three.js/examples/jsm/effects/StereoEffect.js

import {
	StereoCamera,
	Vector2
} from 'three';

class StereoEffect {

	constructor( renderer ) {

		const _stereo = new StereoCamera();
		_stereo.aspect = 0.5;
		const size = new Vector2();

		this.setEyeSeparation = function ( eyeSep ) {

			_stereo.eyeSep = eyeSep;

		};

		this.setSize = function ( width, height ) {

			renderer.setSize( width, height );

		};

		this.render = function ( scene, camera ) {

			if ( scene.matrixWorldAutoUpdate === true ) scene.updateMatrixWorld();

			if ( camera.parent === null && camera.matrixWorldAutoUpdate === true ) camera.updateMatrixWorld();

			_stereo.update( camera );

			const currentAutoClear = renderer.autoClear;
			renderer.getSize( size );

			renderer.autoClear = false;
			renderer.clear();

			renderer.setScissorTest( true );

			renderer.setScissor( 0, 0, size.width / 2, size.height );
			renderer.setViewport( 0, 0, size.width / 2, size.height );
			renderer.render( scene, _stereo.cameraL );

			renderer.setScissor( size.width / 2, 0, size.width / 2, size.height );
			renderer.setViewport( size.width / 2, 0, size.width / 2, size.height );
			renderer.render( scene, _stereo.cameraR );

			renderer.setScissorTest( false );

			renderer.autoClear = currentAutoClear;

		};

	}

}

export { StereoEffect };