render the scene to the canvas
  renderer.render(scene, camera);
</pre>
<p>就是这样啦</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/render-target.html"></iframe></div>
  <a class="threejs_center" href="/manual/examples/render-target.html" target="_blank">点击此处在新标签页中打开</a>
</div>

<p></p>
<p>方块是红色的，这是因为我们设置了 <code class="notranslate" translate="no">rtScene</code> 的 <code class="notranslate" translate="no">background</code> 为红色，所以渲染目标的纹理所处的背景为红色。</p>
<p>渲染目标可以用在各种各样的物体上。<a href="shadows.html">Shadows</a>用了渲染目标，<a href="picking.html">Picking can use a render target</a>，多种效果<a href="post-processing.html">post processing effects</a>需要用到渲染目标。
渲染汽车的后视镜，或者3D场景中的监控实时画面，都可能用到渲染目标。</p>
<p>关于 <a href="/docs/#api/zh/renderers/WebGLRenderTarget"><code class="notranslate" translate="no">WebGLRenderTarget</code></a> 的笔记。</p>
<ul>
<li><p>默认情况下 <a href="/docs/#api/zh/renderers/WebGLRenderTarget"><code class="notranslate" translate="no">WebGLRenderTarget</code></a> 会创建两个纹理。 颜色纹理和深度/模版纹理。如果你不需要深度或者模版纹理，你可以通过可选设置取消创建。例如：</p>
<pre class="prettyprint showlinemods notranslate lang-js" translate="no">  const rt = new THREE.WebGLRenderTarget(width, height, {
    depthBuffer: false,
    stencilBuffer: false,
  });
</pre>
</li>
<li><p>你可能需要改变渲染目标的尺寸</p>
<p>在上面的例子，我们创建了固定尺寸512X512的渲染目标。对于像后处理，你通常需要创建跟画布一样尺寸的渲染目标。在我们的代码中意味着，当我们改变画布的尺寸，会同时更新渲染目标尺寸，和渲染目标中正在使用的摄像机。例如：</p>
<pre class="prettyprint showlinemods notranslate notranslate" translate="no">function render(time) {
  time *= 0.001;

  if (resizeRendererToDisplaySize(renderer)) {
    const canvas = renderer.domElement;
    camera.aspect = canvas.clientWidth / canvas.clientHeight;
    camera.updateProjectionMatrix();

+    renderTarget.setSize(canvas.width, canvas.height);
+    rtCamera.aspect = camera.aspect;
+    rtCamera.updateProjectionMatrix();
}
</pre></li>
</ul>

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