text: import { Matrix3 } from '../../math/Matrix3';
  import { Vector3 } from '../../math/Vector3';
  import { LineSegments } from '../../objects/LineSegments';
  import { LineBasicMaterial } from '../../materials/LineBasicMaterial';
  import { Float32BufferAttribute } from '../../core/BufferAttribute';
  import { BufferGeometry } from '../../core/BufferGeometry';
  
  /**
   * @author mrdoob / http://mrdoob.com/
   * @author WestLangley / http://github.com/WestLangley
  */
  
  function FaceNormalsHelper( object, size, hex, linewidth ) {
  
  	// FaceNormalsHelper only supports THREE.Geometry
  
  	this.object = object;
  
  	this.size = ( size !== undefined ) ? size : 1;
  
  	var color = ( hex !== undefined ) ? hex : 0xffff00;
  
  	var width = ( linewidth !== undefined ) ? linewidth : 1;
  
  	//
  
  	var nNormals = 0;
  
  	var objGeometry = this.object.geometry;
  
  	if ( objGeometry && objGeometry.isGeometry ) {
  
  		nNormals = objGeometry.faces.length;
  
  	} else {
  
  		console.warn( 'THREE.FaceNormalsHelper: only THREE.Geometry is supported. Use THREE.VertexNormalsHelper, instead.' );
  
  	}
  
  	//
  
  	var geometry = new BufferGeometry();
  
  	var positions = new Float32BufferAttribute( nNormals * 2 * 3, 3 );
  
  	geometry.addAttribute( 'position', positions );
  
  	LineSegments.call( this, geometry, new LineBasicMaterial( { color: color, linewidth: width } ) );
  
  	//
  
  	this.matrixAutoUpdate = false;
  	this.update();
  
  }
  
  FaceNormalsHelper.prototype = Object.create( LineSegments.prototype );
  FaceNormalsHelper.prototype.constructor = FaceNormalsHelper;
  
  FaceNormalsHelper.prototype.update = ( function () {
  
  	var v1 = new Vector3();
  	var v2 = new Vector3();
  	var normalMatrix = new Matrix3();
  
  	return function update() {
  
  		this.object.updateMatrixWorld( true );
  
  		normalMatrix.getNormalMatrix( this.object.matrixWorld );
  
  		var matrixWorld = this.object.matrixWorld;
  
  		var position = this.geometry.attributes.position;
  
  		//
  
  		var objGeometry = this.object.geometry;
  
  		var vertices = objGeometry.vertices;
  
  		var faces = objGeometry.faces;
  
  		var idx = 0;
  
  		for ( var i = 0, l = faces.length; i < l; i ++ ) {
  
  			var face = faces[ i ];
  
  			var normal = face.normal;
  
  			v1.copy( vertices[ face.a ] )
  				.add( vertices[ face.b ] )
  				.add( vertices[ face.c ] )
  				.divideScalar( 3 )
  				.applyMatrix4( matrixWorld );
  
  			v2.copy( normal ).applyMatrix3( normalMatrix ).normalize().multiplyScalar( this.size ).add( v1 );
  
  			position.setXYZ( idx, v1.x, v1.y, v1.z );
  
  			idx = idx + 1;
  
  			position.setXYZ( idx, v2.x, v2.y, v2.z );
  
  			idx = idx + 1;
  
  		}
  
  		position.needsUpdate = true;
  
  		return this;
  
  	};
  
  }() );
  
  
  export { FaceNormalsHelper };
  