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.PlaneGeometry(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 );

		}

	}
}