title: three.js fade in/out post processing shaders
body: <p>I want to fade the scene into grayscale, but this also applies to other post-processing effects I'd like to fade in or out.</p>
  
  <p>So for this example I'm using the THREE.ColorifyShader, but when activating it, the whole scene instantly receives the intended color</p>
  
  <p>How can I "apply opacity/alpha" to post-processing?</p>
  
  <p>Code example:</p>
  
  <pre><code>var effectColorify,
      copyPass;
  
      effectColorify = new THREE.ShaderPass(THREE.ColorifyShader);
      effectColorify.uniforms['color'].value.setRGB(0.8, 0.8, 0.8); // Grayscale
  
      copyPass = new THREE.ShaderPass(THREE.CopyShader);
      copyPass.renderToScreen = true;
  
      this.composerGrayscale = new THREE.EffectComposer(this.engine.renderer);
      this.composerGrayscale.addPass(this.renderPass);
      this.composerGrayscale.addPass(effectColorify);
      this.composerGrayscale.addPass(copyPass);
  </code></pre>