three.js/examples/jsm/helpers/FaceNormalsHelper.js

import {
	BufferGeometry,
	Float32BufferAttribute,
	LineSegments,
	LineBasicMaterial,
	Matrix3,
	Vector3
} from '../../../build/three.module.js';

var _v1 = new Vector3();
var _v2 = new Vector3();
var _normalMatrix = new Matrix3();

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.setAttribute( 'position', positions );

	LineSegments.call( this, geometry, new LineBasicMaterial( { color: color, linewidth: width } ) );

	this.type = 'FaceNormalsHelper';

	//

	this.matrixAutoUpdate = false;
	this.update();

}

FaceNormalsHelper.prototype = Object.create( LineSegments.prototype );
FaceNormalsHelper.prototype.constructor = FaceNormalsHelper;

FaceNormalsHelper.prototype.update = function () {

	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;

};


export { FaceNormalsHelper };