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<h1 id="fog">雾（Fog）</h1>
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<p>在处理更复杂的问题之前，我们将学习如何在游戏引擎中创建雾特效。有了这个特效，就可以模拟遥远的物体变暗，似乎消失在浓雾中。</p>
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<p>让我们来确定定义雾的属性是什么。第一个是雾的颜色。在现实世界中，雾是灰色的，但我们可以利用这个特效来模拟不同颜色的雾覆盖的区域。还有一个属性是雾的浓度。</p>
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<p>因此，为了使用雾特效，我们需要找到一种方法，当3D场景的物体远离摄像机时，使它们褪色到雾的颜色。靠近摄像机的物体不会受到雾的影响，但远处的物体将无法分辨。因此，我们需要计算一个参数，可以用来混合雾的颜色与每个片元的颜色，以模拟雾特效。这个参数取决于与摄像机相距的距离。</p>
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<p>让我们把这个参数命名为<code>fogFactor</code>，并设定它的范围为0到1。当<code>fogFactor</code>为1时，意味着物体完全不会受到雾的影响，也就是说，它是较近的物体。当<code>fogFactor</code>为0时，意味着物体完全隐藏在雾中。</p>
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<p>然后，计算雾色的方程如下：</p>
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<p>
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<script type="math/tex; mode=display">finalColour = (1 - fogFactor) \cdot fogColour + fogFactor \cdot framentColour</script>
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</p>
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<ul>
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<li><strong>finalColour</strong> 是使用雾特效的最终颜色。</li>
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<li><strong>fogFactor</strong> 是控制雾的颜色与片元的颜色如何混合的参数，它基本上控制物体的可见性。</li>
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<li><strong>fogColour</strong> 是雾的颜色。</li>
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<li><strong>fragmentColour</strong> 没有使用雾特效的片元颜色。</li>
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</ul>
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<p>现在我们需要找到一种方法来基于距离计算<strong>fogFactor</strong>。我们可以选择不同的模型，首先使用线性模型。这是一个给定距离以线性方式改变<strong>fogFactor</strong>的模型。</p>
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<p>线性模型由以下参数定义：</p>
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<ul>
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<li><strong>fogStart</strong>: 开始使用雾特效的距离。</li>
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<li><strong>fogFinish</strong>: 雾特效达到最大值的距离。</li>
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<li><strong>distance</strong>: 到摄像机的距离。</li>
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</ul>
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<p>有了这些参数，方程就会是这样的：</p>
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<p>
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<script type="math/tex; mode=display">\displaystyle fogFactor = \frac{(fogFinish - distance)}{(fogFinish - fogStart)}</script>
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</p>
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<p>对于距离低于<strong>fogStart</strong>的物体我们简单地设置<strong>fogFactor</strong>为<strong>1</strong>。下图表明了<strong>fogFactor</strong>是如何随着距离变化而变化的。</p>
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<p><img alt="线性模型" src="../_static/16/linear_model.png" /></p>
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<p>线性模型易于计算，但不太真实，因为它不考虑雾气浓度。实际上雾往往以更平滑的方式增加。所以下一个合适的模型是指数模型。该模型的方程如下：</p>
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<p>
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<script type="math/tex; mode=display">\displaystyle focFactor = e^{-(distance \cdot fogDensity)^{exponent}} = \frac{1}{e^{(distance \cdot fogDensity)^{exponent}}}</script>
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</p>
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<p>其中的新变量如下所述：</p>
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<ul>
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<li><strong>fogDensity</strong> 是雾的厚度或浓度。</li>
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<li><strong>exponent</strong> 用来控制雾随着距离的增加增长的速度。</li>
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</ul>
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<p>下图显示了两个图形，分别设置了不同的<strong>exponent</strong>值（蓝线为<strong>2</strong>，红线为<strong>4</strong>）。</p>
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<p><img alt="指数模型" src="../_static/16/exponential_model.png" /></p>
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<p>在代码中，我们将使用一个公式，让它可以为<strong>exponent</strong>设置不同的值（你可以很容易地修改示例以使用不同的值）。</p>
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<p>既然已经解释过这个原理了，我们就可以实现它。我们将在场景的片元着色器中实现雾特效，因为这有我们需要的所有变量。我们将首先定义一个储存雾属性的结构体。</p>
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<pre><code class="language-glsl">struct Fog
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{
