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  <title>背景与天空盒</title>
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      <h1>背景与天空盒</h1>
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    <div class="lesson">
      <div class="lesson-main">
        <p>这里大部分的文章都使用纯色作为背景。</p>
        <p>添加静态背景就像添加CSS一样简单，参考来自这篇<a href="responsive.html">THREE.js响应式编程</a>中的一个例子，我们只需要改变两个地方。
        </p>
        <p>我们需要在Canvas上添加一些CSS，以将其背景设置为图像。</p>
        <pre class="prettyprint showlinemods notranslate lang-html" translate="no">&lt;style&gt;
body {
    margin: 0;
}
#c {
    width: 100%;
    height: 100%;
    display: block;
+    background: url(resources/images/daikanyama.jpg) no-repeat center center;
+    background-size: cover;
}
&lt;/style&gt;</pre>
        <p>然后我们需要告诉 <a href="/docs/#api/en/renderers/WebGLRenderer"><code class="notranslate"
              translate="no">WebGLRenderer</code></a> 去使用 <code class="notranslate"
            translate="no">alpha</code>，这样我们不绘制的地方都是透明的。
        </p>
        <pre class="prettyprint showlinemods notranslate lang-js" translate="no">function main() {
  const canvas = document.querySelector('#c');
-  const renderer = new THREE.WebGLRenderer({canvas});
+  const renderer = new THREE.WebGLRenderer({
+    canvas,
+    alpha: true,
+  });</pre>
        <p>然后背景就有了。</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/background-css.html"></iframe></div>
          <a class="threejs_center" href="/manual/examples/background-css.html" target="_blank">点击在新窗口打开</a>
        </div>
        <p></p>
        <p>如果我们想让背景被 <a href="post-processing.html">后处理效果</a>影响，我们需要用THREE.js绘制背景。</p>
        <p>THREE.js 让这变得非常简单。我们可以将场景的背景设置为一种纹理。</p>
        <pre class="prettyprint showlinemods notranslate lang-js" translate="no">const loader = new THREE.TextureLoader();
const bgTexture = loader.load('resources/images/daikanyama.jpg');
scene.background = bgTexture;</pre>
        <p>效果如下</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/background-scene-background.html"></iframe>
          </div>
          <a class="threejs_center" href="/manual/examples/background-scene-background.html"
            target="_blank">点击在新窗口打开</a>
        </div>
        <p></p>
        <p>这产生了一个背景图，但它被拉伸以适应屏幕。</p>
        <p>我们可以解决这个问题，通过设置纹理属性 <code class="notranslate" translate="no">repeat</code> 和 <code class="notranslate"
            translate="no">offset</code> 来显示图像的一部分。</p>
        <pre class="prettyprint showlinemods notranslate lang-js" translate="no">function render(time) {

  ...

+  // 设置背景贴图的repeat和offset属性
+  // 来保证图片的比例是正确的
+  // 注意图片有可能还没加载完成
+  const canvasAspect = canvas.clientWidth / canvas.clientHeight;
+  const imageAspect = bgTexture.image ? bgTexture.image.width / bgTexture.image.height : 1;
+  const aspect = imageAspect / canvasAspect;
+
+  bgTexture.offset.x = aspect &gt; 1 ? (1 - 1 / aspect) / 2 : 0;
+  bgTexture.repeat.x = aspect &gt; 1 ? 1 / aspect : 1;
+
+  bgTexture.offset.y = aspect &gt; 1 ? 0 : (1 - aspect) / 2;
+  bgTexture.repeat.y = aspect &gt; 1 ? 1 : aspect;

  ...

