three.js/src/objects/ShadowMesh.js

// File:src/objects/ShadowMesh.js

/**
 * @author erichlof /  http://github.com/erichlof
 */

// a shadow Mesh that follows its parent Mesh in the scene, but is confined to a single plane.

THREE.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.parentMeshMatrix = mesh.matrixWorld;
	
	this.frustumCulled = false;
	
	this.matrixAutoUpdate = false;

};

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

THREE.ShadowMesh.prototype.update = function () {

	// based on https://www.opengl.org/archives/resources/features/StencilTalk/tsld021.htm
	var shadowMatrix = new THREE.Matrix4();

	return function ( plane, lightPosition4D ) {
		
		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.parentMeshMatrix );

	};

}();