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int active;
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vec3 colour;
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float density;
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};
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</code></pre>
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<p><code>active</code>属性用于激活或关闭雾特效。雾属性将通过另一个被称作<code>fog</code>的Uniform传递给着色器。</p>
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<pre><code class="language-glsl">uniform Fog fog;
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</code></pre>
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<p>我们还将创建一个包含着雾属性的名为<code>Fog</code>的新类，它是另一个POJO（Plain Ordinary Java Object，简单的Java对象）。</p>
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<pre><code class="language-java">package org.lwjglb.engine.graph.weather;
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import org.joml.Vector3f;
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public class Fog {
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private boolean active;
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private Vector3f colour;
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private float density;
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public static Fog NOFOG = new Fog();
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public Fog() {
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active = false;
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this.colour = new Vector3f(0, 0, 0);
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this.density = 0;
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}
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public Fog(boolean active, Vector3f colour, float density) {
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this.colour = colour;
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this.density = density;
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this.active = active;
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}
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// Getters and setters here….
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</code></pre>
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<p>我们将在<code>Scene</code>类中添加一个<code>Fog</code>示例。默认情况下，<code>Scene</code>类将初始化一个<code>Fog</code>示例到常量<code>NOFOG</code>，用于模拟关闭雾特效的情况。</p>
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<p>因为添加了一个新的Uniform类型，所以我们需要修改<code>ShaderProgram</code>类来创建和初始化雾的Uniform。</p>
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<pre><code class="language-java">public void createFogUniform(String uniformName) throws Exception {
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createUniform(uniformName + ".active");
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createUniform(uniformName + ".colour");
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createUniform(uniformName + ".density");
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}
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public void setUniform(String uniformName, Fog fog) {
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setUniform(uniformName + ".activeFog", fog.isActive() ? 1 : 0);
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setUniform(uniformName + ".colour", fog.getColour() );
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setUniform(uniformName + ".density", fog.getDensity());
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}
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</code></pre>
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<p>在<code>Renderer</code>类中，我们只需要在<code>setupSceneShader</code>方法中创建Uniform：</p>
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<pre><code class="language-java">sceneShaderProgram.createFogUniform("fog");
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</code></pre>
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<p>然后在<code>renderScene</code>方法中使用它：</p>
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<pre><code class="language-java">sceneShaderProgram.setUniform("fog", scene.getFog());
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</code></pre>
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<p>我们现在可以在游戏中定义雾特效，但是需要回到片元着色器中应用雾特效。我们将创建一个名为<code>calcFog</code>的函数，函数定义如下。</p>
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<pre><code class="language-glsl">vec4 calcFog(vec3 pos, vec4 colour, Fog fog)
