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

/**
 * Blend two textures
 */

THREE.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' )

};