three.js/examples/js/postprocessing/SavePass.js

/**
 * @author alteredq / http://alteredqualia.com/
 */

THREE.SavePass = function ( renderTarget ) {

	THREE.Pass.call( this );

	if ( THREE.CopyShader === undefined )
		console.error( "THREE.SavePass relies on THREE.CopyShader" );

	var shader = THREE.CopyShader;

	this.textureID = "tDiffuse";

	this.uniforms = THREE.UniformsUtils.clone( shader.uniforms );

	this.material = new THREE.ShaderMaterial( {

		uniforms: this.uniforms,
		vertexShader: shader.vertexShader,
		fragmentShader: shader.fragmentShader

	} );

	this.renderTarget = renderTarget;

	if ( this.renderTarget === undefined ) {

		this.renderTargetParameters = { minFilter: THREE.LinearFilter, magFilter: THREE.LinearFilter, format: THREE.RGBFormat, stencilBuffer: false };
		this.renderTarget = new THREE.WebGLRenderTarget( window.innerWidth, window.innerHeight, this.renderTargetParameters );

	}

	this.needsSwap = false;

	this.camera = new THREE.OrthographicCamera( - 1, 1, 1, - 1, 0, 1 );
	this.scene  = new THREE.Scene();

	this.quad = new THREE.Mesh( new THREE.PlaneBufferGeometry( 2, 2 ), null );
	this.scene.add( this.quad );

};

THREE.SavePass.prototype = Object.assign( Object.create( THREE.Pass.prototype ), {

	constructor: THREE.SavePass,

	render: function ( renderer, writeBuffer, readBuffer, delta, maskActive ) {

		if ( this.uniforms[ this.textureID ] ) {

			this.uniforms[ this.textureID ].value = readBuffer.texture;

		}

		this.quad.material = this.material;

		renderer.render( this.scene, this.camera, this.renderTarget, this.clear );

	}

} );