three.js/examples/js/crossfade/transition.js

function Transition ( sceneA, sceneB ) {

	this.scene = new THREE.Scene();
	
	this.cameraOrtho = new THREE.OrthographicCamera( window.innerWidth / - 2, window.innerWidth / 2, window.innerHeight / 2, window.innerHeight / - 2, - 10, 10 );

	this.textures = [];
	for ( var i = 0; i < 6; i ++ )
		this.textures[ i ] = new THREE.ImageUtils.loadTexture( 'textures/transition/transition' + ( i + 1 ) + '.png' );
				
	this.quadmaterial = new THREE.ShaderMaterial( {

		uniforms: {

			tDiffuse1: {
				type: "t",
				value: null
			},
			tDiffuse2: {
				type: "t",
				value: null
			},
			mixRatio: {
				type: "f",
				value: 0.0
			},
			threshold: {
				type: "f",
				value: 0.1
			},
			useTexture: {
				type: "i",
				value: 1,
			},
			tMixTexture: {
				type: "t",
				value: this.textures[ 0 ]
			}
		},
		vertexShader: [

			"varying vec2 vUv;",

			"void main() {",

			"vUv = vec2( uv.x, uv.y );",
			"gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

			"}"

		].join( "\n" ),
		fragmentShader: [

			"uniform float mixRatio;",

			"uniform sampler2D tDiffuse1;",
			"uniform sampler2D tDiffuse2;",
			"uniform sampler2D tMixTexture;",
			
			"uniform int useTexture;",
			"uniform float threshold;",

			"varying vec2 vUv;",

			"void main() {",

			"vec4 texel1 = texture2D( tDiffuse1, vUv );",
			"vec4 texel2 = texture2D( tDiffuse2, vUv );",
			
			"if (useTexture==1) {",
				
				"vec4 transitionTexel = texture2D( tMixTexture, vUv );",
				"float r = mixRatio * (1.0 + threshold * 2.0) - threshold;",
				"float mixf=clamp((transitionTexel.r - r)*(1.0/threshold), 0.0, 1.0);",
				
				"gl_FragColor = mix( texel1, texel2, mixf );",
			"} else {",
				
				"gl_FragColor = mix( texel2, texel1, mixRatio );",
				
			"}",
		"}"

		].join( "\n" )

	} );

	quadgeometry = new THREE.PlaneBufferGeometry( window.innerWidth, window.innerHeight );
	
	this.quad = new THREE.Mesh( quadgeometry, this.quadmaterial );
	this.scene.add( this.quad );

	// Link both scenes and their FBOs
	this.sceneA = sceneA;
	this.sceneB = sceneB;

	this.quadmaterial.uniforms.tDiffuse1.value = sceneA.fbo;
	this.quadmaterial.uniforms.tDiffuse2.value = sceneB.fbo;

	this.needChange = false;
	
	this.setTextureThreshold = function ( value ) {
	
		this.quadmaterial.uniforms.threshold.value = value;

	};
	
	this.useTexture = function ( value ) {
	
		this.quadmaterial.uniforms.useTexture.value = value ? 1 : 0;

	};
	
	this.setTexture = function ( i ) {
		
		this.quadmaterial.uniforms.tMixTexture.value = this.textures[ i ];
		
	};
	
	this.render = function( delta ) {
		
		// Transition animation
		if ( transitionParams.animateTransition ) {

			var t = ( 1 + Math.sin( transitionParams.transitionSpeed * clock.getElapsedTime() / Math.PI ) ) / 2;
			transitionParams.transition = THREE.Math.smoothstep( t, 0.3, 0.7 );
			
			// Change the current alpha texture after each transition
			if ( transitionParams.loopTexture && ( transitionParams.transition == 0 || transitionParams.transition == 1 ) ) {

				if ( this.needChange ) {

					transitionParams.texture = ( transitionParams.texture + 1 ) % this.textures.length;
					this.quadmaterial.uniforms.tMixTexture.value = this.textures[ transitionParams.texture ];
					this.needChange = false;

				}

			} else
				this.needChange = true;
				
		}
		
		this.quadmaterial.uniforms.mixRatio.value = transitionParams.transition;

		// Prevent render both scenes when it's not necessary
		if ( transitionParams.transition == 0 ) {
			
			this.sceneB.render( delta, false );
		
		} else if ( transitionParams.transition == 1 ) {
		
			this.sceneA.render( delta, false );
			
		} else {
			
			// When 0<transition<1 render transition between two scenes
			
			this.sceneA.render( delta, true );
			this.sceneB.render( delta, true );
			renderer.render( this.scene, this.cameraOrtho, null, true );

		}

	}

}