  renderer.render(scene, camera);

  requestAnimationFrame(render);
}</pre>
        <p>现在由 THREE.js 绘制背景。这个上面CSS的版本没有明显的区别，不过如果我们应用<a href="post-processing.html">后处理效果</a>，背景也会被影响。</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/background-scene-background-fixed-aspect.html"></iframe>
          </div>
          <a class="threejs_center" href="/manual/examples/background-scene-background-fixed-aspect.html"
            target="_blank">点击在新窗口打开</a>
          <p></p>
          <p>当然，静态背景通常不是我们在 3D 场景中想要的，相对的我们需要某种<em>天空盒</em>效果。就如字面意思，天空盒就是绘制了一片天空在盒子上。我们把相机放在盒子里，看起来里面有天空背景。</p>
          <p>
            实现天空盒最常见的方法是制作一个立方体，应用纹理，绘制在它的内部。在立方体的每一面绘制一个纹理（使用纹理坐标），看起来像地平线的一些图像。通常使用带有纹理的天空球体或天空穹顶。你也许自己能想象得出来。只用一个立方体或球体,
            <a href="textures.html">应用纹理</a>, 标记它是 <code class="notranslate" translate="no">THREE.BackSide</code>
            ，这样我们渲染内部面而不是外部面，然后也把它直接放到场景里。或者使用两个场景，一个特殊的场景用于绘制天空盒/球体/圆顶，另一个画正常的一切。你可以用你正常的 <a
              href="/docs/#api/en/cameras/PerspectiveCamera"><code class="notranslate"
                translate="no">PerspectiveCamera</code></a> 去绘制，不需要<a
              href="/docs/#api/en/cameras/OrthographicCamera"><code class="notranslate"
                translate="no">OrthographicCamera</code></a>。
          </p>
          <p>另一个方法是使用<em>立方体贴图(Cubemap)</em>. 立方体贴图是一种特殊的纹理，它有6个立方体面，它不使用标准的纹理坐标，使用从中心指向外部的方向来决定绘制的颜色。</p>
          <p>这是来自加利福利亚，山景城的计算机历史博物馆的6张立方体图像。</p>
          <div class="threejs_center">
            <img src="../examples/resources/images/cubemaps/computer-history-museum/pos-x.jpg" style="width: 200px"
              class="border">
            <img src="../examples/resources/images/cubemaps/computer-history-museum/neg-x.jpg" style="width: 200px"
              class="border">
            <img src="../examples/resources/images/cubemaps/computer-history-museum/pos-y.jpg" style="width: 200px"
              class="border">
          </div>
          <div class="threejs_center">
            <img src="../examples/resources/images/cubemaps/computer-history-museum/neg-y.jpg" style="width: 200px"
              class="border">
            <img src="../examples/resources/images/cubemaps/computer-history-museum/pos-z.jpg" style="width: 200px"
              class="border">
            <img src="../examples/resources/images/cubemaps/computer-history-museum/neg-z.jpg" style="width: 200px"
              class="border">
          </div>
          <p>为了使用它们，我们使用<a href="/docs/#api/en/loaders/CubeTextureLoader"><code class="notranslate"
                translate="no">CubeTextureLoader</code></a> 加载它们，然后将其用作场景的背景。</p>
          <pre class="prettyprint showlinemods notranslate lang-js" translate="no">{
  const loader = new THREE.CubeTextureLoader();
  const texture = loader.load([
    'resources/images/cubemaps/computer-history-museum/pos-x.jpg',
    'resources/images/cubemaps/computer-history-museum/neg-x.jpg',
    'resources/images/cubemaps/computer-history-museum/pos-y.jpg',
    'resources/images/cubemaps/computer-history-museum/neg-y.jpg',
    'resources/images/cubemaps/computer-history-museum/pos-z.jpg',
    'resources/images/cubemaps/computer-history-museum/neg-z.jpg',
  ]);
  scene.background = texture;
}</pre>
          <p>在渲染时，我们不需要像上面那样调整纹理：</p>
          <pre class="prettyprint showlinemods notranslate lang-js" translate="no">function render(time) {

  ...

-  // 设置背景贴图的repeat和offset属性
-  // 来保证图片的比例是正确的
-  // 注意图片有可能还没加载完成
-  const canvasAspect = canvas.clientWidth / canvas.clientHeight;
-  const imageAspect = bgTexture.image ? bgTexture.image.width / bgTexture.image.height : 1;
-  const aspect = imageAspect / canvasAspect;
-
-  bgTexture.offset.x = aspect &gt; 1 ? (1 - 1 / aspect) / 2 : 0;
-  bgTexture.repeat.x = aspect &gt; 1 ? 1 / aspect : 1;
-
-  bgTexture.offset.y = aspect &gt; 1 ? 0 : (1 - aspect) / 2;
-  bgTexture.repeat.y = aspect &gt; 1 ? 1 : aspect;

