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

( function () {

	/**
 * Blend two textures
 */
	const 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 );

		}`,
		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 );

		}`
	};

	THREE.BlendShader = BlendShader;

} )();