Three.js: Convert face normal from local space to world space

Question

I have a THREE.PlaneGeometry, with ComputeFaceNormals(). I create two meshes using this geometry, with different rotations applied to them. I want to compute the two angles between the camera and the two meshes central face normal. It should be :

vLocalCamera = vCamera.position - mesh1.position;

mesh1.normal() . vLocalCamera = cos(angle);

The problem is that I don't know how to get the mesh normal in world coordinate (from geometry and mesh rotation) with three.js API

Answer 1

If you want to convert a face or vertex normal, normal, from local space to world space, you do it like so:

var normalMatrix = new THREE.Matrix3(); // create once and reuse
var worldNormal = new THREE.Vector3(); // create once and reuse

...

normalMatrix.getNormalMatrix( object.matrixWorld );

worldNormal.copy( normal ).applyMatrix3( normalMatrix ).normalize();

three.js r.107

Answer 2

I finaly found an old post saying that THREE.js does not compute the world normal for you, so I did it by hand, and it works fine. Although, There might be nicer way to do it. In case other people are interested :

    var rotationMatrix = new THREE.Matrix4().extractRotation( this.mesh.matrixWorld );
    this.normalWorld = this.geometry.faces[0].normal.clone().applyMatrix4(rotationMatrix);

    var vLocalCamera = new THREE.Vector3();
    vLocalCamera.subVectors(wbEngine.wb_getCamera().getPosition(), this.mesh.position);
    var cosTheta = Math.abs(this.normalWorld.dot(vLocalCamera.normalize()));