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.
 */

	const _shadowMatrix = new THREE.Matrix4();

	class ShadowMesh extends THREE.Mesh {

		constructor( mesh ) {

			const shadowMaterial = new THREE.MeshBasicMaterial( {
				color: 0x000000,
				transparent: true,
				opacity: 0.6,
				depthWrite: false,
				stencilWrite: true,
				stencilFunc: THREE.EqualStencilFunc,
				stencilRef: 0,
				stencilZPass: THREE.IncrementStencilOp
			} );
			super( mesh.geometry, shadowMaterial );
			this.isShadowMesh = true;
			this.meshMatrix = mesh.matrixWorld;
			this.frustumCulled = false;
			this.matrixAutoUpdate = false;

		}

		update( plane, lightPosition4D ) {

			// based on https://www.opengl.org/archives/resources/features/StencilTalk/tsld021.htm
			const dot = plane.normal.x * lightPosition4D.x + plane.normal.y * lightPosition4D.y + plane.normal.z * lightPosition4D.z + - plane.constant * lightPosition4D.w;
			const 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;

} )();