three.js/examples/js/shaders/BlendShader.js

( function () {

	/**
 * Blend two textures
 */
	var BlendShader = {
		uniforms: {
			'tDiffuse1': {
				value: null
			},
			'tDiffuse2': {
				value: null
			},
			'mixRatio': {
				value: 0.5
			},
			'opacity': {
				value: 1.0
			}
		},
		vertexShader: [ 'varying vec2 vUv;', 'void main() {', '	vUv = uv;', '	gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );', '}' ].join( '\n' ),
		fragmentShader: [ 'uniform float opacity;', 'uniform float mixRatio;', 'uniform sampler2D tDiffuse1;', 'uniform sampler2D tDiffuse2;', 'varying vec2 vUv;', 'void main() {', '	vec4 texel1 = texture2D( tDiffuse1, vUv );', '	vec4 texel2 = texture2D( tDiffuse2, vUv );', '	gl_FragColor = opacity * mix( texel1, texel2, mixRatio );', '}' ].join( '\n' )
	};

	THREE.BlendShader = BlendShader;

} )();