  ...

  renderer.render(scene, camera);

  requestAnimationFrame(render);
}</pre>
          <p>让我们添加一些控件，以便我们可以旋转相机。</p>
          <pre class="prettyprint showlinemods notranslate lang-js"
            translate="no">import {OrbitControls} from '/examples/jsm/controls/OrbitControls.js';</pre>
          <pre class="prettyprint showlinemods notranslate lang-js" translate="no">const fov = 75;
const aspect = 2;  // Canvas默认值
const near = 0.1;
-const far = 5;
+const far = 100;
const camera = new THREE.PerspectiveCamera(fov, aspect, near, far);
-camera.position.z = 2;
+camera.position.z = 3;

+const controls = new OrbitControls(camera, canvas);
+controls.target.set(0, 0, 0);
+controls.update();</pre>
          <p>来尝试下，拖动以旋转示例里的相机，观察围绕着我们的立方体贴图。</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/background-cubemap.html"></iframe>
            </div>
            <a class="threejs_center" href="/manual/examples/background-cubemap.html" target="_blank">点击在新窗口打开</a>
          </div>
          <p></p>
          <p>另一种选择是使用等距矩形贴图(Equirectangular map)， 这是被
            <a href="https://google.com/search?q=360+camera">360 全景相机</a> 拍摄的一种特殊类型的图片。
          </p>
          <p><a href="https://hdrihaven.com/hdri/?h=tears_of_steel_bridge">这是一张图片</a>，我从
            <a href="https://hdrihaven.com">这个网站</a>找到的。
          </p>
          <div class="threejs_center"><img
              src="../examples/resources/images/equirectangularmaps/tears_of_steel_bridge_2k.jpg" style="width: 600px">
          </div>
          <p>这并不难，首先，我们将等距矩形图片加载为纹理, 我们可以调用 <a
              href="/docs/#api/en/renderers/WebGLCubeRenderTarget.fromEquirectangularTexture"><code class="notranslate"
                translate="no">WebGLCubeRenderTarget.fromEquirectangularTexture</code></a>
            这将为我们从等距矩形纹理中生成一个立方体贴图，我们传入预期的立方体贴图的大小给 <a href="/docs/#api/en/renderers/WebGLCubeRenderTarget"><code
                class="notranslate" translate="no">WebGLCubeRenderTarget</code></a>，
            使用等距矩形图片的高度似乎是一个不错的假设。</p>
          <pre class="prettyprint showlinemods notranslate lang-js" translate="no">{
-  const loader = new THREE.CubeTextureLoader();
-  const texture = loader.load([
-    'resources/images/cubemaps/computer-history-museum/pos-x.jpg',
-    'resources/images/cubemaps/computer-history-museum/neg-x.jpg',
-    'resources/images/cubemaps/computer-history-museum/pos-y.jpg',
-    'resources/images/cubemaps/computer-history-museum/neg-y.jpg',
-    'resources/images/cubemaps/computer-history-museum/pos-z.jpg',
-    'resources/images/cubemaps/computer-history-museum/neg-z.jpg',
-  ]);
-  scene.background = texture;
+  const loader = new THREE.TextureLoader();
+  const texture = loader.load(
+    'resources/images/equirectangularmaps/tears_of_steel_bridge_2k.jpg',
+    () =&gt; {
+      const rt = new THREE.WebGLCubeRenderTarget(texture.image.height);
+      rt.fromEquirectangularTexture(renderer, texture);
+      scene.background = rt.texture;
+    });
}</pre>
          <p>这就是全部要做的事。</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/background-equirectangularmap.html"></iframe>
            </div>
            <a class="threejs_center" href="/manual/examples/background-equirectangularmap.html"
              target="_blank">点击在新窗口打开</a>
          </div>
          <p></p>
          <p>除了在加载时执行此操作，你还可以提前转换等距矩形到立方体贴图。<a href="https://matheowis.github.io/HDRI-to-CubeMap/">这是一个可以为你做这件事的网站</a>。
          </p>
        </div>
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