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<h1>How to use post-processing</h1>
<p>
Many three.js applications render their 3D objects directly to the screen. Sometimes, however, you want to apply one or more graphical
effects like Depth-Of-Field, Bloom, Film Grain or various types of Anti-aliasing. Post-processing is a widely used approach
to implement such effects. First, the scene is rendered to a render target which represents a buffer in the video card's memory.
In the next step one ore more post-processing passes apply filters and effects to the image buffer before it is eventually rendered to
the screen.
</p>
<p>
three.js provides a complete post-processing solution via [page:EffectComposer] to implement such a workflow.
</p>
<h2>Workflow</h2>
<p>
The first step in the process is to import all necessary files from the examples directory. The guide assumes your are using the official
[link:https://www.npmjs.com/package/three npm package] of three.js. For our basic demo in this guide we need the following files.
</p>
<code>
import { EffectComposer } from 'three/examples/jsm/postprocessing/EffectComposer.js';
import { RenderPass } from 'three/examples/jsm/postprocessing/RenderPass.js';
import { GlitchPass } from 'three/examples/jsm/postprocessing/GlitchPass.js';
</code>
<p>
After all files are successfully imported, we can create our composer by passing in an instance of [page:WebGLRenderer].
</p>
<code>
var composer = new EffectComposer( renderer );
</code>
<p>
When using a composer, it's necessary to change the application's animation loop. Instead of calling the render method of
[page:WebGLRenderer], we now use the respective counterpart of [page:EffectComposer].
</p>
<code>
function animate() {
requestAnimationFrame( animate );
composer.render();
}
</code>
<p>
Our composer is now ready so it's possible to configure the chain of post-processing passes. These passes are responsible for creating
the final visual output of the application. They are processed in order of their addition/insertion. In our example, the instance of *RenderPass*
is executed first and then the instance of *GlitchPass*. The last enabled pass in the chain is automatically rendered to the screen. The setup
of the passes looks like so:
</p>
<code>
var renderPass = new RenderPass( scene, camera );
composer.addPass( renderPass );
var glitchPass = new GlitchPass();
composer.addPass( glitchPass );
</code>
<p>
*RenderPass* is normally placed at the beginning of the chain in order to provide the rendered scene as an input for the next post-processing step. In our case,
*GlitchPass* is going to use these image data to apply a wild glitch effect. Check out this [link:https://threejs.org/examples/webgl_postprocessing_glitch live example]
to see it in action.
</p>
<h2>Built-in Passes</h2>
<p>
You can use a wide range of pre-defined post-processing passes provided by the engine. They are located in the
[link:https://github.com/mrdoob/three.js/tree/dev/examples/jsm/postprocessing postprocessing] directory.
</p>
<h2>Custom Passes</h2>
<p>
Sometimes you want to write a custom post-processing shader and include it into the chain of post-processing passes. For this scenario,
you can utilize *ShaderPass*. After importing the file and your custom shader, you can use the following code to setup the pass.
</p>
<code>
import { ShaderPass } from 'three/examples/jsm/postprocessing/ShaderPass.js';
import { LuminosityShader } from 'three/examples/jsm/shaders/LuminosityShader.js';
// later in your init routine
var luminosityPass = new ShaderPass( LuminosityShader );
composer.addPass( luminosityPass );
</code>
<p>
The repository provides a file called [link:https://github.com/mrdoob/three.js/blob/master/examples/jsm/shaders/CopyShader.js CopyShader] which is a
good starting code for your own custom shader. *CopyShader* just copies the image contents of the [page:EffectComposer]'s read buffer
to its write buffer without applying any effects.
</p>
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