Three.js DoubleSided material doesn't cast shadow on both sides of planar parametric geometry

Question

See this jfiddle: http://jsfiddle.net/blwoodley/5Tr4D/1/

I have a blue spot light that shines on a rotating rotating square. This casts a shadow to the underlying ground. Except that it only casts a shadow on one side of the square.

I saw this discussion: https://github.com/mrdoob/three.js/issues/3544 which indicates that face culling on planar surfaces is the cause. the recommendation is to give my square some depth, i.e. make it a cube.

I can do that with this simple example, but I'm encountering the same problem with a parametric geometry that is a surface. Is there a way to get both sides to cast a shadow without having to give my geometry a depth in doesn't have or need?

Here is the main function in the fiddle that replicates the problem with a plane:

function init() {

container = document.createElement('div');
document.body.appendChild(container);

camera = new THREE.PerspectiveCamera(30, window.innerWidth / window.innerHeight, 1, 100000);
camera.position.x = 100;
camera.position.y = 100;
camera.position.z = 100;
camera.lookAt({x: 0,y: 0,z: 0});

scene = new THREE.Scene();

var groundMaterial = new THREE.MeshLambertMaterial({
    color: 0xffffff, side:THREE.DoubleSide 
});
plane = new THREE.Mesh(new THREE.PlaneGeometry(2000,2000,10,10), groundMaterial);
plane.rotation.x = Math.PI / 2;
plane.position.y = -40;
plane.receiveShadow = true;

scene.add(plane);

var light;

light = new THREE.SpotLight(0x0000ff);
light.position.set(40, 40, 0);
light.castShadow = true;
light.shadowCameraVisible = true;
light.shadowMapWidth = 2048;
light.shadowMapHeight = 2048;
light.position.set(24, 20, 0);
light.lookAt(plane);    
light.castShadow = true;
light.angle = .8;
light.intensity = 30;
light.distance=0;
light.shadowCameraNear = 2;
light.shadowCameraFar = 100;
light.shadowCameraFov = 100;
light.shadowDarkness = 1;
light.shadowCameraVisible = true;
scene.add(light);

var planeGeo = new THREE.PlaneGeometry(20,20,20,20)
_planeMesh = new THREE.Mesh(planeGeo, new THREE.MeshLambertMaterial( { color: 0x00ff00, side:THREE.DoubleSide } ) );
_planeMesh.castShadow = true;
scene.add( _planeMesh );


// RENDERER
webglRenderer = new THREE.WebGLRenderer();
webglRenderer.setSize(SCREEN_WIDTH, SCREEN_HEIGHT);
webglRenderer.domElement.style.position = "relative";
webglRenderer.shadowMapEnabled = true;
webglRenderer.shadowMapSoft = true;

container.appendChild(webglRenderer.domElement);
window.addEventListener('resize', onWindowResize, false);

}

Answer 1

Yes, this is a feature.

WebGLRenderer, by default, culls front faces when rendering shadows. This is OK, because it is assumed that objects have depth. You can cull back faces, instead, if you want:

renderer.shadowMapCullFace = THREE.CullFaceBack;

... but culling neither is not an option.

The material.side property is not taken into consideration when casting shadows.

three.js r.63