real-zoom.js

import * as THREE from 'three';

export class RealZoom {
    constructor({ width, height }, diag) {
        var diagPX = Math.sqrt(width ** 2 + height ** 2);
        var screenWidthInch = width / diagPX * diag;
        var screenWidthCM = 2.54 * screenWidthInch;
        var viewWidthPX = window.innerWidth;
        this.viewWidthCM = viewWidthPX / width * screenWidthCM;
    }
    
    getDelta(camera) {
        var vFOV = THREE.Math.degToRad( camera.fov ); 
        var worldViewHeight = 2 * Math.tan(vFOV / 2) * camera.position.length();
        var viewWidthUnit = worldViewHeight * camera.aspect;
        var viewWidthMM = this.viewWidthCM * 10;

        return viewWidthMM / viewWidthUnit;
    }
}