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{
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float distance = length(pos);
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float fogFactor = 1.0 / exp( (distance * fog.density)* (distance * fog.density));
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fogFactor = clamp( fogFactor, 0.0, 1.0 );
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vec3 resultColour = mix(fog.colour, colour.xyz, fogFactor);
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return vec4(resultColour.xyz, colour.w);
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}
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</code></pre>
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<p>如你所见，我们首先计算到顶点的距离。顶点坐标定义在<code>pos</code>变量中，我们只需要计算长度。然后利用<strong>exponent</strong>为2的指数模型（相当于乘以两次）计算雾参数。我们得到的<code>fogFactor</code>的范围在<strong>0</strong>到<strong>1</strong>之间，并使用<code>mix</code>函数。在GLSL中，<code>min</code>函数被用于混合雾色和图元颜色（由颜色变量定义）。相当于使用如下方程：</p>
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<p>
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<script type="math/tex; mode=display">resultColour = (1 - fogFactor) \cdot fog.colour + fogFactor \cdot colour</script>
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</p>
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<p>我们还为颜色保留了<strong>w</strong>元素，即透明度。我们不希望这个元素受到影响，片元应该保持它的透明程度不变。</p>
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<p>在应用了所有的光效之后，在片元着色器的最后，如果雾特效启用的话，我们将简单地把返回值设置为片元颜色。</p>
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<pre><code class="language-glsl">if ( fog.activeFog == 1 )
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{
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fragColor = calcFog(mvVertexPos, fragColor, fog);
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}
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</code></pre>
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<p>所有这些代码完成后，我们可以用下面的数据设置一个雾特效：</p>
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<pre><code class="language-java">scene.setFog(new Fog(true, new Vector3f(0.5f, 0.5f, 0.5f), 0.15f));
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</code></pre>
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<p>然后我们将获得像这样的效果：</p>
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<p><img alt="雾特效" src="../_static/16/fog_effect.png" /></p>
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<p>你会看到远处的物体褪色，当你靠近它们时，雾开始消失。但有一个问题，天空盒看起来有点奇怪，地平线不受雾的影响。有几种方法可以解决这个问题：</p>
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<ul>
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<li>使用只能看到天空的另一个不同的天空盒。</li>
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<li>删除天空盒，因为有浓雾，你不应该能够看到一个背景。</li>
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</ul>
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<p>也可能这两个解决方案没有适合你的，你可以试着将雾色与天空盒的背景相匹配，但这样你会做复杂的计算，结果也许不会更好。</p>
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<p>如果你运行这个示例，你会感到平行光变得暗淡，场景变暗，但雾看起来有问题，因为它不受光的影响，会看到如下图所示的结果。</p>
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<p><img alt="发光的雾" src="../_static/16/glowing_fog.png" /></p>
|
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<p>远处的物体变为雾色，这是一个不受光影响的常数。这造成了一个在黑暗中发光的效果（这可能并不好）。我们需要修改计算雾的函数，让其考虑光照。该函数将接收环境光和平行光来调整雾色。</p>
|
||
<pre><code class="language-glsl">vec4 calcFog(vec3 pos, vec4 colour, Fog fog, vec3 ambientLight, DirectionalLight dirLight)
|
||
{
|
||
vec3 fogColor = fog.colour * (ambientLight + dirLight.colour * dirLight.intensity);
|
||
float distance = length(pos);
|
||
float fogFactor = 1.0 / exp( (distance * fog.density)* (distance * fog.density));
|
||
fogFactor = clamp( fogFactor, 0.0, 1.0 );
|
||
|
||
vec3 resultColour = mix(fogColor, colour.xyz, fogFactor);
|
||
return vec4(resultColour.xyz, 1);
|
||
}
|
||
</code></pre>
|
||
<p>如你所见，平行光我们仅使用了颜色和强度，我们不需要关注它的方向。这样，我们只需要稍微修改函数的调用：</p>
|
||
<pre><code class="language-glsl">if ( fog.active == 1 )
|
||
{
|
||
fragColor = calcFog(mvVertexPos, fragColor, fog, ambientLight, directionalLight);
|
||
}
|
||
</code></pre>
|
||
<p>在夜晚时，我们会看到这样的效果。</p>
|
||
<p><img alt="夜晚的雾" src="../_static/16/fog_at_night.png" /></p>
|
||
<p>一个要强调的重要的事情是，我们必须聪明地选择雾色。这是很重要的，当我们没有天空盒，但有固定的颜色背景，应该把雾色设置为背景色。如果你删除了天空盒的代码并重新运行示例代码，你会得到这样的结果。</p>
|
||
<p><img alt="黑色背景" src="../_static/16/fog_clear_colour_black.png" /></p>
|
||
<p>但如果我们把背景色修改为（0.5, 0.5, 0.5），最终结果看起来就是如下所示。</p>
|
||
<p><img alt="雾灰色背景" src="../_static/16/fog_clear_colour_grey.png" /></p>
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