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

THREE.StereoEffect = function ( renderer ) {

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

	this.setEyeSeparation = function ( eyeSep ) {

		_stereo.eyeSep = eyeSep;

	};

	this.setSize = function ( width, height ) {

		renderer.setSize( width, height );

	};

	this.render = function ( scene, camera ) {

		scene.updateMatrixWorld();

		if ( camera.parent === null ) camera.updateMatrixWorld();

		_stereo.update( camera );

		renderer.getSize( size );

		if ( renderer.autoClear ) 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 );

	};

};