import {
	Matrix4,
	Mesh,
	MeshBasicMaterial
} from 'three';

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

const _shadowMatrix = new Matrix4();

class ShadowMesh extends Mesh {

	constructor( mesh ) {

		const shadowMaterial = new MeshBasicMaterial( {

			color: 0x000000,
			transparent: true,
			opacity: 0.6,
			depthWrite: false

		} );

		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 );

	}

}

export { ShadowMesh };

@@ -37,16 +37,56 @@
 Material
+,%0A%09EqualStencilFunc,%0A%09IncrementStencilOp
 %0A%7D from 
@@ -430,16 +430,161 @@
 e: false
+,%0A%09%09%09stencilWrite: true,%0A            %09%09stencilFunc: EqualStencilFunc,%0A            %09%09stencilRef: 0,%0A            %09%09stencilZPass: IncrementStencilOp
 %0A%0A%09%09%7D );

Improve ShadowMesh.js and Prevent Shadow Overlap (#24180)