ppi.js

// Identifies the correct screen density (i.e., PPI) -- DO NOT CHANGE!
// https://gist.github.com/scryptonite/5242987

function Display(facts, screen) { this.poll(facts, screen); }

Display.prototype.poll = function(facts, screen) {
    screen = screen || window.screen;
    this.facts = facts || this.facts || {};

    this.screen = {};
    this.screen.ppi = (screen.devicePixelRatio || devicePixelRatio) * 100;
    this.screen.width = screen.width;
    this.screen.height = screen.height;
    this.screen.diagonal = Math.sqrt(Math.pow(this.screen.width, 2) + Math.pow(this.screen.height, 2));

    this.guess = {};
    this.guess.ppi = function() {
        try {
            var el = document.createElement("div");
            el.style.width = "72pt";
            document.body.appendChild(el);
            var _ppi = (this.screen.ppi / 100) * (el.offsetWidth || 96);
            document.body.removeChild(el);
            return _ppi;
        } catch (e) {
            return 96;
        }
    }.bind(this)();
    this.guess.width = this.screen.width / this.guess.ppi;
    this.guess.height = this.screen.height / this.guess.ppi;
    this.guess.diagonal = Math.sqrt(Math.pow(this.guess.width, 2) + Math.pow(this.guess.height, 2));

    if ("diagonal" in this.facts) {
        this.facts.width =
            Math.sqrt(Math.pow(this.facts.diagonal, 2) -
                      Math.pow(this.facts.diagonal, 2) * Math.pow(this.screen.height / this.screen.width, 2));
        this.facts.height = this.facts.width * (this.screen.height / this.screen.width);
        this.facts.diagonal = this.facts.diagonal + 0;
        this.facts.ppi = Math.round(this.screen.width /
                                    Math.sqrt(Math.pow(this.facts.diagonal, 2) *
                                              (Math.pow(this.facts.width, 2) /
                                               (Math.pow(this.facts.width, 2) + Math.pow(this.facts.height, 2)))));
    } else if ("width" in this.facts) {
        this.facts.width = this.facts.width + 0;
        this.facts.height = (this.screen.height / this.screen.width) * this.facts.width;
        this.facts.diagonal = Math.sqrt(Math.pow(this.facts.width, 2) + Math.pow(this.facts.height, 2));
        this.facts.ppi = Math.round(this.screen.width /
                                    Math.sqrt(Math.pow(this.facts.diagonal, 2) *
                                              (Math.pow(this.facts.width, 2) /
                                               (Math.pow(this.facts.width, 2) + Math.pow(this.facts.height, 2)))));
    } else if ("height" in this.facts) {
        this.facts.width = (this.screen.width / this.screen.height) * this.facts.height;
        this.facts.height = this.facts.height + 0;
        this.facts.diagonal = Math.sqrt(Math.pow(this.facts.width, 2) + Math.pow(this.facts.height, 2));
        this.facts.ppi = Math.round(this.screen.width /
                                    Math.sqrt(Math.pow(this.facts.diagonal, 2) *
                                              (Math.pow(this.facts.width, 2) /
                                               (Math.pow(this.facts.width, 2) + Math.pow(this.facts.height, 2)))));
    } else if ("ppi" in this.facts) {
        this.facts.width = this.screen.width / this.facts.ppi;
        this.facts.height = this.screen.height / this.facts.ppi;
        this.facts.diagonal = Math.sqrt(Math.pow(this.facts.width, 2) + Math.pow(this.facts.height, 2));
        this.facts.ppi = this.facts.ppi + 0;
    } else {
        this.facts.width = this.guess.width;
        this.facts.height = this.guess.height;
        this.facts.diagonal = this.guess.diagonal;
        this.facts.ppi = this.guess.height;
    }

    this.width = this.facts.width || this.guess.width;
    this.height = this.facts.height || this.guess.width;
    this.diagonal = this.facts.diagonal || this.guess.diagonal;
    this.ppi = this.facts.ppi || this.guess.ppi;

    if (!this.facts.__halt) {
        this.poll({ppi : this.ppi, __halt : true}, screen);
    }

    delete this.facts.__halt;
    return this;
};