three.js/examples/js/shaders/MirrorShader.js

( function () {

	/**
 * Mirror Shader
 * Copies half the input to the other half
 *
 * side: side of input to mirror (0 = left, 1 = right, 2 = top, 3 = bottom)
 */
	const MirrorShader = {
		uniforms: {
			'tDiffuse': {
				value: null
			},
			'side': {
				value: 1
			}
		},
		vertexShader: `varying vec2 vUv;

		void main() {

			vUv = uv;
			gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );

		}`,
		fragmentShader: `uniform sampler2D tDiffuse;
		uniform int side;

		varying vec2 vUv;

		void main() {

			vec2 p = vUv;
			if (side == 0){
				if (p.x > 0.5) p.x = 1.0 - p.x;
			}else if (side == 1){
				if (p.x < 0.5) p.x = 1.0 - p.x;
			}else if (side == 2){
				if (p.y < 0.5) p.y = 1.0 - p.y;
			}else if (side == 3){
				if (p.y > 0.5) p.y = 1.0 - p.y;
			}
			vec4 color = texture2D(tDiffuse, p);
			gl_FragColor = color;

		}`
	};

	THREE.MirrorShader = MirrorShader;

} )();