Flip (mirror) any object with Three.js

Question

Update 2019

Since r89 three.js will also adjust the faces and normals. To flip/mirror an object simply use:

object3D.applyMatrix(new THREE.Matrix4().makeScale(-1, 1, 1));

No other adjustments as shown in the original solution are required. Link to the ticket: Support reflection matrices. #12787

Original question

I'm trying to create an utility that would flip any object in Three.js scene. The flipping itself is the easy bit:

object3D.applyMatrix(new THREE.Matrix4().makeScale(-1, 1, 1));

What is proving difficult is fixing faces and normals after the flip. The result looks quite messed up. Images of the source and flipped object: https://dl.dropboxusercontent.com/u/20925853/Flipped.png

I have seen a number of threads where this and similar issues were discussed but did not find anything usable. Does anybody know what I'm missing there? - Thanks!

Sample code on Js fiddle: https://jsfiddle.net/7dwh084w/

var renderer;
var scene;
var camera;

function render() {

    renderer.render(scene, camera);
};

function load(callback) {

    new THREE.ColladaLoader().load("https://dl.dropboxusercontent.com/u/20925853/Kitchen.dae", function (result) {

        var mesh = result.scene.children[0].children[0].clone();
        if (callback) callback(mesh);
    });
}

function init() {

    scene = new THREE.Scene();
    camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.1, 10000);
    camera.position.z = 1000;

    var light = new THREE.DirectionalLight(0xffffff);
    light.position.set(2, 1, 1).normalize();
    scene.add(light);
    var ambient = new THREE.AmbientLight(0x777777);
    scene.add(ambient);

    var controls = new THREE.OrbitControls(camera);
    controls.damping = 0.2;
    controls.addEventListener('change', render);

    renderer = new THREE.WebGLRenderer();
    renderer.setSize(window.innerWidth, window.innerHeight);
    document.body.appendChild(renderer.domElement);

    load(function (mesh) {

        flipMesh(mesh);

        scene.add(mesh);
        render();
    });

    render();
}

function flipMesh(object3D) {

    object3D.applyMatrix(new THREE.Matrix4().makeScale(-1, 1, 1));
    reverseWindingOrder(object3D);
}

function reverseWindingOrder(object3D) {

    // TODO: Something is missing, the objects are flipped alright but the light reflection on them is somehow broken

    if (object3D.type === "Mesh") {

        var geometry = object3D.geometry;

        for (var i = 0, l = geometry.faces.length; i < l; i++) {

            var face = geometry.faces[i];
            var temp = face.a;
            face.a = face.c;
            face.c = temp;

        }

        var faceVertexUvs = geometry.faceVertexUvs[0];
        for (i = 0, l = faceVertexUvs.length; i < l; i++) {

            var vector2 = faceVertexUvs[i][0];
            faceVertexUvs[i][0] = faceVertexUvs[i][2];
            faceVertexUvs[i][2] = vector2;
        }

        geometry.computeFaceNormals();
        geometry.computeVertexNormals();
    }

    if (object3D.children) {

        for (var j = 0, jl = object3D.children.length; j < jl; j++) {

            reverseWindingOrder(object3D.children[j]);
        }
    }
}

init();

Answer 1

I use this code in my project to flip objects:

    const scale = new THREE.Vector3(1, 1, 1);
    if (flipX) {
        scale.x *= -1;
    }
    if (flipY) {
        scale.z *= -1;
    }
    object.scale.multiply(scale);

Hope this helps.