What goes wrong when I compute screen-space position?

I want to compute the screen-space size of a bounding box.
Here is my code(heightOnScreen means the pixels which the box occupies longitudinally on screen space, I try to compute it by the diagonal lines of the box to take projection into account):

let center = new THREE.Vector3((box.min.x + box.max.x) / 2, (box.min.y + box.max.y) / 2, (box.min.z + box.max.z) / 2);
let extents = new THREE.Vector3((box.max.x - box.min.x) / 2, (box.max.y - box.min.y) / 2, (box.max.z - box.min.z) / 2);

let a = center
  .clone()
  .add(new THREE.Vector3(-extents.x, -extents.y, -extents.z))
  .project(this._mainCamera);
let b = center
  .clone()
  .add(new THREE.Vector3(extents.x, -extents.y, extents.z))
  .project(this._mainCamera);
let c = center
  .clone()
  .add(new THREE.Vector3(extents.x, extents.y, extents.z))
  .project(this._mainCamera);
let d = center
  .clone()
  .add(new THREE.Vector3(-extents.x, extents.y, -extents.z))
  .project(this._mainCamera);

a.x = 0;
b.x = 0;
c.x = 0;
d.x = 0;
let heightOnScreen = Math.max(a.distanceTo(c), b.distanceTo(d));

I found that for a same box, a's value doesn’t change wherever the camera is, so my heightOnScreen(doesn’t change of course) is meaningless. Any kind of help is welcome.:blush:

I make a simpler experiment:

this._mainCamera = new THREE.PerspectiveCamera(70, window.innerWidth / window.innerHeight, 1, 200);
this._mainCamera.position.copy(new THREE.Vector3(0, 0, 50)); // init pos
this._mainCamera.lookAt(new THREE.Vector3(0, 0, 0));
let zero = new THREE.Vector3(0, 0, 0);
console.log("zero", zero.project(this._mainCamera));

And this give me Vector3 {x: NaN, y: NaN, z: -Infinity}. I think it shouble be 0,0,0.

I need to add this._mainCamera.updateMatrixWorld(); after change camera’s properties.:sweat_smile: