Three.js SoftwareRenderer - transparent background

I am playing with one of the three.js examples http://threejs.org/examples/#software_geometry_earth

How do I make the background transparent? Doesn't work for SoftwareRenderer (but it works for WebGLRenderer, but I need to use SoftwareRenderer).

renderer = new THREE.SoftwareRenderer( { alpha: true } );
renderer.setClearColor( 0x000000, 0 );

      

        
        
        
      

    

Any ideas?

Complete code:

        var container, stats;
        var camera, scene, renderer;
        var group;
        var mouseX = 0, mouseY = 0;

        var windowHalfX = window.innerWidth / 2;
        var windowHalfY = window.innerHeight / 2;

        init();
        animate();

        function init() {

            container = document.getElementById( 'container' );

            camera = new THREE.PerspectiveCamera( 60, window.innerWidth / window.innerHeight, 1, 2000 );
            camera.position.z = 500;

            scene = new THREE.Scene();

            group = new THREE.Group();
            scene.add( group );

            // earth

            var loader = new THREE.TextureLoader();
            loader.load( 'textures/land_ocean_ice_cloud_2048.jpg', function ( texture ) {

                var geometry = new THREE.SphereGeometry( 200, 20, 20 );

                var material = new THREE.MeshLambertMaterial( { map: texture, overdraw: 0.5 } );
                var mesh = new THREE.Mesh( geometry, material );
                group.add( mesh );

            } );

            // shadow

            var canvas = document.createElement( 'canvas' );
            canvas.width = 128;
            canvas.height = 128;

            var texture = new THREE.Texture( canvas );
            texture.needsUpdate = true;

            var geometry = new THREE.PlaneBufferGeometry( 0, 0, 0, 0 );
            var material = new THREE.MeshBasicMaterial( { map: texture, overdraw: 0.5 } );

            var mesh = new THREE.Mesh( geometry, material );
            mesh.position.y = - 250;
            mesh.rotation.x = - Math.PI / 2;
            group.add( mesh );

            renderer = new THREE.SoftwareRenderer( { alpha: true } );
            renderer.setClearColor( 0x000000, 0 );
            renderer.setSize( window.innerWidth, window.innerHeight );

            container.appendChild( renderer.domElement );

            document.addEventListener( 'mousemove', onDocumentMouseMove, false );

            //

            window.addEventListener( 'resize', onWindowResize, false );

        }

        function onWindowResize() {

            windowHalfX = window.innerWidth / 2;
            windowHalfY = window.innerHeight / 2;

            camera.aspect = window.innerWidth / window.innerHeight;
            camera.updateProjectionMatrix();

            renderer.setSize( window.innerWidth, window.innerHeight );

        }

        function onDocumentMouseMove( event ) {

            mouseX = ( event.clientX - windowHalfX );
            mouseY = ( event.clientY - windowHalfY );

        } 

        //

        function animate() {

            requestAnimationFrame( animate );

            render();
            // stats.update(); 

        }

        function render() {

            camera.position.x += ( mouseX - camera.position.x ) * 0.05;
            camera.position.y += ( - mouseY - camera.position.y ) * 0.05;
            camera.lookAt( scene.position );

            group.rotation.y -= 0.005;

            renderer.render( scene, camera );

        }

      

        
        
        
      

    

** edit: here's a link to the softwareRenderer.js file I used. I am opening it with Sublime Text.