three.js/src/extras/helpers/SpotLightHelper.js

/**
 * @author alteredq / http://alteredqualia.com/
 * @author mrdoob / http://mrdoob.com/
 * @author WestLangley / http://github.com/WestLangley
*/

THREE.SpotLightHelper = function ( light ) {

	THREE.Object3D.call( this );

	this.light = light;
	this.light.updateMatrixWorld();

	this.matrix = light.matrixWorld;
	this.matrixAutoUpdate = false;

	var geometry = new THREE.BufferGeometry();

	var positions = [
		0, 0, 0,   0,   0,   1,
		0, 0, 0,   1,   0,   1,
		0, 0, 0, - 1,   0,   1,
		0, 0, 0,   0,   1,   1,
		0, 0, 0,   0, - 1,   1
	];

	for ( var i = 0, j = 1, l = 32; i < l; i ++, j ++ ) {

		var p1 = ( i / l ) * Math.PI * 2;
		var p2 = ( j / l ) * Math.PI * 2;

		positions.push(
			Math.cos( p1 ), Math.sin( p1 ), 1,
			Math.cos( p2 ), Math.sin( p2 ), 1
		);

	}

	geometry.addAttribute( 'position', new THREE.Float32Attribute( positions, 3 ) );

	var material = new THREE.LineBasicMaterial( { fog: false } );

	this.cone = new THREE.LineSegments( geometry, material );
	this.add( this.cone );

	this.update();

};

THREE.SpotLightHelper.prototype = Object.create( THREE.Object3D.prototype );
THREE.SpotLightHelper.prototype.constructor = THREE.SpotLightHelper;

THREE.SpotLightHelper.prototype.dispose = function () {

	this.cone.geometry.dispose();
	this.cone.material.dispose();

};

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

	var vector = new THREE.Vector3();
	var vector2 = new THREE.Vector3();

	return function () {

		var coneLength = this.light.distance ? this.light.distance : 1000;
		var coneWidth = coneLength * Math.tan( this.light.angle );

		this.cone.scale.set( coneWidth, coneWidth, coneLength );

		vector.setFromMatrixPosition( this.light.matrixWorld );
		vector2.setFromMatrixPosition( this.light.target.matrixWorld );

		this.cone.lookAt( vector2.sub( vector ) );

		this.cone.material.color.copy( this.light.color ).multiplyScalar( this.light.intensity );

	};

}();