three.js/examples/js/objects/ShadowMesh.js

( function () {

	/**
 * A shadow THREE.Mesh that follows a shadow-casting THREE.Mesh in the scene, but is confined to a single plane.
 */

	var ShadowMesh = function ( mesh ) {

		var shadowMaterial = new THREE.MeshBasicMaterial( {
			color: 0x000000,
			transparent: true,
			opacity: 0.6,
			depthWrite: false
		} );
		THREE.Mesh.call( this, mesh.geometry, shadowMaterial );
		this.meshMatrix = mesh.matrixWorld;
		this.frustumCulled = false;
		this.matrixAutoUpdate = false;

	};

	ShadowMesh.prototype = Object.create( THREE.Mesh.prototype );
	ShadowMesh.prototype.constructor = ShadowMesh;

	ShadowMesh.prototype.update = function () {

		var shadowMatrix = new THREE.Matrix4();
		return function ( plane, lightPosition4D ) {

			// based on https://www.opengl.org/archives/resources/features/StencilTalk/tsld021.htm
			var dot = plane.normal.x * lightPosition4D.x + plane.normal.y * lightPosition4D.y + plane.normal.z * lightPosition4D.z + - plane.constant * lightPosition4D.w;
			var sme = shadowMatrix.elements;
			sme[ 0 ] = dot - lightPosition4D.x * plane.normal.x;
			sme[ 4 ] = - lightPosition4D.x * plane.normal.y;
			sme[ 8 ] = - lightPosition4D.x * plane.normal.z;
			sme[ 12 ] = - lightPosition4D.x * - plane.constant;
			sme[ 1 ] = - lightPosition4D.y * plane.normal.x;
			sme[ 5 ] = dot - lightPosition4D.y * plane.normal.y;
			sme[ 9 ] = - lightPosition4D.y * plane.normal.z;
			sme[ 13 ] = - lightPosition4D.y * - plane.constant;
			sme[ 2 ] = - lightPosition4D.z * plane.normal.x;
			sme[ 6 ] = - lightPosition4D.z * plane.normal.y;
			sme[ 10 ] = dot - lightPosition4D.z * plane.normal.z;
			sme[ 14 ] = - lightPosition4D.z * - plane.constant;
			sme[ 3 ] = - lightPosition4D.w * plane.normal.x;
			sme[ 7 ] = - lightPosition4D.w * plane.normal.y;
			sme[ 11 ] = - lightPosition4D.w * plane.normal.z;
			sme[ 15 ] = dot - lightPosition4D.w * - plane.constant;
			this.matrix.multiplyMatrices( shadowMatrix, this.meshMatrix );

		};

	}();

	THREE.ShadowMesh = ShadowMesh;

} )();